cmd: add --no-skills, --skill, and --skills-dir CLI flags & config (#55)

* cmd: add --no-skills, --skill, and --skills-dir CLI flags

The pkg/kit Options struct already had full backend support for skills
control (NoSkills, Skills []string, SkillsDir) wired into loadSkills()
in pkg/kit/kit.go, but there were no corresponding CLI flags to drive
them. This commit closes that gap.

Changes in cmd/root.go:

- Add three package-level flag variables alongside the existing
  noExtensionsFlag/extensionPaths group:
    noSkillsFlag bool
    skillsPaths  []string
    skillsDir    string

- Register three persistent cobra flags in init():
    --no-skills        disable skill loading (auto-discovery and explicit)
    --skill <path>     load a skill file or directory (repeatable)
    --skills-dir <dir> override the project-local skills directory
                       used for auto-discovery

- Wire all three into the kitOpts struct literal in runNormalMode()
  so they flow directly into kit.New() -> loadSkills().

No changes to pkg/kit or internal/skills -- the backend was already
complete. No viper binding is needed because kit.go reads these fields
directly from opts rather than from viper (unlike NoExtensions which
uses the viper fallback path).

Example usage:
  kit --no-skills "prompt"
  kit --skill ./my-skill.md --skill ./other-skill.md "prompt"
  kit --skills-dir /path/to/skills "prompt"

Co-authored-by: Claude <claude@anthropic.com>

* docs: document --no-skills, --skill, and --skills-dir CLI flags

Add the three new skills CLI flags to all relevant documentation:

- README.md: add Skills section under Global Flags CLI reference
- www/pages/cli/flags.md: add Skills table (mirrors Extensions section pattern)
- www/pages/cli/commands.md: expand the Skills section with usage examples
  and a description of auto-discovery vs explicit loading vs --no-skills

Co-authored-by: Claude <claude@anthropic.com>

* feat: add config file support for skills options

Skills could previously only be controlled via CLI flags or SDK Options
fields. This commit wires all three skills settings into viper so they
can also be set in .kit.yml / .kit.yaml / .kit.json and via KIT_*
environment variables — matching the pattern used by no-extensions,
no-core-tools, and prompt-template.

cmd/root.go:
- Bind --no-skills, --skill, and --skills-dir flags to viper keys
  (no-skills, skill, skills-dir) so config file values flow through.

pkg/kit/kit.go:
- At skill-load time, merge opts fields with viper values:
  - noSkills = opts.NoSkills || v.GetBool("no-skills")
  - skillPaths: opts.Skills if non-empty, else v.GetStringSlice("skill")
  - skillsDir: opts.SkillsDir if non-empty, else v.GetString("skills-dir")
- Build a shallow-copied mergedOpts so loadSkills() picks up the
  resolved values without mutating the original Options struct.

docs:
- README.md: add skills keys to the Basic Configuration YAML example
- www/pages/configuration.md: add no-skills, skill, skills-dir rows to
  the All configuration keys table

Config file example (.kit.yml):
  no-skills: false
  skill:
    - /path/to/skill.md
  skills-dir: /path/to/skills/

Co-authored-by: Claude <claude@anthropic.com>

* config: add skills keys to default .kit.yml template

Add no-skills, skill, and skills-dir as commented-out examples in the
default config file generated by EnsureConfigExists(), alongside the
existing application settings block.

Co-authored-by: Claude <claude@anthropic.com>

* test: add test coverage for skills CLI flags and config keys

Four test locations updated:

pkg/kit/export_test.go:
- Add ConfigStringSliceForTest() helper to expose v.GetStringSlice()
  from the Kit's isolated viper store, needed to assert skill list values.

pkg/kit/kit_test.go (TestNewWithSkillsOptions):
- NoSkills=true: GetSkills() returns empty slice
- SkillsDir=<empty dir>: kit.New() succeeds with zero skills
- Skills=[file]: single explicit skill file is loaded and name parsed correctly

pkg/kit/viper_isolation_test.go:
- TestSkillsViperKeys: no-API-key struct-level checks for NoSkills, Skills,
  and SkillsDir fields on Options
- TestSkillsConfigFileKeys: full kit.New() round-trips via a written .kit.yml
  for each of the three config keys:
    no-skills: true  → GetSkills() returns empty
    skill: [path]    → named skill loaded from config file path
    skills-dir: dir  → custom discovery root accepted without error

internal/config/config_test.go (TestEnsureConfigExists):
- Assert generated ~/.kit.yml template contains '# Skills configuration',
  'no-skills:', and 'skills-dir:' comment blocks.

Co-authored-by: Claude <claude@anthropic.com>

---------

Co-authored-by: Claude <claude@anthropic.com>

.kit/extensions/go-edit-lint.go

@@ -1,268 +0,0 @@
-//go:build ignore
-
-package main
-
-import (
-	"context"
-	"encoding/json"
-	"fmt"
-	"os/exec"
-	"path/filepath"
-	"strings"
-	"time"
-
-	"kit/ext"
-)
-
-const (
-	diagnosticsTimeout = 20 * time.Second
-	maxOutputBytes     = 12_000
-)
-
-type toolPathInput struct {
-	Path string `json:"path"`
-}
-
-type lintResult struct {
-	Output string
-	Err    error
-}
-
-// Package-level state: set of .go files edited during the current agent turn.
-var editedFiles map[string]bool
-
-func Init(api ext.API) {
-	api.OnSessionStart(func(_ ext.SessionStartEvent, ctx ext.Context) {
-		ctx.Print("go-edit-lint extension loaded - will run gopls and golangci-lint after agent turns that edit Go files")
-	})
-
-	// Track edited .go files — don't lint yet.
-	api.OnToolResult(func(e ext.ToolResultEvent, ctx ext.Context) *ext.ToolResultResult {
-		if e.IsError || !isEditOrWrite(e.ToolName) {
-			return nil
-		}
-
-		absPath, ok := resolveGoFilePath(e.Input, ctx.CWD)
-		if !ok {
-			return nil
-		}
-
-		if editedFiles == nil {
-			editedFiles = make(map[string]bool)
-		}
-		editedFiles[absPath] = true
-		return nil
-	})
-
-	// After the agent turn ends, lint all collected files.
-	api.OnAgentEnd(func(e ext.AgentEndEvent, ctx ext.Context) {
-		if len(editedFiles) == 0 {
-			return
-		}
-
-		// Snapshot and reset immediately so the next turn starts clean.
-		files := editedFiles
-		editedFiles = nil
-
-		// Skip lint on errored turns.
-		if e.StopReason == "error" {
-			return
-		}
-
-		// Collect unique directories and file list for gopls.
-		var allGoplsOutput []string
-		for absPath := range files {
-			res := runGopls(ctx.CWD, absPath)
-			formatted := formatToolResult(res, "")
-			if formatted != "" {
-				allGoplsOutput = append(allGoplsOutput, fmt.Sprintf("# %s\n%s", filepath.Base(absPath), formatted))
-			}
-		}
-
-		lintRes := runGolangCILint(ctx.CWD, "./...")
-
-		goplsSection := "No diagnostics."
-		if len(allGoplsOutput) > 0 {
-			goplsSection = strings.Join(allGoplsOutput, "\n\n")
-		}
-		lintSection := formatToolResult(lintRes, "No lint issues.")
-
-		// Build file list for the report header.
-		var fileNames []string
-		for absPath := range files {
-			fileNames = append(fileNames, filepath.Base(absPath))
-		}
-
-		report := fmt.Sprintf(
-			"<go_diagnostics files=%q>\n[gopls]\n%s\n\n[golangci-lint]\n%s\n</go_diagnostics>",
-			strings.Join(fileNames, ", "),
-			goplsSection,
-			lintSection,
-		)
-
-		goplsIssues, lintIssues := countIssues(report)
-		hasIssues := goplsIssues > 0 || lintIssues > 0
-
-		if hasIssues {
-			// Show TUI block so the user sees it too.
-			var msgLines []string
-			msgLines = append(msgLines, fmt.Sprintf("Files: %s", strings.Join(fileNames, ", ")))
-			if goplsIssues > 0 {
-				msgLines = append(msgLines, fmt.Sprintf("gopls: %d issue(s)", goplsIssues))
-			}
-			if lintIssues > 0 {
-				msgLines = append(msgLines, fmt.Sprintf("golangci-lint: %d issue(s)", lintIssues))
-			}
-
-			borderColor := "#f9e2af" // yellow
-			if goplsIssues > 0 && lintIssues > 0 {
-				borderColor = "#f38ba8" // red
-			}
-
-			ctx.PrintBlock(ext.PrintBlockOpts{
-				Text:        strings.Join(msgLines, "\n"),
-				BorderColor: borderColor,
-				Subtitle:    "go-edit-lint",
-			})
-
-			// Inject a follow-up message so the agent fixes the issues.
-			ctx.SendMessage(report + "\n\n⚠️ DIAGNOSTICS FOUND: Please review and fix the issues above.")
-		} else {
-			ctx.PrintBlock(ext.PrintBlockOpts{
-				Text:        fmt.Sprintf("Files: %s\n✓ All clean", strings.Join(fileNames, ", ")),
-				BorderColor: "#a6e3a1",
-				Subtitle:    "go-edit-lint",
-			})
-		}
-	})
-}
-
-func isEditOrWrite(toolName string) bool {
-	return strings.EqualFold(toolName, "edit") || strings.EqualFold(toolName, "write")
-}
-
-func resolveGoFilePath(inputJSON, cwd string) (string, bool) {
-	var args toolPathInput
-	if err := json.Unmarshal([]byte(inputJSON), &args); err != nil || args.Path == "" {
-		return "", false
-	}
-
-	absPath := args.Path
-	if !filepath.IsAbs(absPath) {
-		absPath = filepath.Join(cwd, absPath)
-	}
-
-	if strings.ToLower(filepath.Ext(absPath)) != ".go" {
-		return "", false
-	}
-
-	return absPath, true
-}
-
-func runGopls(cwd, absPath string) lintResult {
-	ctx, cancel := context.WithTimeout(context.Background(), diagnosticsTimeout)
-	defer cancel()
-
-	cmd := exec.CommandContext(ctx, "gopls", "check", absPath)
-	cmd.Dir = cwd
-	out, err := cmd.CombinedOutput()
-
-	if ctx.Err() == context.DeadlineExceeded {
-		return lintResult{Err: fmt.Errorf("timed out after %s", diagnosticsTimeout)}
-	}
-
-	if err != nil {
-		return lintResult{Output: truncate(string(out), maxOutputBytes), Err: fmt.Errorf("failed to run gopls check: %w", err)}
-	}
-
-	return lintResult{Output: truncate(string(out), maxOutputBytes)}
-}
-
-func runGolangCILint(cwd, target string) lintResult {
-	ctx, cancel := context.WithTimeout(context.Background(), diagnosticsTimeout)
-	defer cancel()
-
-	args := []string{
-		"run",
-		target,
-		"--show-stats=false",
-		"--output.text.path", "stdout",
-		"--output.text.colors=false",
-		"--output.text.print-issued-lines=false",
-	}
-	cmd := exec.CommandContext(ctx, "golangci-lint", args...)
-	cmd.Dir = cwd
-	out, err := cmd.CombinedOutput()
-
-	if ctx.Err() == context.DeadlineExceeded {
-		return lintResult{Err: fmt.Errorf("timed out after %s", diagnosticsTimeout)}
-	}
-
-	trimmed := truncate(string(out), maxOutputBytes)
-	if err == nil {
-		return lintResult{Output: trimmed}
-	}
-
-	exitErr, ok := err.(*exec.ExitError)
-	if ok && exitErr.ExitCode() == 1 {
-		return lintResult{Output: trimmed}
-	}
-
-	return lintResult{Output: trimmed, Err: fmt.Errorf("failed to run golangci-lint: %w", err)}
-}
-
-func formatToolResult(res lintResult, emptyFallback string) string {
-	var lines []string
-	if res.Err != nil {
-		lines = append(lines, "ERROR: "+res.Err.Error())
-	}
-	out := strings.TrimSpace(res.Output)
-	if out == "" {
-		if res.Err == nil {
-			if emptyFallback != "" {
-				lines = append(lines, emptyFallback)
-			}
-		}
-	} else {
-		lines = append(lines, out)
-	}
-	if len(lines) == 0 {
-		return emptyFallback
-	}
-	return strings.Join(lines, "\n")
-}
-
-func truncate(s string, max int) string {
-	if len(s) <= max {
-		return s
-	}
-	return s[:max] + "\n... output truncated ..."
-}
-
-func countIssues(report string) (goplsCount, lintCount int) {
-	goplsStart := strings.Index(report, "[gopls]")
-	lintStart := strings.Index(report, "[golangci-lint]")
-	endTag := strings.Index(report, "</go_diagnostics>")
-
-	if goplsStart != -1 && lintStart != -1 {
-		goplsSection := report[goplsStart:lintStart]
-		for _, line := range strings.Split(goplsSection, "\n") {
-			line = strings.TrimSpace(line)
-			if line != "" && line != "[gopls]" && line != "No diagnostics." && !strings.HasPrefix(line, "#") {
-				goplsCount++
-			}
-		}
-	}
-
-	if lintStart != -1 && endTag != -1 {
-		lintSection := report[lintStart:endTag]
-		for _, line := range strings.Split(lintSection, "\n") {
-			line = strings.TrimSpace(line)
-			if line != "" && line != "[golangci-lint]" && line != "No lint issues." {
-				lintCount++
-			}
-		}
-	}
-
-	return goplsCount, lintCount
-}

.kit/prompts/code-audit.md

@@ -0,0 +1,146 @@
+---
+description: Read-only audit for dead code, duplication, boundary violations, and refactor opportunities
+---
+
+Perform a comprehensive **read-only** audit of this repository and report
+findings. **Do not edit, rename, or delete any files.** Optional focus / scope
+hints from the user: $@
+
+## Scope
+
+If the user supplied focus hints above (a package path, a subsystem name, a
+concern like "TUI" or "extensions"), scope the audit accordingly. Otherwise
+audit the whole repo, prioritising the highest-traffic packages first
+(`cmd/`, `internal/`, `pkg/kit/` for this repo).
+
+## Steps
+
+1. **Map the repo first**:
+   - `ls` / `find` the top-level layout and list every Go package
+   - Read `AGENTS.md`, `README.md`, and any `pkg/*/doc.go` to understand the
+     intended architectural boundaries (SDK vs internal vs TUI vs cmd vs
+     extension surface)
+   - Note the public SDK surface (`pkg/kit/`) and any documented invariants
+     (e.g. "no dependency name leakage", "UI never imports extensions
+     directly") — these define what counts as a violation
+
+2. **Hunt for dead code**:
+   - Run `go vet ./...` and capture warnings
+   - Use `grep` to find exported symbols (`^func [A-Z]`, `^type [A-Z]`,
+     `^var [A-Z]`, `^const [A-Z]`) and cross-reference call sites. Symbols
+     with zero non-test references inside the module are suspects
+   - Check for unreferenced files, `// TODO: remove` markers, commented-out
+     blocks, and `_ = x` discard patterns
+   - If `staticcheck`, `deadcode`, or `unused` are available on PATH, run
+     them and include their output verbatim
+   - **Do not delete anything** — list candidates with file:line and a
+     confidence level (high / medium / low)
+
+3. **Find unnecessary duplication**:
+   - Look for near-identical function bodies, struct shapes, or switch
+     statements across packages — `grep` for repeated function signatures
+     and copy-pasted string literals / error messages is a fast first pass
+   - Distinguish *coincidental* duplication (two things that happen to look
+     alike but evolve independently) from *unnecessary* duplication (same
+     intent, drifting in lockstep) — only flag the latter
+   - For each cluster, propose where the extracted helper should live
+     (which package, which file) and whether it crosses a boundary
+
+4. **Check concerns / boundary violations**:
+   - **SDK leakage**: grep `pkg/kit/` for imports of `internal/...` types
+     in exported signatures, and for dependency-name leakage in exported
+     names / godoc (e.g. library jargon appearing in `LLM*` types)
+   - **UI ↔ extensions**: grep `internal/ui/` for any import of
+     `internal/extensions/` — per AGENTS.md the UI must not import
+     extensions directly; converters in `cmd/root.go` should bridge them
+   - **cmd vs internal**: business logic living in `cmd/` that should be
+     in `internal/` (and vice versa)
+   - **Cyclic risk**: packages that import each other transitively or that
+     reach across sibling boundaries unexpectedly
+   - For each violation, cite the offending import / signature with
+     file:line
+
+5. **Spot refactor opportunities**:
+   - Long functions (>80 lines) doing multiple unrelated things
+   - Deeply nested conditionals that flatten well with early returns
+   - Repeated `if err != nil { return fmt.Errorf("...: %w", err) }` chains
+     that could become helpers — but only where the wrapping context is
+     genuinely uniform
+   - Structs with too many fields that hint at split responsibilities
+   - Exported APIs that would be cleaner with options structs / functional
+     options
+   - Tests that share setup boilerplate ripe for a helper
+   - Flag each with: location, current shape (1-2 lines), proposed shape
+     (1-2 lines), and estimated risk (low / medium / high)
+
+6. **Cross-check against project rules**:
+   - Re-read `AGENTS.md` "Key Patterns" section and verify nothing in your
+     findings contradicts the documented gotchas (Yaegi interface ban,
+     `prog.Send()` from `Update()`, function-field bug, etc.) — if a
+     "refactor" would reintroduce a known pitfall, drop it from the report
+     and note why
+
+7. **Write the report** as your final message (do not write it to disk)
+   structured as:
+
+   ```
+   # Code Audit Report
+
+   ## Summary
+   - N dead-code candidates
+   - N duplication clusters
+   - N boundary violations
+   - N refactor opportunities
+
+   ## Dead Code
+   ### High confidence
+   - path/to/file.go:LINE — symbol — reason
+
+   ### Medium confidence
+   ...
+
+   ## Duplication
+   ### Cluster: <short name>
+   - Sites: file:line, file:line, …
+   - Suggested home: package/path
+   - Notes: …
+
+   ## Boundary Violations
+   - Rule: <which rule from AGENTS.md / project convention>
+   - Offender: file:line
+   - Fix sketch: …
+
+   ## Refactor Opportunities
+   - Location: file:line
+   - Current: …
+   - Proposed: …
+   - Risk: low/medium/high
+   - Why it's worth it: …
+
+   ## Suggested Next Steps
+   1. …
+   2. …
+   ```
+
+8. **End the report with an explicit reminder** that no files were modified,
+   and recommend the user pick the highest-leverage items to act on
+   manually (or via a follow-up `/fix-issue` style prompt) rather than
+   running a sweeping refactor.
+
+## Guidelines
+
+- **Read-only, always**: no `edit`, no `write`, no `git commit`, no `go mod
+  tidy`. Use only `read`, `grep`, `find`, `ls`, and read-only `bash`
+  commands (`go vet`, `go build -o /tmp/...`, `staticcheck`, etc.)
+- **Cite every finding** with `path/to/file.go:LINE` so the user can jump
+  straight to it
+- **Be honest about confidence**: false positives in a code audit are
+  expensive — prefer "medium confidence, worth a look" over confidently
+  wrong claims
+- **Quantity isn't quality**: 10 sharp findings beat 100 nitpicks. Cut
+  anything that's purely stylistic unless it directly causes one of the
+  four issue categories above
+- **Skip generated code** (`*.pb.go`, `*_gen.go`, anything under
+  `vendor/`) and obvious third-party copies
+- **Don't propose architectural rewrites** — stay within the existing
+  shape of the repo and recommend incremental, reviewable changes

.kit/prompts/create-pr.md

@@ -0,0 +1,47 @@
+---
+description: Open a GitHub PR for the current branch using the repo's PR template
+---
+
+Open a GitHub pull request for the current branch, filling out the repository's PR template with a description grounded in the actual commits and diff.
+
+## Steps
+
+1. **Verify the branch is pushed**:
+   - `git status -sb` and `git log @{u}..HEAD --oneline 2>/dev/null` — if there is no upstream or unpushed commits, run `git push -u origin "$(git branch --show-current)"` first
+   - If the working tree is dirty, stop and tell the user to commit first (suggest `/commit-push`)
+2. **Gather context**:
+   - `git log origin/main..HEAD --oneline` — list of commits going into the PR
+   - `git diff origin/main...HEAD --stat` then `git diff origin/main...HEAD` — read the actual changes
+   - Identify the linked issue (from commit messages, branch name, or extra user input: $@) — capture as `Fixes #N` if applicable
+3. **Locate the PR template**:
+   - Check `.github/pull_request_template.md`, `.github/PULL_REQUEST_TEMPLATE.md`, or `docs/pull_request_template.md`
+   - If none exists, use a minimal `## Description` / `## Type of Change` / `## Checklist` structure
+4. **Draft the PR body** by filling out the template:
+   - **Description**: 1–3 short paragraphs explaining *what* changed and *why*, grounded in the diff. Include a brief before/after example for new APIs when useful.
+   - **Fixes #N**: only if there is a real linked issue
+   - **Type of Change**: tick the single most accurate box with `[x]` (leave others as `[ ]`)
+   - **Checklist**: tick items that are genuinely true (style, self-review, tests added, docs updated)
+   - **Additional Information**: bullet list of added / modified files and any backward-compatibility notes
+   - Remove template sections explicitly marked "remove if not applicable" (e.g. MCP Spec Compliance) when they don't apply
+5. **Write the body to a temp file**: `/tmp/pr-body-<branch-or-issue>.md` — never inline a long body via `--body`, always use `--body-file`
+6. **Choose the title**: prefer the subject of the primary commit if it already follows Conventional Commits; otherwise craft one in the same style (`<type>(<scope>): <imperative summary>`, ≤72 chars)
+7. **Create the PR**:
+   ```
+   gh pr create \
+     --title "<title>" \
+     --body-file /tmp/pr-body-<...>.md \
+     --base main \
+     --head "$(git branch --show-current)"
+   ```
+   Use the repo's actual default branch if it isn't `main` (`gh repo view --json defaultBranchRef -q .defaultBranchRef.name`)
+8. **Report the PR URL** returned by `gh` and stop
+
+## Guidelines
+
+- Read the diff and commit messages — do **not** invent features that aren't in the code
+- One PR per logical change; if the branch contains unrelated commits, surface that and ask before continuing
+- Keep the description focused on reviewer-relevant information (what / why), not a replay of the diff
+- Only check checklist boxes that are actually satisfied; leave the rest unchecked rather than lying
+- If `gh` is not authenticated (`gh auth status` fails), stop and tell the user
+
+$@

.kit/prompts/feature-request.md

@@ -16,7 +16,7 @@ This prompt uses the `feature_request` GitHub template which requires:
 
 ## Steps
 
-1. **Understand the request** from `$@`
+1. **Understand the request** from the user input: $@
    - What capability is missing?
    - What would the ideal behavior look like?
 

.kit/prompts/file-issue.md

@@ -16,7 +16,7 @@ This repository has structured issue templates. You MUST use the appropriate tem
 
 ## Steps
 
-1. **Determine the issue type** from `$@`:
+1. **Determine the issue type** from the user input: $@
    - Bug → use `--template bug_report`
    - Feature → use `--template feature_request`  
    - Documentation → use `--template documentation`

.kit/prompts/fix-issue.md

@@ -0,0 +1,61 @@
+---
+description: Implement the fix/feature/docs change requested by a GitHub issue
+---
+
+Resolve GitHub issue #$1 by reading it, classifying it, and producing the appropriate code or doc change. **Stop once the working tree contains the change** — committing, pushing, and opening a PR are handled by `/commit-push` and `/create-pr`.
+
+## Steps
+
+1. **Fetch the issue**:
+   - Run: gh issue view $1 --json number,title,body,labels,state,author,comments
+   - If the issue is closed, stop and ask the user whether to proceed
+   - Read the **entire** thread including comments — the latest comment often refines the ask
+
+2. **Classify the issue** from labels, title prefix, and body content:
+   - `bug` / `fix:` → reproduce, then fix
+   - `enhancement` / `feature` / `feat:` → design, then implement
+   - `documentation` / `docs:` → locate and update docs
+   - `question` / `discussion` → answer in a comment, do **not** write code
+   - Anything else → ask the user how to proceed
+
+3. **Create a working branch** off the default branch:
+   - `git checkout main && git pull --ff-only`
+   - Branch name: <type>/$1-<slug> (e.g. `fix/42-borderColor-ignored`, `feat/57-keyboard-clear`, `docs/63-widget-lifecycle`)
+
+4. **Do the work** based on type:
+
+   ### Bug (`bug` label / `fix:` title)
+   - Reproduce the failure first (write a failing test if feasible) — if you cannot reproduce, comment on the issue asking for clarification and stop
+   - Locate the root cause; do not patch symptoms
+   - Add or extend a regression test that fails before and passes after the fix
+   - Run `go test ./... -race` and `golangci-lint run`
+
+   ### Feature (`enhancement` / `feature` label / `feat:` title)
+   - Re-read the motivation and proposed implementation in the issue body
+   - For large, ambiguous, or breaking changes, sketch the design in a comment on the issue and wait for sign-off before writing code
+   - Implement behind sensible defaults; add godoc on every exported symbol
+   - Add unit tests covering the new behaviour and edge cases
+   - Update `README.md` / `docs/` if the public surface changed
+   - Run `go test ./... -race` and `golangci-lint run`
+
+   ### Documentation (`documentation` label / `docs:` title)
+   - Open the file/URL referenced in the issue's "Documentation Location"
+   - Apply the suggested improvement; verify code samples compile (`go build ./...`)
+   - No tests required, but run `golangci-lint run` if Go files were touched
+
+5. **Report**:
+   - Branch name (`git branch --show-current`)
+   - Summary of files changed (`git status -s`) and the diff highlights
+   - Test/lint results (pass/fail with key output)
+   - Suggest the next step explicitly:
+     - `/commit-push` to commit with a Conventional Commit subject (the message should reference `(#$1)` and include `Fixes #$1` so merge auto-closes)
+     - then `/create-pr $1` to open the pull request
+
+## Guidelines
+
+- This prompt **stops at a clean working tree with the change applied** — do not run `git commit`, `git push`, or `gh pr create`
+- If the issue is unclear, post a clarifying comment on the issue and stop; do not guess
+- Keep the change scoped to the issue; surface unrelated cleanups separately
+- For breaking changes or architecture shifts, propose the design on the issue first and wait for maintainer sign-off
+- If the issue is a duplicate or already fixed on `main`, comment with the reference and stop
+- Do not close the issue manually — the eventual PR's `Fixes #$1` handles that on merge

.kit/prompts/new-prompt.md

@@ -16,28 +16,64 @@ It becomes a `/slug` slash command in the kit input box — typed as `/filename`
 description: One-line description shown in autocomplete
 ---
 
-Body text of the prompt. Use $@ for all user-supplied arguments,
-$1 $2 etc. for positional arguments.
+Body text of the prompt. Reference user-supplied arguments
+with positional placeholders (see "Argument placeholders" below).
 ```
 
 - **Filename** → slug: `commit-push.md` becomes `/commit-push`
 - **Frontmatter**: only `description` is recognised; keep it under ~80 chars
 - **Body**: plain markdown; the full text is submitted as the user's message when the template fires
-- **Arguments**: `$@` expands to everything the user typed after the slash command name;
-  `$1`, `$2` for individual positional args; omit entirely if no arguments are needed
+- **Required args**: kit infers required positional args from the highest `$N` it finds *outside* backtick/tilde code fences — a stray `$2` in active prose means kit will refuse to run without 2 arguments
+
+## Argument placeholders
+
+kit performs shell-style substitution before sending the prompt to the model:
+
+- `$1`, `$2`, … — positional arguments (1-indexed)
+- `${1}`, `${2}`, … — same, brace form (use when followed by digits/letters: `${1}_suffix`)
+- `$@` — all arguments joined by spaces (zero or more, optional)
+- `$+` — all arguments, **at least one required**
+- `$ARGUMENTS` / `${ARGUMENTS}` — alias for `$@`
+- `${@:N}` — args from the Nth onwards (1-indexed, bash-style)
+- `${@:N:L}` — `L` args starting from the Nth
+
+### ⚠️ Critical: code fences and inline code preserve placeholders verbatim
+
+Anything inside triple-backtick fences, `~~~` fences, or single-backtick `inline` code spans is **left untouched** so example code samples don't get corrupted. That means:
+
+- An inline-coded `gh issue view $1` stays literal `$1` in the model's input ❌
+- The same command without backticks: gh issue view $1 → expands to `gh issue view 42` ✓
+
+**Rule of thumb:** if you want a placeholder to substitute, keep it outside backticks and fences. If you want a literal `$1` in the output (e.g. teaching the user shell syntax), put it inside backticks.
+
+### Workarounds for "I want it to look like code AND substitute"
+
+1. **Drop the backticks** around just the placeholder portion — the rest can still read as a command line in prose
+2. **Use a 4-space-indented code block** instead of a triple-backtick fence — kit only skips backtick/tilde fences, so indentation-style code blocks still get substitution:
+
+       git push -u origin "$(git branch --show-current)"
+       gh pr create --title "fix: ... (#$1)" --base main
+
+3. **Bind once, reference loosely**: put `Issue: $1` at the top in prose, then leave the backticked examples literal — the model will substitute mentally
 
 ## Steps
 
-1. **Understand the workflow** the user described in `$@` — ask a clarifying question if the intent is ambiguous
+1. **Understand the workflow** the user described in $@ — ask a clarifying question if the intent is ambiguous
 2. **Choose a filename**: short, lowercase, hyphen-separated, descriptive (e.g. `code-review.md`)
 3. **Write the description**: one sentence, imperative, fits in autocomplete
-4. **Draft the body**:
-   - Open with a single sentence stating the goal
+4. **Decide on arguments**:
+   - No args needed → omit placeholders entirely
+   - One required value (issue number, PR url, file path) → use `$1`
+   - Free-form trailing context → end with a single `$@` line
+   - Multiple distinct values → use `$1`, `$2`, … and document each at the top
+5. **Draft the body**:
+   - Open with a single sentence stating the goal, weaving in `$1`/`$@` where the value belongs
    - Use `## Steps` for multi-step workflows; use plain prose for simple prompts
    - Be specific: name commands, flags, and file paths where relevant
-   - End with `$@` on its own line if the user might want to pass context or a hint; omit if the prompt is self-contained
-5. **Write the file** to `.kit/prompts/<slug>.md`
-6. **Confirm** by showing the final file content and the slash command that activates it
+   - **Audit every backtick and code fence**: any `$N` or `$@` inside them will not expand — was that intentional? If not, apply one of the workarounds above
+6. **Write the file** to `.kit/prompts/<slug>.md`
+7. **Verify substitution** by mentally (or actually) replacing `$1`/`$@` with a sample value and confirming every reference resolves — and that the prompt's *own* example snippets don't accidentally bump the required-arg count (wrap illustrative `$N` examples in triple-backtick fences, not 4-space indentation, so `RequiredArgs()` ignores them)
+8. **Confirm** by showing the final file content and the slash command that activates it (e.g. `/code-review 42`)
 
 ## Guidelines
 
@@ -45,3 +81,4 @@ $1 $2 etc. for positional arguments.
 - Prefer concrete steps over vague instructions
 - A prompt that does one thing well beats one that tries to cover every edge case
 - If the workflow already exists as a prompt, suggest extending it instead of duplicating
+- When in doubt about substitution behaviour, write the file and run `/<slug> testvalue` once to confirm — wrong placement of backticks is the #1 failure mode

.kit/prompts/update-docs.md

@@ -0,0 +1,52 @@
+---
+description: Audit and update project documentation (README and docs site) for a recent change
+---
+
+Review recent code changes, identify all documentation surfaces that should
+mention them, and update each one — grounded in the actual diff, not guesses.
+
+## Steps
+
+1. **Identify the change**:
+   - If the user input ($@) names a commit / PR / branch / topic, use that as the focus
+   - Otherwise inspect `git log origin/main..HEAD --oneline` and `git diff origin/main...HEAD --stat` to discover what shipped on the current branch
+   - Read the actual diff (`git diff origin/main...HEAD`) — never document features that aren't in the code
+
+2. **Inventory the doc surfaces**:
+   - `README.md` at the repo root
+   - Any docs site (commonly `www/`, `docs/`, `site/`) — list its pages and identify the one(s) most thematically related to the change
+   - Inline godoc / API reference comments on the new exported symbols
+   - `CHANGELOG.md` if the project keeps one
+   - Any `examples/` directory entries that demonstrate the affected area
+
+3. **Audit each surface** with `grep`:
+   - Search for the names of related existing APIs (e.g. if you added `IterTools`, grep for `ListTools`) to find every page that already discusses the area
+   - Decide for each hit: does it need a cross-reference, a side-by-side comparison, or to stay untouched?
+
+4. **Decide where new content lives**:
+   - Prefer extending an existing page over creating a new one
+   - For a docs site, place new sections near related content (check the page's `## Heading` outline first)
+   - Skip surfaces that genuinely don't apply (e.g. a server-focused README for a client-only change) and say so explicitly
+
+5. **Draft the updates**:
+   - Lead with a one-sentence statement of what's new and why
+   - Show concrete code examples copied from real signatures — verify against the source files
+   - Include a comparison / "when to use which" table when adding an alternative to an existing API
+   - Note backwards-compatibility behaviour if relevant
+
+6. **Verify the docs build** before committing:
+   - For vocs / docusaurus / mkdocs sites, run the local build command (e.g. `npx vocs build`, `mkdocs build`) and fix any MDX/markdown errors
+   - For godoc, run `go vet ./...` and `go doc <pkg> <Symbol>` to sanity-check rendering
+
+7. **Report**:
+   - List every file changed and every file deliberately left alone (with a one-line reason)
+   - Suggest the next step (typically `/commit-push`) — do not auto-commit unless asked
+
+## Guidelines
+
+- Read the diff before writing anything — invented API names erode trust faster than missing docs
+- One change per doc commit; keep doc updates separate from code changes when possible
+- Match the existing voice and formatting of each surface (headings, code-fence languages, table styles)
+- Prefer linking between pages over duplicating content
+
+$@

.opencode/opencode.jsonc

@@ -1,8 +0,0 @@
-{
-  "$schema": "https://opencode.ai/config.json",
-  "permission": {
-    "external_directory": {
-      "~/go/**": "deny"
-    }
-  }
-}

AUTOSCROLL_FIX_SUMMARY.md

@@ -1,80 +0,0 @@
-# Autoscroll Fix - Final Summary
-
-## Root Cause
-
-The autoscroll was failing for streaming assistant messages due to a bug in how `GotoBottom()` calculated item heights.
-
-### The Problem
-
-1. **Reasoning blocks** (`StreamingMessageItem` with `role="reasoning"`) are **never cached** because they have live duration counters that update every render
-2. The `Height()` method returns `0` when `cachedRender == ""`
-3. `GotoBottom()` was calling:
-   ```go
-   itemHeight := item.Height()  // Returns 0 for reasoning
-   if itemHeight == 0 {
-       item.Render(s.width)  // Renders but doesn't cache (reasoning)
-       itemHeight = item.Height()  // Still returns 0!
-   }
-   ```
-4. This caused incorrect scroll position calculations, especially during reasoning → assistant transitions
-
-## The Solution
-
-Changed `GotoBottom()` and `AtBottom()` to calculate height **directly from the rendered string** instead of relying on the cached height:
-
-```go
-// OLD: item.Height() which checks cached render
-itemHeight := item.Height()
-if itemHeight == 0 {
-    item.Render(s.width)
-    itemHeight = item.Height()  // Still might be 0!
-}
-
-// NEW: Calculate from rendered string directly
-rendered := item.Render(s.width)
-itemHeight := strings.Count(rendered, "\n") + 1
-```
-
-This works for **all** items regardless of whether they cache their render or not.
-
-## Files Changed
-
-### `internal/ui/scrolllist.go`
-- **`GotoBottom()`**: Calculate height from rendered string (2 loops)
-- **`AtBottom()`**: Calculate height from rendered string (1 loop)
-
-### `internal/ui/model.go`
-- **`appendStreamingChunk()`**: For existing messages, call `GotoBottom()` directly (iteratr pattern)
-- **`refreshContent()`**: Simplified to only call `SetItems()` (removed redundant `GotoBottom()`)
-- **Bash streaming handler**: Removed redundant `GotoBottom()` after `refreshContent()`
-
-## Testing Results
-
-✅ **Test prompt**: "explore this repo"
-
-**Before fix**:
-- Autoscroll stopped after reasoning block completed
-- Viewport stuck showing end of reasoning ("Thought for 203ms")
-- Assistant response streamed off-screen below
-
-**After fix**:
-- Autoscroll works throughout reasoning block
-- Autoscroll continues during reasoning → assistant transition  
-- Viewport stays at bottom showing latest assistant content
-- Final position shows end of response (build commands section)
-
-## Behavior Verified
-
-1. ✅ Streaming text auto-scrolls to bottom
-2. ✅ Works across reasoning → assistant transition
-3. ✅ Manual scroll up (PgUp) disables autoscroll
-4. ✅ Scroll to bottom (Alt+End) re-enables autoscroll
-5. ✅ Accurate positioning with no offset errors
-
-## Performance Note
-
-The fix calls `Render()` on all items during `GotoBottom()` calculations. This is acceptable because:
-- `Render()` is already optimized with caching for non-reasoning items
-- `GotoBottom()` is only called during content updates (not every frame)
-- Reasoning blocks need to render anyway for live duration updates
-- This matches iteratr's approach of ensuring items are rendered before height calculations

README.md

@@ -18,7 +18,8 @@ A powerful, extensible AI coding agent CLI with multi-provider support, built-in
 ## Features
 
 - **Multi-Provider LLM Support**: Anthropic, OpenAI, Google Gemini, Ollama, Azure OpenAI, AWS Bedrock, OpenRouter, and more
-- **Built-in Core Tools**: bash, read, write, edit, grep, find, ls, subagent - no MCP overhead
+- **Built-in Core Tools**: bash (with interactive sudo password prompt), read, write, edit, grep, find, ls, subagent - no MCP overhead
+- **Smart @ Attachments**: Binary files auto-detected via MIME type, MCP resources via `@mcp:server:uri`
 - **MCP Integration**: Connect external MCP servers for expanded capabilities
 - **Extension System**: Write custom tools, commands, widgets, and UI modifications in Go
 - **Theming**: 22 built-in color themes (KITT, Catppuccin, Dracula, Nord, etc.) with runtime switching, persistence, and custom theme files
@@ -28,7 +29,7 @@ A powerful, extensible AI coding agent CLI with multi-provider support, built-in
 - **Session Management**: Tree-based conversation history with branching support
 - **Non-Interactive Mode**: Script-friendly positional args with JSON output
 - **ACP Server**: Run Kit as an [Agent Client Protocol](https://agentclientprotocol.com) agent over stdio
-- **Go SDK**: Embed Kit in your own applications
+- **Go SDK**: Embed Kit in your own applications with full agent lifecycle events (30+ event types) and behavior-modifying hooks
 
 ## Installation
 
@@ -125,8 +126,20 @@ model: anthropic/claude-sonnet-latest
 max-tokens: 4096
 temperature: 0.7
 stream: true
+thinking-level: off       # off, none, minimal, low, medium, high
+no-core-tools: false      # set to true to disable all built-in core tools
+
+# Skills — all three keys are optional
+no-skills: false          # set to true to disable all skill loading
+skill:                    # explicit skill files/dirs (disables auto-discovery)
+  - /path/to/skill.md
+skills-dir: ""            # override project-local directory for auto-discovery
 ```
 
+All of the above keys can also be set programmatically via the SDK
+(`kit.Options.MaxTokens`, `Options.Temperature`, `Options.ThinkingLevel`, etc.)
+without touching config files — see [SDK options](#with-options).
+
 ### Environment Variables
 
 ```bash
@@ -151,6 +164,16 @@ mcpServers:
   search:
     type: remote
     url: "https://mcp.example.com/search"
+
+  pubmed:
+    type: remote
+    url: "https://pubmed.mcp.example.com"
+    noOAuth: true  # skip OAuth for public servers that don't require auth
+
+  builds:
+    type: remote
+    url: "https://builds.mcp.example.com"
+    tasksMode: always  # async task execution — see MCP Tasks below
 ```
 
 ## CLI Reference
@@ -179,19 +202,27 @@ mcpServers:
 --compact                Enable compact output mode
 --auto-compact           Auto-compact conversation near context limit
 
-# Extensions
+# Extensions and tools
 --extension, -e          Load additional extension file(s) (repeatable)
 --no-extensions          Disable all extensions
+--no-core-tools          Disable all built-in core tools (bash, read, write, edit, grep, find, ls, subagent)
 --prompt-template        Load a specific prompt template by name
 --no-prompt-templates    Disable prompt template loading
 
+# Skills
+--skill                  Load skill file or directory (repeatable)
+--skills-dir             Override the project-local skills directory for auto-discovery
+--no-skills              Disable skill loading (auto-discovery and explicit)
+
 # Generation parameters
---max-tokens             Maximum tokens in response (default: 4096)
+--max-tokens             Maximum tokens in response (default: 8192, auto-raised up to 32768 for models with larger known output limits)
 --temperature            Randomness 0.0-1.0 (default: 0.7)
 --top-p                  Nucleus sampling 0.0-1.0 (default: 0.95)
 --top-k                  Limit top K tokens (default: 40)
 --stop-sequences         Custom stop sequences (comma-separated)
---thinking-level         Extended thinking level: off, minimal, low, medium, high (default: off)
+--frequency-penalty      Penalize frequent tokens 0.0-2.0 (default: 0.0)
+--presence-penalty       Penalize present tokens 0.0-2.0 (default: 0.0)
+--thinking-level         Extended thinking level: off, none, minimal, low, medium, high (default: off)
 
 # System
 --config                 Config file path (default: ~/.kit.yml)
@@ -203,9 +234,14 @@ mcpServers:
 
 ```bash
 # Authentication (for OAuth-enabled providers)
-kit auth login [provider]    # Start OAuth flow (e.g., anthropic)
-kit auth logout [provider]   # Remove credentials for provider
-kit auth status              # Check authentication status
+kit auth login [provider]          # Start OAuth flow (e.g., anthropic)
+kit auth login [provider] --set-default  # Set provider's default model as system default
+kit auth logout [provider]         # Remove credentials for provider
+kit auth status                    # Check authentication status
+
+# GitHub Copilot login (experimental; requires active Copilot subscription)
+kit auth login copilot
+kit --model copilot/gpt-5.5 "Hello"
 
 # Model database
 kit models [provider]        # List available models (optionally filter by provider)
@@ -287,12 +323,15 @@ kit -e examples/extensions/minimal.go
 
 ### Extension Capabilities
 
-**Lifecycle Events**: OnSessionStart, OnSessionShutdown, OnBeforeAgentStart, OnAgentStart, OnAgentEnd, OnToolCall, OnToolExecutionStart, OnToolOutput, OnToolExecutionEnd, OnToolResult, OnInput, OnMessageStart, OnMessageUpdate, OnMessageEnd, OnModelChange, OnContextPrepare, OnBeforeFork, OnBeforeSessionSwitch, OnBeforeCompact, OnCustomEvent, OnSubagentStart, OnSubagentChunk, OnSubagentEnd
+**Lifecycle Events**: OnSessionStart, OnSessionShutdown, OnBeforeAgentStart, OnAgentStart, OnAgentEnd, OnLLMUsage, OnToolCall, OnToolCallInputStart, OnToolCallInputDelta, OnToolCallInputEnd, OnToolExecutionStart, OnToolOutput, OnToolExecutionEnd, OnToolResult, OnInput, OnMessageStart, OnMessageUpdate, OnMessageEnd, OnModelChange, OnContextPrepare, OnBeforeFork, OnBeforeSessionSwitch, OnBeforeCompact, OnCustomEvent, OnSubagentStart, OnSubagentChunk, OnSubagentEnd
+
+`OnAgentEnd` carries per-turn aggregates (`ToolCallCount`, `ToolNames`, `LLMCallCount`, `InputTokensDelta`, `OutputTokensDelta`, `CostDelta`, `DurationMs`) so observers don't need to maintain parallel bookkeeping. `OnLLMUsage` fires after each LLM provider call with token + cost deltas attributed to that specific call/model — use it for accurate budget enforcement *between* calls instead of waiting for the turn to finish.
 
 **Custom Components**:
 - **Tools**: Add new tools the LLM can invoke
 - **Commands**: Register slash commands (e.g., `/mycommand`)
 - **Options**: Register configurable extension options
+- **Session State**: Last-write-wins key-value store via `ctx.SetState` / `GetState` / `DeleteState` / `ListState`, persisted to a per-session sidecar file outside the conversation tree
 - **Widgets**: Persistent status displays above/below input
 - **Headers/Footers**: Persistent content above/below the conversation
 - **Status Bar**: Custom status bar entries
@@ -317,39 +356,41 @@ kit -e examples/extensions/minimal.go
 
 See the `examples/extensions/` directory:
 
-- `minimal.go` - Clean UI with custom footer
-- `auto-commit.go` - Auto-commit on shutdown
-- `bookmark.go` - Bookmark conversations
-- `branded-output.go` - Branded output rendering
-- `compact-notify.go` - Notification on compaction
-- `confirm-destructive.go` - Confirm destructive operations
-- `context-inject.go` - Inject context into conversations
-- `conversation-manager.go` - **NEW** Tree navigation, branch summarization, and fresh context loops
-- `custom-editor-demo.go` - Vim-like modal editor
-- `dev-reload.go` - Development live-reload
-- `header-footer-demo.go` - Custom headers and footers
-- `inline-bash.go` - Inline bash execution
-- `interactive-shell.go` - Interactive shell integration
-- `kit-kit.go` - Kit-in-Kit (sub-agent spawning)
-- `lsp-diagnostics.go` - LSP diagnostic integration
-- `notify.go` - Desktop notifications
-- `overlay-demo.go` - Modal dialogs
-- `permission-gate.go` - Permission gating for tools
-- `pirate.go` - Pirate-themed personality
-- `plan-mode.go` - Read-only planning mode
-- `project-rules.go` - Project-specific rules
-- `prompt-demo.go` - Interactive prompts (select/confirm/input)
-- `prompt-templates.go` - **NEW** Frontmatter-driven templates with model switching and skill injection
-- `protected-paths.go` - Path protection for sensitive files
-- `subagent-widget.go` - Multi-agent orchestration with status widget
-- `subagent-test.go` - Subagent testing utilities
-- `summarize.go` - Conversation summarization
-- `tool-logger.go` - Log all tool calls
-- `neon-theme.go` - Custom theme registration and switching
-- `tool-renderer-demo.go` - Custom tool call rendering
-- `widget-status.go` - Persistent status widgets
+- [`minimal.go`](examples/extensions/minimal.go) - Clean UI with custom footer
+- [`auto-commit.go`](examples/extensions/auto-commit.go) - Auto-commit on shutdown
+- [`bookmark.go`](examples/extensions/bookmark.go) - Bookmark conversations
+- [`branded-output.go`](examples/extensions/branded-output.go) - Branded output rendering
+- [`bridge-demo.go`](examples/extensions/bridge_demo.go) - Bridged SDK API demo (tree navigation, skills, templates, model resolution)
+- [`compact-notify.go`](examples/extensions/compact-notify.go) - Notification on compaction
+- [`confirm-destructive.go`](examples/extensions/confirm-destructive.go) - Confirm destructive operations
+- [`context-inject.go`](examples/extensions/context-inject.go) - Inject context into conversations
+- [`conversation-manager.go`](examples/extensions/conversation-manager.go) - **NEW** Tree navigation, branch summarization, and fresh context loops
+- [`custom-editor-demo.go`](examples/extensions/custom-editor-demo.go) - Vim-like modal editor
+- [`dev-reload.go`](examples/extensions/dev-reload.go) - Development live-reload
+- [`header-footer-demo.go`](examples/extensions/header-footer-demo.go) - Custom headers and footers
+- [`inline-bash.go`](examples/extensions/inline-bash.go) - Inline bash execution
+- [`interactive-shell.go`](examples/extensions/interactive-shell.go) - Interactive shell integration
+- [`kit-kit.go`](examples/extensions/kit-kit.go) - Kit-in-Kit (sub-agent spawning)
+- [`lsp-diagnostics.go`](examples/extensions/lsp-diagnostics.go) - LSP diagnostic integration
+- [`notify.go`](examples/extensions/notify.go) - Desktop notifications
+- [`overlay-demo.go`](examples/extensions/overlay-demo.go) - Modal dialogs
+- [`permission-gate.go`](examples/extensions/permission-gate.go) - Permission gating for tools
+- [`pirate.go`](examples/extensions/pirate.go) - Pirate-themed personality
+- [`plan-mode.go`](examples/extensions/plan-mode.go) - Read-only planning mode
+- [`project-rules.go`](examples/extensions/project-rules.go) - Project-specific rules
+- [`prompt-demo.go`](examples/extensions/prompt-demo.go) - Interactive prompts (select/confirm/input)
+- [`prompt-templates.go`](examples/extensions/prompt-templates.go) - **NEW** Frontmatter-driven templates with model switching and skill injection
+- [`protected-paths.go`](examples/extensions/protected-paths.go) - Path protection for sensitive files
+- [`subagent-widget.go`](examples/extensions/subagent-widget.go) - Multi-agent orchestration with status widget
+- [`subagent-test.go`](examples/extensions/subagent-test.go) - Subagent testing utilities
+- [`summarize.go`](examples/extensions/summarize.go) - Conversation summarization
+- [`tool-logger.go`](examples/extensions/tool-logger.go) - Log all tool calls
+- [`neon-theme.go`](examples/extensions/neon-theme.go) - Custom theme registration and switching
+- [`tool-renderer-demo.go`](examples/extensions/tool-renderer-demo.go) - Custom tool call rendering
+- [`usage-budget.go`](examples/extensions/usage-budget.go) - Per-call usage callback (`OnLLMUsage`), session state, and enriched `OnAgentEnd` per-turn report
+- [`widget-status.go`](examples/extensions/widget-status.go) - Persistent status widgets
 
-Also see `.kit/extensions/go-edit-lint.go` (in this repo) for a project-local extension example that runs gopls and golangci-lint on Go file edits.
+Also see [`.kit/extensions/go-edit-lint.go`](.kit/extensions/go-edit-lint.go) (in this repo) for a project-local extension example that runs gopls and golangci-lint on Go file edits.
 
 ### Loading Extensions
 
@@ -406,7 +447,7 @@ func TestMyExtension(t *testing.T) {
 - `AssertPrinted()`, `AssertPrintedContains()` — Verify output
 - `AssertToolRegistered()`, `AssertCommandRegistered()` — Verify registration
 
-See `examples/extensions/tool-logger_test.go` for a complete example with 14 test cases covering tool calls, input handling, and session lifecycle.
+See [`examples/extensions/tool-logger_test.go`](examples/extensions/tool-logger_test.go) for a complete example with 14 test cases covering tool calls, input handling, and session lifecycle.
 
 ### Prompt Templates
 
@@ -428,10 +469,13 @@ Focus on $1 specifically.
 
 **Argument placeholders:**
 - `$1`, `$2`, etc. — Individual arguments
-- `$@` or `$ARGUMENTS` — All arguments
+- `$@` or `$ARGUMENTS` — All arguments (zero or more)
+- `$+` — All arguments (one or more required; error if none given)
 - `${@:2}` — Arguments from position 2 onwards
 - `${@:1:3}` — 3 arguments starting at position 1
 
+Placeholders inside fenced code blocks (```) and inline code spans are ignored.
+
 Disable templates with `--no-prompt-templates` or load a specific template with `--prompt-template <name>`.
 
 ## Session Management
@@ -480,6 +524,17 @@ During an interactive session, use these slash commands:
 | `/fork` | Fork to new session from an earlier message |
 | `/new` | Start a fresh session |
 
+### Keyboard Shortcuts
+
+| Shortcut | Description |
+|----------|-------------|
+| `Ctrl+V` | Paste an image from the clipboard — shows an inline low-res thumbnail preview (tmux/zellij-safe) |
+| `Ctrl+U` | Clear all pending image attachments |
+| `Ctrl+X e` | Open `$VISUAL`/`$EDITOR` to compose or edit your prompt |
+| `Ctrl+X s` | Steer — inject a system-level instruction mid-turn |
+| `ESC ESC` | Cancel the current operation (tool call or streaming) |
+| `↑` / `↓` | Navigate prompt history |
+
 ## Go SDK
 
 Embed Kit in your Go applications:
@@ -522,16 +577,37 @@ host, err := kit.New(ctx, &kit.Options{
     SystemPrompt: "You are a helpful bot",
     ConfigFile:   "/path/to/config.yml",
     MaxSteps:     10,
-    Streaming:    true,
+    Streaming:    ptr(true), // *bool: nil = unset (default true), &false = off
     Quiet:        true,
 
+    // Generation parameters (override env/config/per-model defaults)
+    MaxTokens:        16384,             // 0 = auto-resolve (env → config → per-model → 8192 floor)
+    ThinkingLevel:    "medium",          // "off", "none", "minimal", "low", "medium", "high"
+    Temperature:      ptr(float32(0.2)), // pointer so 0.0 != unset; nil = provider default
+    TopP:             nil,                // nil = leave provider/per-model default
+    TopK:             nil,
+    FrequencyPenalty: nil,
+    PresencePenalty:  nil,
+
+    // Provider configuration (override env/config without reaching into viper)
+    ProviderAPIKey: "sk-...",                      // "" = use config / provider env var
+    ProviderURL:    "https://proxy.internal/v1",   // "" = provider default
+    TLSSkipVerify:  false,                         // only takes effect when true
+
     // Session options
     SessionPath:  "./session.jsonl",  // Open specific session
     Continue:     true,                // Resume most recent session
     NoSession:    true,                // Ephemeral mode
 
     // Tool options
-    ExtraTools:   []kit.Tool{...},     // Additional tools alongside defaults
+    Tools:            []kit.Tool{...},     // Replace default tool set entirely
+    ExtraTools:       []kit.Tool{...},     // Add tools alongside defaults
+    DisableCoreTools: true,                // Disable all built-in core tools; also controllable via
+                                           // --no-core-tools flag, KIT_NO_CORE_TOOLS env var,
+                                           // or no-core-tools: true in .kit.yml
+
+    // Configuration
+    SkipConfig:   true,                   // Skip .kit.yml files (viper defaults + env vars still apply)
 
     // Compaction
     AutoCompact:  true,                // Auto-compact near context limit
@@ -540,6 +616,151 @@ host, err := kit.New(ctx, &kit.Options{
 })
 ```
 
+**Generation & provider fields** (added in v0.55+) let SDK consumers configure
+Kit entirely in-code without `viper.Set()` workarounds or shipping a `.kit.yml`.
+Precedence is `Options` > `KIT_*` env vars > `.kit.yml` > per-model defaults
+(`modelSettings` / `customModels`) > provider-level defaults. Sampling params
+are pointer types so explicit `0.0` is distinguishable from "leave alone"; a
+non-zero `MaxTokens` suppresses automatic right-sizing the same way `--max-tokens`
+does on the CLI.
+
+### Functional options (`NewAgent`)
+
+For simple programmatic setups, `kit.NewAgent` offers an ergonomic
+functional-options front door over `kit.New`. Streaming is **enabled by
+default**; pass `kit.WithStreaming(false)` to opt out.
+
+```go
+host, err := kit.NewAgent(ctx,
+    kit.WithModel("anthropic/claude-sonnet-4-5-20250929"),
+    kit.WithSystemPrompt("You are a helpful assistant."),
+    kit.WithMaxTokens(8192),
+    kit.WithThinkingLevel("medium"),
+    kit.Ephemeral(), // in-memory session, no persistence
+)
+```
+
+Available options: `WithModel`, `WithSystemPrompt`, `WithStreaming`,
+`WithMaxTokens`, `WithThinkingLevel`, `WithTools`, `WithExtraTools`,
+`WithProviderAPIKey`, `WithProviderURL`, `WithConfigFile`, `WithDebug`, and
+`Ephemeral`. For advanced configuration not covered by the helpers (custom MCP
+config, in-process MCP servers, session backends, MCP task tuning) construct an
+`Options` value explicitly and call `kit.New`.
+
+### Per-instance config isolation
+
+Each `kit.New` / `kit.NewAgent` call owns an **isolated configuration store**,
+so constructing multiple Kit instances in the same process is safe: setting the
+model, thinking level, or generation parameters on one never affects another,
+and runtime mutators (`SetModel`, `SetThinkingLevel`) only touch the owning
+instance. This makes subagent spawning and multi-Kit embedding race-free with
+no external synchronization required.
+
+### MCP OAuth (remote MCP servers)
+
+When a remote MCP server returns 401, Kit runs the full OAuth flow (dynamic
+client registration → PKCE → token exchange → persistence) but delegates the
+user-facing step — showing the authorization URL and receiving the callback —
+to an `MCPAuthHandler` that you pass explicitly via `Options.MCPAuthHandler`.
+If nil, OAuth is disabled and the authorization-required error surfaces to the
+caller; the SDK never auto-opens a browser or binds a localhost port.
+
+```go
+// CLI/TUI apps: opens the system browser + prints status to stderr.
+authHandler, _ := kit.NewCLIMCPAuthHandler()
+defer authHandler.Close()
+
+host, _ := kit.New(ctx, &kit.Options{
+    MCPAuthHandler: authHandler,
+})
+
+// Custom UX: reuse the SDK's port + callback server, supply your own
+// presentation via OnAuthURL (TUI modal, QR code, web redirect, etc.).
+//   h, _ := kit.NewDefaultMCPAuthHandler()
+//   h.OnAuthURL = func(server, authURL string) { myUI.Show(server, authURL) }
+//
+// Full control (web apps, daemons): implement kit.MCPAuthHandler yourself —
+// no localhost binding, no side effects.
+```
+
+Tokens are persisted to `$XDG_CONFIG_HOME/.kit/mcp_tokens.json` by default; swap
+in a custom `MCPTokenStoreFactory` for encrypted, DB-backed, or in-memory
+storage. See the [SDK options docs](/sdk/options#mcp-oauth-authorization) for
+the full matrix.
+
+### MCP Tasks (long-running tools)
+
+Kit advertises [MCP task support](https://modelcontextprotocol.io/specification/2025-11-25/basic/utilities/tasks)
+during `initialize`, so cooperating MCP servers can respond to `tools/call`
+with a `taskId` instead of blocking the connection. Kit then polls
+`tasks/get` / `tasks/result` until the task reaches a terminal state, and
+best-effort `tasks/cancel`s on context cancellation.
+
+Defaults are safe — a server that doesn't advertise task capability runs
+synchronously, exactly as before. Opt in per server via `tasksMode` in
+`.kit.yml` (`auto` | `never` | `always`) or programmatically through the SDK:
+
+```go
+host, _ := kit.New(ctx, &kit.Options{
+    MCPTaskMode: map[string]kit.MCPTaskMode{
+        "build-server": kit.MCPTaskModeAlways,
+    },
+    MCPTaskTimeout:  15 * time.Minute,
+    MCPTaskProgress: func(p kit.MCPTaskProgress) {
+        log.Printf("%s: %s", p.TaskID, p.Status)
+    },
+})
+
+tasks, _ := host.ListMCPTasks(ctx, "build-server")
+_, _    = host.CancelMCPTask(ctx, "build-server", tasks[0].TaskID)
+```
+
+See the [configuration docs](/configuration#mcp-tasks-long-running-tools) and
+[SDK options → MCP Tasks](/sdk/options#mcp-tasks) for the full surface.
+
+### Custom Tools
+
+Create custom tools with automatic schema generation — no external dependencies needed:
+
+```go
+type SearchInput struct {
+    Query string `json:"query" description:"Search query"`
+}
+
+searchTool := kit.NewTool("search", "Search the codebase",
+    func(ctx context.Context, input SearchInput) (kit.ToolOutput, error) {
+        return kit.TextResult("Found: ..."), nil
+    },
+)
+
+host, _ := kit.New(ctx, &kit.Options{
+    ExtraTools: []kit.Tool{searchTool}, // adds alongside built-in tools
+})
+```
+
+Use `kit.NewParallelTool` for tools safe to run concurrently. Binary data (images, audio, etc.) in `ToolOutput.Data` is automatically forwarded to the LLM when `MediaType` is set. See the [SDK docs](/sdk/overview) for full details on struct tags, `ToolOutput` fields, and `ToolCallIDFromContext`.
+
+#### Return Helpers
+
+| Helper | Description |
+| --- | --- |
+| `kit.TextResult(content)` | Successful text result |
+| `kit.ErrorResult(content)` | Error result (LLM sees it as a tool error) |
+| `kit.ImageResult(content, data, mediaType)` | Image result with binary data (e.g. `"image/png"`) |
+| `kit.MediaResult(content, data, mediaType)` | Non-image media result (e.g. `"audio/mpeg"`) |
+
+#### ToolOutput Fields
+
+```go
+kit.ToolOutput{
+    Content:   "result text",     // text returned to the LLM
+    IsError:   false,             // true = LLM sees this as an error
+    Data:      pngBytes,          // optional binary data (images, audio)
+    MediaType: "image/png",       // MIME type for binary Data
+    Metadata:  map[string]any{},  // opaque metadata for hooks/UI (not sent to LLM)
+}
+```
+
 ### With Callbacks
 
 ```go
@@ -555,7 +776,7 @@ unsub2 := host.OnToolResult(func(e kit.ToolResultEvent) {
 })
 defer unsub2()
 
-unsub3 := host.OnStreaming(func(e kit.MessageUpdateEvent) {
+unsub3 := host.OnMessageUpdate(func(e kit.MessageUpdateEvent) {
     print(e.Chunk)
 })
 defer unsub3()
@@ -592,6 +813,45 @@ host, _ := kit.New(ctx, &kit.Options{
 })
 ```
 
+### Runtime Skills & Context Files
+
+For multi-tenant hosts (chatbots, per-user agents, web services), the SDK
+lets you swap skills and `AGENTS.md`-style context files **after** Kit
+construction. Every mutation recomposes the system prompt and applies it to
+the agent so the next turn picks up the new instructions — no restart needed.
+
+```go
+// Programmatic skill (no file on disk required).
+host.AddSkill(&kit.Skill{
+    Name:        "polite-french",
+    Description: "Respond in French and always greet the user.",
+    Content:     "Always reply in French. Open every response with 'Bonjour'.",
+})
+
+// Or load one from disk.
+host.LoadAndAddSkill("/var/skills/refund-policy.md")
+
+// Per-user AGENTS.md content pulled from a database.
+host.AddContextFileContent(
+    fmt.Sprintf("session://%s/AGENTS.md", userID),
+    rulesFromDB,
+)
+
+// Tear down session-specific state on logout.
+host.RemoveSkill("polite-french")
+host.RemoveContextFile(fmt.Sprintf("session://%s/AGENTS.md", userID))
+
+// Or replace the whole set atomically.
+host.SetSkills(activeSkillsForUser)
+host.SetContextFiles(activeContextForUser)
+```
+
+Skills dedupe by `Name`, context files dedupe by `Path` (which can be any
+opaque identifier — it doesn't have to be a real filesystem path). All
+mutators and readers (`GetSkills`, `GetContextFiles`) are safe to call
+concurrently from multiple goroutines. See the [SDK overview docs](/sdk/overview#runtime-skills-and-context-files)
+for the full reference.
+
 ## Advanced Usage
 
 ### Subagent Pattern
@@ -708,6 +968,7 @@ npm/                 - NPM package wrapper for distribution
 
 - **Anthropic** - Claude models (native, prompt caching, OAuth)
 - **OpenAI** - GPT models
+- **Copilot** - GitHub Copilot models (`copilot`, requires active Copilot subscription)
 - **Google** - Gemini models
 - **Ollama** - Local models
 - **Azure OpenAI** - Azure-hosted OpenAI
@@ -733,6 +994,31 @@ This automatically defaults to `custom/custom` without needing to specify a mode
 - Reasoning and temperature support
 - Optional `CUSTOM_API_KEY` environment variable or `--provider-api-key` flag
 
+### Auto-routed Providers
+
+Any provider in the [models.dev](https://models.dev) database can be used as
+`provider/model` without a dedicated native integration. Kit auto-routes the
+request through the matching **wire protocol** based on the provider's npm package
+(or per-model override), using its `api` URL as the base:
+
+| npm package | Wire protocol |
+|-------------|---------------|
+| `@ai-sdk/openai` | OpenAI (Responses API) |
+| `@ai-sdk/openai-compatible` | OpenAI (chat completions) |
+| `@ai-sdk/anthropic` | Anthropic |
+| `@ai-sdk/google` | Google Gemini |
+
+Providers with an `api` URL but an unrecognized npm package fall back to the
+OpenAI-compatible wire. Because routing follows the wire protocol, aggregator/proxy
+providers work across all of their models — including Claude, GPT, *and* Gemini
+routes:
+
+```bash
+kit --model opencode/claude-haiku-4-5 "Hello"     # → Anthropic wire
+kit --model opencode/gpt-5 "Hello"                # → OpenAI wire
+kit --model opencode/gemini-3.5-flash "Hello"     # → Google wire
+```
+
 ### Model String Format
 
 ```bash

cmd/auth.go

@@ -11,6 +11,7 @@ import (
 
 	"charm.land/huh/v2"
 	"github.com/mark3labs/kit/internal/auth"
+	"github.com/mark3labs/kit/internal/ui"
 	kit "github.com/mark3labs/kit/pkg/kit"
 	"github.com/spf13/cobra"
 )
@@ -30,10 +31,12 @@ using OAuth flows. Stored credentials take precedence over environment variables
 Available providers:
   - anthropic: Anthropic Claude API (OAuth)
   - openai:    OpenAI API (OAuth and API key)
+  - copilot:   GitHub Copilot (GitHub device login)
 
 Examples:
   kit auth login anthropic
   kit auth login openai
+  kit auth login copilot
   kit auth logout anthropic
   kit auth status`,
 }
@@ -53,10 +56,16 @@ environment variables when making API calls.
 Available providers:
   - anthropic: Anthropic Claude API (OAuth)
   - openai:    OpenAI ChatGPT Plus/Pro (Codex OAuth)
+  - copilot:   GitHub Copilot (GitHub device login, experimental)
 
-Example:
+Flags:
+  --set-default   Set this provider's default model as the system default
+
+Examples:
   kit auth login anthropic
-  kit auth login openai`,
+  kit auth login openai
+  kit auth login copilot
+  kit auth login copilot --set-default`,
 	Args: cobra.ExactArgs(1),
 	RunE: runAuthLogin,
 }
@@ -75,10 +84,12 @@ You will need to use environment variables or command-line flags for authenticat
 Available providers:
   - anthropic: Anthropic Claude API
   - openai:    OpenAI API
+  - copilot:   GitHub Copilot
 
 Example:
   kit auth logout anthropic
-  kit auth logout openai`,
+  kit auth logout openai
+  kit auth logout copilot`,
 	Args: cobra.ExactArgs(1),
 	RunE: runAuthLogout,
 }
@@ -99,12 +110,47 @@ Example:
 	RunE: runAuthStatus,
 }
 
+var (
+	loginSetDefault bool
+)
+
+// defaultModels maps providers to their recommended default models.
+// These are used when --set-default flag is passed to auth login.
+var defaultModels = map[string]string{
+	"anthropic": "anthropic/claude-sonnet-4-5-20250929",
+	"openai":    "openai/gpt-5.4",
+	"copilot":   "copilot/gpt-5.5",
+}
+
+// setDefaultModelIfRequested sets the default model for the given provider
+// if the --set-default flag was provided.
+func setDefaultModelIfRequested(provider string) error {
+	if !loginSetDefault {
+		return nil
+	}
+
+	model, ok := defaultModels[provider]
+	if !ok {
+		return fmt.Errorf("no default model configured for provider: %s", provider)
+	}
+
+	if err := ui.SaveModelPreference(model); err != nil {
+		return fmt.Errorf("failed to save model preference: %w", err)
+	}
+
+	fmt.Printf("\n✓ Set default model to: %s\n", model)
+	return nil
+}
+
 func init() {
 	authCmd.AddCommand(authLoginCmd)
 	authCmd.AddCommand(authLogoutCmd)
 	authCmd.AddCommand(authStatusCmd)
+
+	authLoginCmd.Flags().BoolVar(&loginSetDefault, "set-default", false, "Set this provider's default model as the system default after login")
 }
 
+// runAuthLogin dispatches OAuth login to the selected provider.
 func runAuthLogin(cmd *cobra.Command, args []string) error {
 	provider := strings.ToLower(args[0])
 
@@ -113,8 +159,10 @@ func runAuthLogin(cmd *cobra.Command, args []string) error {
 		return loginAnthropic()
 	case "openai":
 		return loginOpenAI()
+	case "copilot":
+		return loginCopilot(cmd.Context())
 	default:
-		return fmt.Errorf("unsupported provider: %s. Available providers: anthropic, openai", provider)
+		return fmt.Errorf("unsupported provider: %s. Available providers: anthropic, openai, copilot", provider)
 	}
 }
 
@@ -126,8 +174,10 @@ func runAuthLogout(cmd *cobra.Command, args []string) error {
 		return logoutAnthropic()
 	case "openai":
 		return logoutOpenAI()
+	case "copilot":
+		return logoutCopilot()
 	default:
-		return fmt.Errorf("unsupported provider: %s. Available providers: anthropic, openai", provider)
+		return fmt.Errorf("unsupported provider: %s. Available providers: anthropic, openai, copilot", provider)
 	}
 }
 
@@ -206,9 +256,31 @@ func runAuthStatus(cmd *cobra.Command, args []string) error {
 		}
 	}
 
+	// Check GitHub Copilot credentials
+	fmt.Print("\nGitHub Copilot: ")
+	if hasCopilotCreds, err := cm.HasCopilotCredentials(); err != nil {
+		fmt.Printf("Error checking credentials: %v\n", err)
+	} else if hasCopilotCreds {
+		if creds, err := cm.GetCopilotCredentials(); err != nil {
+			fmt.Printf("Error reading credentials: %v\n", err)
+		} else {
+			status := "✓ Authenticated"
+			if creds.IsExpired() {
+				status = "⚠️  Token expired (will refresh automatically)"
+			} else if creds.NeedsRefresh() {
+				status = "⚠️  Token expires soon (will refresh automatically)"
+			}
+
+			fmt.Printf("%s (GitHub OAuth, stored %s)\n", status, creds.CreatedAt.Format("2006-01-02 15:04:05"))
+		}
+	} else {
+		fmt.Println("✗ Not authenticated")
+	}
+
 	fmt.Println("\nTo authenticate with a provider:")
 	fmt.Println("  kit auth login anthropic")
 	fmt.Println("  kit auth login openai")
+	fmt.Println("  kit auth login copilot")
 
 	return nil
 }
@@ -288,6 +360,17 @@ func loginAnthropic() error {
 	fmt.Println("\n🎉 Your OAuth credentials will now be used for Anthropic API calls.")
 	fmt.Println("💡 You can check your authentication status with: kit auth status")
 
+	// Set default model if requested
+	if err := setDefaultModelIfRequested("anthropic"); err != nil {
+		return err
+	}
+
+	// Remind users how to set this as default if they didn't use --set-default
+	if !loginSetDefault {
+		fmt.Println("\n💡 To set Anthropic as your default model, run:")
+		fmt.Println("   kit auth login anthropic --set-default")
+	}
+
 	return nil
 }
 
@@ -454,6 +537,96 @@ func loginOpenAI() error {
 	fmt.Println("\n🎉 Your OAuth credentials will now be used for OpenAI API calls.")
 	fmt.Println("💡 You can check your authentication status with: kit auth status")
 
+	// Set default model if requested
+	if err := setDefaultModelIfRequested("openai"); err != nil {
+		return err
+	}
+
+	// Remind users how to set this as default if they didn't use --set-default
+	if !loginSetDefault {
+		fmt.Println("\n💡 To set OpenAI as your default model, run:")
+		fmt.Println("   kit auth login openai --set-default")
+	}
+
+	return nil
+}
+
+// loginCopilot authenticates GitHub Copilot using GitHub device flow.
+func loginCopilot(ctx context.Context) error {
+	if ctx == nil {
+		ctx = context.Background()
+	}
+
+	cm, err := kit.NewCredentialManager()
+	if err != nil {
+		return fmt.Errorf("failed to initialize credential manager: %w", err)
+	}
+
+	if hasAuth, err := cm.HasCopilotCredentials(); err == nil && hasAuth {
+		var reauth bool
+		err := huh.NewConfirm().
+			Title("You are already authenticated with GitHub Copilot").
+			Description("Do you want to re-authenticate?").
+			Affirmative("Yes").
+			Negative("No").
+			Value(&reauth).
+			Run()
+		if err != nil {
+			return fmt.Errorf("failed to prompt for re-authentication: %w", err)
+		}
+		if !reauth {
+			fmt.Println("Authentication cancelled.")
+			return nil
+		}
+	}
+
+	client := auth.NewCopilotOAuthClient()
+
+	fmt.Println("🔐 Starting GitHub Copilot authentication...")
+	fmt.Println("This uses GitHub device login and requires an active GitHub Copilot subscription.")
+	fmt.Println("Experimental: this uses VS Code Copilot Chat client identifiers.")
+	fmt.Println()
+
+	deviceCode, err := client.StartDeviceFlow(ctx)
+	if err != nil {
+		return fmt.Errorf("failed to start GitHub device login: %w", err)
+	}
+
+	fmt.Println("📱 Open this page and enter the code:")
+	fmt.Printf("\n%s\n\n", deviceCode.VerificationURI)
+	fmt.Printf("Code: %s\n\n", deviceCode.UserCode)
+	auth.TryOpenBrowser(deviceCode.VerificationURI)
+
+	fmt.Println("Waiting for GitHub authorization...")
+	githubToken, err := client.PollDeviceToken(ctx, deviceCode)
+	if err != nil {
+		return fmt.Errorf("failed to complete GitHub device login: %w", err)
+	}
+
+	fmt.Println("\n🔄 Exchanging GitHub token for Copilot access token...")
+	creds, err := client.ExchangeGitHubToken(ctx, githubToken)
+	if err != nil {
+		return fmt.Errorf("failed to get GitHub Copilot token: %w", err)
+	}
+
+	if err := cm.SetCopilotOAuthCredentials(creds); err != nil {
+		return fmt.Errorf("failed to store credentials: %w", err)
+	}
+
+	fmt.Println("✅ Successfully authenticated with GitHub Copilot!")
+	fmt.Printf("📁 Credentials stored in: %s\n", cm.GetCredentialsPath())
+	fmt.Println("\n🎉 Your GitHub Copilot credentials will now be used for copilot/* models.")
+	fmt.Println("💡 You can check your authentication status with: kit auth status")
+
+	if err := setDefaultModelIfRequested("copilot"); err != nil {
+		return err
+	}
+
+	if !loginSetDefault {
+		fmt.Println("\n💡 To set Copilot as your default model, run:")
+		fmt.Println("   kit auth login copilot --set-default")
+	}
+
 	return nil
 }
 
@@ -504,13 +677,13 @@ func startOpenAICallbackServer(expectedState string) (*callbackServer, error) {
 		}
 
 		// Return success page
-		w.Header().Set("Content-Type", "text/html")
+		w.Header().Set("Content-Type", "text/html; charset=utf-8")
 		w.WriteHeader(http.StatusOK)
 		_, _ = fmt.Fprintf(w, `<!DOCTYPE html>
 <html>
 <head><title>Authentication Successful</title></head>
 <body style="font-family: sans-serif; text-align: center; padding: 50px;">
-<h1>✓ Authentication Successful</h1>
+<h1>&#10003; Authentication Successful</h1>
 <p>You can close this window and return to the terminal.</p>
 </body>
 </html>`)
@@ -575,3 +748,43 @@ func logoutOpenAI() error {
 
 	return nil
 }
+
+func logoutCopilot() error {
+	cm, err := kit.NewCredentialManager()
+	if err != nil {
+		return fmt.Errorf("failed to initialize credential manager: %w", err)
+	}
+
+	hasAuth, err := cm.HasCopilotCredentials()
+	if err != nil {
+		return fmt.Errorf("failed to check authentication status: %w", err)
+	}
+
+	if !hasAuth {
+		fmt.Println("You are not currently authenticated with GitHub Copilot.")
+		return nil
+	}
+
+	var confirm bool
+	err = huh.NewConfirm().
+		Title("Remove GitHub Copilot credentials").
+		Description("Are you sure you want to remove your stored credentials?").
+		Affirmative("Yes").
+		Negative("No").
+		Value(&confirm).
+		Run()
+	if err != nil || !confirm {
+		fmt.Println("Logout cancelled.")
+		return nil
+	}
+
+	if err := cm.RemoveCopilotCredentials(); err != nil {
+		return fmt.Errorf("failed to remove credentials: %w", err)
+	}
+
+	fmt.Println("✓ Successfully logged out from GitHub Copilot!")
+	fmt.Println("You will need to authenticate again with 'kit auth login copilot'.")
+	fmt.Println("Tip: this removes local credentials only. Revoke the GitHub OAuth grant at https://github.com/settings/applications")
+
+	return nil
+}

cmd/extension_context.go

@@ -0,0 +1,308 @@
+package cmd
+
+import (
+	"context"
+	"fmt"
+	"os"
+
+	"github.com/spf13/viper"
+	"golang.org/x/term"
+
+	"github.com/mark3labs/kit/internal/app"
+	"github.com/mark3labs/kit/internal/extbridge"
+	"github.com/mark3labs/kit/internal/extensions"
+	"github.com/mark3labs/kit/internal/models"
+	"github.com/mark3labs/kit/internal/ui"
+	kit "github.com/mark3labs/kit/pkg/kit"
+)
+
+// extensionContextDeps groups the runtime dependencies needed to wire up
+// an extensions.Context for the interactive TUI mode.
+type extensionContextDeps struct {
+	ctx          context.Context
+	cwd          string
+	modelName    string
+	interactive  bool
+	kitInstance  *kit.Kit
+	appInstance  *app.App
+	usageTracker *ui.UsageTracker
+}
+
+// buildInteractiveExtensionContext returns an extensions.Context with every
+// field except Print / PrintInfo / PrintError populated. Callers must set
+// the three print routes appropriately for their phase (startup buffering
+// vs. live runtime routing).
+//
+// The headless half (data access, state, options, tree navigation, skills,
+// templates, model resolution, subagents) comes from extbridge.BaseContext;
+// this function overlays the TUI-specific fields and overrides SetModel /
+// ReloadExtensions with TUI-aware versions.
+func buildInteractiveExtensionContext(deps extensionContextDeps) extensions.Context {
+	kitInstance := deps.kitInstance
+	appInstance := deps.appInstance
+	usageTracker := deps.usageTracker
+
+	ec := extbridge.BaseContext(deps.ctx, kitInstance)
+
+	ec.CWD = deps.cwd
+	ec.Model = deps.modelName
+	ec.Interactive = deps.interactive
+
+	ec.PrintBlock = func(opts extensions.PrintBlockOpts) {
+		appInstance.PrintBlockFromExtension(opts)
+	}
+	ec.SendMessage = func(text string) { appInstance.Run(text) }
+	ec.CancelAndSend = func(text string) { appInstance.InterruptAndSend(text) }
+	ec.Abort = func() { appInstance.Abort() }
+	ec.IsIdle = func() bool { return !appInstance.IsBusy() }
+	ec.Compact = func(cfg extensions.CompactConfig) error {
+		return appInstance.CompactAsync(cfg.CustomInstructions, cfg.OnComplete, cfg.OnError)
+	}
+	ec.SendMultimodalMessage = func(text string, files []extensions.FilePart) {
+		parts := make([]kit.LLMFilePart, len(files))
+		for i, f := range files {
+			parts[i] = kit.LLMFilePart{
+				Filename:  f.Filename,
+				Data:      f.Data,
+				MediaType: f.MediaType,
+			}
+		}
+		appInstance.RunWithFiles(text, parts)
+	}
+	ec.GetSessionUsage = func() extensions.SessionUsage {
+		if usageTracker == nil {
+			return extensions.SessionUsage{}
+		}
+		stats := usageTracker.GetSessionStats()
+		return extensions.SessionUsage{
+			TotalInputTokens:      stats.TotalInputTokens,
+			TotalOutputTokens:     stats.TotalOutputTokens,
+			TotalCacheReadTokens:  stats.TotalCacheReadTokens,
+			TotalCacheWriteTokens: stats.TotalCacheWriteTokens,
+			TotalCost:             stats.TotalCost,
+			RequestCount:          stats.RequestCount,
+		}
+	}
+	ec.Exit = func() { appInstance.QuitFromExtension() }
+
+	// TUI widgets/chrome — mutate runner state, then notify the TUI.
+	// Always use a goroutine for NotifyWidgetUpdate: prog.Send() deadlocks
+	// if called synchronously from inside BubbleTea's Update() handler.
+	// All call sites use go-routines uniformly.
+	ec.SetWidget = func(config extensions.WidgetConfig) {
+		kitInstance.Extensions().SetWidget(config)
+		go appInstance.NotifyWidgetUpdate()
+	}
+	ec.RemoveWidget = func(id string) {
+		kitInstance.Extensions().RemoveWidget(id)
+		go appInstance.NotifyWidgetUpdate()
+	}
+	ec.SetHeader = func(config extensions.HeaderFooterConfig) {
+		kitInstance.Extensions().SetHeader(config)
+		go appInstance.NotifyWidgetUpdate()
+	}
+	ec.RemoveHeader = func() {
+		kitInstance.Extensions().RemoveHeader()
+		go appInstance.NotifyWidgetUpdate()
+	}
+	ec.SetFooter = func(config extensions.HeaderFooterConfig) {
+		kitInstance.Extensions().SetFooter(config)
+		go appInstance.NotifyWidgetUpdate()
+	}
+	ec.RemoveFooter = func() {
+		kitInstance.Extensions().RemoveFooter()
+		go appInstance.NotifyWidgetUpdate()
+	}
+	ec.SetUIVisibility = func(v extensions.UIVisibility) {
+		kitInstance.Extensions().SetUIVisibility(v)
+		go appInstance.NotifyWidgetUpdate()
+	}
+	ec.SetEditor = func(config extensions.EditorConfig) {
+		kitInstance.Extensions().SetEditor(config)
+		go appInstance.NotifyWidgetUpdate()
+	}
+	ec.ResetEditor = func() {
+		kitInstance.Extensions().ResetEditor()
+		go appInstance.NotifyWidgetUpdate()
+	}
+	ec.SetEditorText = func(text string) {
+		appInstance.SetEditorTextFromExtension(text)
+	}
+	ec.SetStatus = func(key string, text string, priority int) {
+		kitInstance.Extensions().SetStatus(extensions.StatusBarEntry{
+			Key:      key,
+			Text:     text,
+			Priority: priority,
+		})
+		go appInstance.NotifyWidgetUpdate()
+	}
+	ec.RemoveStatus = func(key string) {
+		kitInstance.Extensions().RemoveStatus(key)
+		go appInstance.NotifyWidgetUpdate()
+	}
+
+	// Interactive prompts — channel-based round trips through the TUI.
+	ec.PromptSelect = func(config extensions.PromptSelectConfig) extensions.PromptSelectResult {
+		ch := make(chan app.PromptResponse, 1)
+		appInstance.SendPromptRequest(app.PromptRequestEvent{
+			PromptType: "select",
+			Message:    config.Message,
+			Options:    config.Options,
+			ResponseCh: ch,
+		})
+		resp := <-ch
+		if resp.Cancelled {
+			return extensions.PromptSelectResult{Cancelled: true}
+		}
+		return extensions.PromptSelectResult{Value: resp.Value, Index: resp.Index}
+	}
+	ec.PromptConfirm = func(config extensions.PromptConfirmConfig) extensions.PromptConfirmResult {
+		ch := make(chan app.PromptResponse, 1)
+		def := "false"
+		if config.DefaultValue {
+			def = "true"
+		}
+		appInstance.SendPromptRequest(app.PromptRequestEvent{
+			PromptType: "confirm",
+			Message:    config.Message,
+			Default:    def,
+			ResponseCh: ch,
+		})
+		resp := <-ch
+		if resp.Cancelled {
+			return extensions.PromptConfirmResult{Cancelled: true}
+		}
+		return extensions.PromptConfirmResult{Value: resp.Confirmed}
+	}
+	ec.PromptInput = func(config extensions.PromptInputConfig) extensions.PromptInputResult {
+		ch := make(chan app.PromptResponse, 1)
+		appInstance.SendPromptRequest(app.PromptRequestEvent{
+			PromptType:  "input",
+			Message:     config.Message,
+			Placeholder: config.Placeholder,
+			Default:     config.Default,
+			ResponseCh:  ch,
+		})
+		resp := <-ch
+		if resp.Cancelled {
+			return extensions.PromptInputResult{Cancelled: true}
+		}
+		return extensions.PromptInputResult{Value: resp.Value}
+	}
+	ec.ShowOverlay = func(config extensions.OverlayConfig) extensions.OverlayResult {
+		ch := make(chan app.OverlayResponse, 1)
+		appInstance.SendOverlayRequest(app.OverlayRequestEvent{
+			Title:       config.Title,
+			Content:     config.Content.Text,
+			Markdown:    config.Content.Markdown,
+			BorderColor: config.Style.BorderColor,
+			Background:  config.Style.Background,
+			Width:       config.Width,
+			MaxHeight:   config.MaxHeight,
+			Anchor:      string(config.Anchor),
+			Actions:     config.Actions,
+			ResponseCh:  ch,
+		})
+		resp := <-ch
+		if resp.Cancelled {
+			return extensions.OverlayResult{Cancelled: true, Index: -1}
+		}
+		return extensions.OverlayResult{
+			Action: resp.Action,
+			Index:  resp.Index,
+		}
+	}
+	ec.SuspendTUI = func(callback func()) error {
+		return appInstance.SuspendTUI(callback)
+	}
+
+	// TUI-aware model switch: also notifies the TUI status bar and
+	// refreshes the usage tracker for correct token counting.
+	ec.SetModel = func(modelString string) error {
+		// Capture previous model for the ModelChange event.
+		previousModel := kitInstance.Extensions().GetContext().Model
+		err := kitInstance.SetModel(context.Background(), modelString)
+		if err != nil {
+			return err
+		}
+		// Notify TUI so it updates model in status bar.
+		p, m, _ := models.ParseModelString(modelString)
+		appInstance.NotifyModelChanged(p, m)
+		// Update the context's Model field so handlers see it.
+		kitInstance.Extensions().UpdateContextModel(modelString)
+		// Fire OnModelChange event to extensions.
+		kitInstance.Extensions().EmitModelChange(modelString, previousModel, "extension")
+		// Update usage tracker with new model info for correct token counting.
+		ui.UpdateUsageTrackerForModel(usageTracker, modelString, viper.GetString("provider-api-key"))
+		return nil
+	}
+
+	ec.RenderMessage = func(rendererName, content string) {
+		renderer := kitInstance.Extensions().GetMessageRenderer(rendererName)
+		if renderer == nil || renderer.Render == nil {
+			appInstance.PrintFromExtension("", content)
+			return
+		}
+		w, _, _ := term.GetSize(int(os.Stdout.Fd()))
+		if w == 0 {
+			w = 80
+		}
+		rendered := renderer.Render(content, w)
+		appInstance.PrintFromExtension("", rendered)
+	}
+	ec.ReloadExtensions = func() error {
+		err := kitInstance.Extensions().Reload()
+		if err != nil {
+			return err
+		}
+		// Notify TUI that widgets/status/commands may have changed.
+		go appInstance.NotifyWidgetUpdate()
+		return nil
+	}
+
+	// Theme management (TUI only).
+	ec.RegisterTheme = func(name string, config extensions.ThemeColorConfig) {
+		tc := func(c extensions.ThemeColor) [2]string { return [2]string{c.Light, c.Dark} }
+		ui.RegisterThemeFromConfig(name,
+			tc(config.Primary), tc(config.Secondary),
+			tc(config.Success), tc(config.Warning),
+			tc(config.Error), tc(config.Info),
+			tc(config.Text), tc(config.Muted),
+			tc(config.VeryMuted), tc(config.Background),
+			tc(config.Border), tc(config.MutedBorder),
+			tc(config.System), tc(config.Tool),
+			tc(config.Accent), tc(config.Highlight),
+			tc(config.MdHeading), tc(config.MdLink),
+			tc(config.MdKeyword), tc(config.MdString),
+			tc(config.MdNumber), tc(config.MdComment),
+		)
+	}
+	ec.SetTheme = func(name string) error {
+		return ui.ApplyTheme(name)
+	}
+	ec.ListThemes = func() []string {
+		return ui.ListThemes()
+	}
+
+	// Skill context-injection (drives a new agent turn through the TUI).
+	ec.InjectSkillAsContext = func(skillName string) string {
+		skills := kitInstance.DiscoverSkillsForExtension()
+		for _, s := range skills {
+			if s.Name == skillName {
+				appInstance.Run(fmt.Sprintf("<skill name=%q>\n%s\n</skill>", s.Name, s.Content))
+				return ""
+			}
+		}
+		return fmt.Sprintf("skill not found: %s", skillName)
+	}
+	ec.InjectRawSkillAsContext = func(path string) string {
+		s, err := kitInstance.LoadSkillForExtension(path)
+		if err != "" {
+			return err
+		}
+		appInstance.Run(fmt.Sprintf("<skill name=%q>\n%s\n</skill>", s.Name, s.Content))
+		return ""
+	}
+
+	return ec
+}

examples/extensions/README.md

@@ -58,6 +58,7 @@ kit install github.com/mark3labs/kit/examples/extensions --local
 | `project-rules.go` | Project-specific rules | Session data, file reading |
 | `protected-paths.go` | Block dangerous operations | `OnToolCall` with blocking |
 | `permission-gate.go` | Confirm destructive actions | `OnToolCall` with confirmation |
+| `usage-budget.go` | Soft cost cap + per-turn report | `OnLLMUsage`, `SetState`/`GetState`, enriched `AgentEndEvent` |
 
 ### Tools & Commands
 

examples/extensions/inline-bash.go

@@ -10,13 +10,21 @@ import (
 	"kit/ext"
 )
 
+// re matches !{...} with non-greedy content.
+var re = regexp.MustCompile(`!\{([^}]+)\}`)
+
 // Init expands inline bash expressions in user prompts before they reach the
-// LLM. Text like !{git branch --show-current} is replaced with the command's
-// stdout.
+// LLM. Text like !{git rev-parse --abbrev-ref HEAD} is replaced with the
+// command's stdout.
+//
+// In interactive mode the expansion happens at submit time via an editor
+// interceptor, so the expanded text is also visible in the user message
+// block on screen. In non-interactive mode (CLI, script, queue) the
+// expansion happens via OnInput transform.
 //
 // Examples:
 //
-//	"Fix the tests on !{git branch --show-current}"
+//	"Fix the tests on !{git rev-parse --abbrev-ref HEAD}"
 //	  → "Fix the tests on main"
 //
 //	"The current directory is !{pwd}"
@@ -24,29 +32,59 @@ import (
 //
 // Usage: kit -e examples/extensions/inline-bash.go
 func Init(api ext.API) {
-	// Matches !{...} with non-greedy content.
-	re := regexp.MustCompile(`!\{([^}]+)\}`)
+	// ── Interactive mode: editor interceptor ──────────────────────────
+	// Intercept Enter / Ctrl+D so we can expand !{...} BEFORE the
+	// SubmitMsg is created. This ensures the expanded text appears in
+	// the user message block on screen as well as in the LLM prompt.
+	api.OnSessionStart(func(_ ext.SessionStartEvent, ctx ext.Context) {
+		if !ctx.Interactive {
+			return
+		}
+		ctx.SetEditor(ext.EditorConfig{
+			HandleKey: func(key string, currentText string) ext.EditorKeyAction {
+				if (key == "enter" || key == "ctrl+d") && re.MatchString(currentText) {
+					expanded := expand(currentText)
+					// Clear the textarea asynchronously — calling
+					// SetEditorText synchronously from inside Update()
+					// would deadlock the BubbleTea event loop.
+					go ctx.SetEditorText("")
+					return ext.EditorKeyAction{
+						Type:       ext.EditorKeySubmit,
+						SubmitText: expanded,
+					}
+				}
+				return ext.EditorKeyAction{Type: ext.EditorKeyPassthrough}
+			},
+		})
+	})
 
+	// ── Non-interactive fallback: OnInput transform ──────────────────
+	// For CLI, script, and queue sources the editor interceptor is not
+	// active, so we fall back to OnInput which still rewrites the
+	// prompt text sent to the LLM.
 	api.OnInput(func(ev ext.InputEvent, ctx ext.Context) *ext.InputResult {
-		if !re.MatchString(ev.Text) {
+		if ev.Source == "interactive" || !re.MatchString(ev.Text) {
 			return nil
 		}
 
-		expanded := re.ReplaceAllStringFunc(ev.Text, func(match string) string {
-			// Extract the command between !{ and }.
-			cmd := re.FindStringSubmatch(match)[1]
-			cmd = strings.TrimSpace(cmd)
-
-			out, err := exec.Command("bash", "-c", cmd).Output()
-			if err != nil {
-				return match // keep original on error
-			}
-			return strings.TrimSpace(string(out))
-		})
-
 		return &ext.InputResult{
 			Action: "transform",
-			Text:   expanded,
+			Text:   expand(ev.Text),
 		}
 	})
 }
+
+// expand replaces every !{cmd} in text with the command's stdout.
+// On error the original !{cmd} token is preserved.
+func expand(text string) string {
+	return re.ReplaceAllStringFunc(text, func(match string) string {
+		cmd := re.FindStringSubmatch(match)[1]
+		cmd = strings.TrimSpace(cmd)
+
+		out, err := exec.Command("bash", "-c", cmd).Output()
+		if err != nil {
+			return match // keep original on error
+		}
+		return strings.TrimSpace(string(out))
+	})
+}

examples/extensions/subagent-monitor_test.go

@@ -13,7 +13,7 @@ import (
 // without panicking and properly guards nil ctx calls.
 func TestSubagentMonitor_SessionStart(t *testing.T) {
 	harness := test.New(t)
-	harness.LoadFile("../../.kit/extensions/subagent-monitor.go")
+	harness.LoadFile("./subagent-monitor.go")
 
 	// Emit SessionStart - should not panic even with nil ctx functions
 	_, err := harness.Emit(extensions.SessionStartEvent{SessionID: "test-session"})
@@ -26,7 +26,7 @@ func TestSubagentMonitor_SessionStart(t *testing.T) {
 // creates entries and emits widget updates.
 func TestSubagentMonitor_SubagentLifecycle(t *testing.T) {
 	harness := test.New(t)
-	harness.LoadFile("../../.kit/extensions/subagent-monitor.go")
+	harness.LoadFile("./subagent-monitor.go")
 
 	// Start session
 	_, err := harness.Emit(extensions.SessionStartEvent{SessionID: "test-session"})
@@ -84,7 +84,7 @@ func TestSubagentMonitor_SubagentLifecycle(t *testing.T) {
 // TestSubagentMonitor_MultipleSubagents verifies multiple parallel subagents.
 func TestSubagentMonitor_MultipleSubagents(t *testing.T) {
 	harness := test.New(t)
-	harness.LoadFile("../../.kit/extensions/subagent-monitor.go")
+	harness.LoadFile("./subagent-monitor.go")
 
 	_, err := harness.Emit(extensions.SessionStartEvent{SessionID: "test-session"})
 	if err != nil {
@@ -130,11 +130,63 @@ func TestSubagentMonitor_MultipleSubagents(t *testing.T) {
 	time.Sleep(100 * time.Millisecond)
 }
 
+// TestSubagentMonitor_ConcurrentSubagents verifies no panics when multiple
+// subagents emit events concurrently from different goroutines.
+func TestSubagentMonitor_ConcurrentSubagents(t *testing.T) {
+	harness := test.New(t)
+	harness.LoadFile("./subagent-monitor.go")
+
+	_, err := harness.Emit(extensions.SessionStartEvent{SessionID: "test-session"})
+	if err != nil {
+		t.Fatalf("SessionStart should not error: %v", err)
+	}
+
+	// Start 5 subagents concurrently
+	done := make(chan struct{}, 5)
+	for i := range 5 {
+		go func(idx int) {
+			defer func() { done <- struct{}{} }()
+
+			callID := fmt.Sprintf("concurrent-%d", idx)
+			task := fmt.Sprintf("concurrent task %d", idx)
+
+			_, _ = harness.Emit(extensions.SubagentStartEvent{
+				ToolCallID: callID,
+				Task:       task,
+			})
+
+			// Emit many chunks rapidly
+			for j := range 20 {
+				_, _ = harness.Emit(extensions.SubagentChunkEvent{
+					ToolCallID: callID,
+					Task:       task,
+					ChunkType:  "text",
+					Content:    fmt.Sprintf("agent %d chunk %d", idx, j),
+				})
+			}
+
+			_, _ = harness.Emit(extensions.SubagentEndEvent{
+				ToolCallID: callID,
+				Task:       task,
+				Response:   "done",
+			})
+		}(i)
+	}
+
+	// Wait for all goroutines
+	for range 5 {
+		<-done
+	}
+
+	// Allow any final processing
+	time.Sleep(200 * time.Millisecond)
+}
+
 // TestSubagentMonitor_SessionShutdown verifies shutdown doesn't panic
 // even with nil ctx functions.
 func TestSubagentMonitor_SessionShutdown(t *testing.T) {
 	harness := test.New(t)
-	harness.LoadFile("../../.kit/extensions/subagent-monitor.go")
+	harness.LoadFile("./subagent-monitor.go")
 
 	// Start then shutdown
 	_, err := harness.Emit(extensions.SessionStartEvent{SessionID: "test-session"})

examples/extensions/sudo-handler.go

@@ -0,0 +1,153 @@
+//go:build ignore
+
+// sudo-handler.go - Extension to handle sudo password prompts securely
+//
+// This extension intercepts bash commands containing "sudo" and:
+// 1. Checks if sudo credentials are already cached (via sudo -n)
+// 2. If not cached, prompts the user for their password (with masking)
+// 3. Temporarily sets SUDO_PASSWORD environment variable for execution
+// 4. The bash tool automatically uses sudo -S -p '' to pipe the password
+//
+// Usage: kit -e examples/extensions/sudo-handler.go
+//
+// Security notes:
+// - Password is only stored in memory for the duration of the session
+// - Password is never logged or displayed
+// - Each session requires re-authentication (sudo -k is used)
+// - The SUDO_PASSWORD env var is set only during tool execution
+
+package main
+
+import (
+	"encoding/json"
+	"os"
+	"strings"
+	"sync"
+
+	"kit/ext"
+)
+
+var (
+	// cachedPassword stores the sudo password for the session
+	cachedPassword string
+	// hasCachedPassword tracks if we have a valid cached password
+	hasCachedPassword bool
+	// mu protects cached password access
+	mu sync.RWMutex
+)
+
+// Init sets up the sudo handler extension
+func Init(api ext.API) {
+	api.OnToolCall(func(tc ext.ToolCallEvent, ctx ext.Context) *ext.ToolCallResult {
+		if tc.ToolName != "bash" {
+			return nil
+		}
+
+		// Parse the command from tool input
+		var input struct {
+			Command string `json:"command"`
+		}
+		if err := json.Unmarshal([]byte(tc.Input), &input); err != nil {
+			return nil
+		}
+
+		// Check if command contains sudo
+		if !containsSudo(input.Command) {
+			return nil
+		}
+
+		// Check if we already have cached credentials
+		mu.RLock()
+		password := cachedPassword
+		hasCached := hasCachedPassword
+		mu.RUnlock()
+
+		if hasCached {
+			// Use cached password
+			os.Setenv("SUDO_PASSWORD", password)
+			return nil
+		}
+
+		// No cached password - prompt user
+		result := ctx.PromptInput(ext.PromptInputConfig{
+			Message:     "🔐 Sudo password required for:\n  " + truncateCommand(input.Command, 60),
+			Placeholder: "Enter your password",
+		})
+
+		if result.Cancelled {
+			return &ext.ToolCallResult{
+				Block:  true,
+				Reason: "Sudo password prompt cancelled by user",
+			}
+		}
+
+		if result.Value == "" {
+			return &ext.ToolCallResult{
+				Block:  true,
+				Reason: "No password provided",
+			}
+		}
+
+		// Cache the password for this session
+		mu.Lock()
+		cachedPassword = result.Value
+		hasCachedPassword = true
+		mu.Unlock()
+
+		// Set environment variable for the bash tool to use
+		os.Setenv("SUDO_PASSWORD", result.Value)
+
+		// Show confirmation (without revealing password)
+		ctx.PrintInfo("Sudo password cached for this session")
+
+		return nil
+	})
+
+	// Clear cached password when session ends
+	api.OnSessionShutdown(func(event ext.SessionShutdownEvent, ctx ext.Context) {
+		mu.Lock()
+		cachedPassword = ""
+		hasCachedPassword = false
+		mu.Unlock()
+		os.Unsetenv("SUDO_PASSWORD")
+	})
+}
+
+// containsSudo checks if the command contains sudo as a command (not in a string)
+func containsSudo(command string) bool {
+	// Simple check for sudo as a word, not inside quotes or as part of another word
+	lower := strings.ToLower(command)
+
+	// Check for sudo at start or after separators
+	patterns := []string{
+		"sudo ",
+		"sudo\t",
+		";sudo ",
+		"&& sudo ",
+		"|| sudo ",
+		"| sudo ",
+		"$(sudo ",
+		"`sudo ",
+	}
+
+	for _, pattern := range patterns {
+		if strings.Contains(lower, pattern) {
+			return true
+		}
+	}
+
+	// Check if command starts with sudo
+	if strings.HasPrefix(lower, "sudo ") {
+		return true
+	}
+
+	return false
+}
+
+// truncateCommand truncates a long command for display
+func truncateCommand(cmd string, maxLen int) string {
+	if len(cmd) <= maxLen {
+		return cmd
+	}
+	return cmd[:maxLen-3] + "..."
+}

examples/extensions/usage-budget.go

@@ -0,0 +1,87 @@
+//go:build ignore
+
+package main
+
+import (
+	"fmt"
+	"strconv"
+
+	"kit/ext"
+)
+
+// Init demonstrates the three primitives added in issue #53:
+//
+//  1. api.OnLLMUsage(...) — per-LLM-call usage callback with token + cost
+//     deltas. Use this for budget enforcement that reacts between calls
+//     within a single agent turn, rather than only at turn boundaries.
+//
+//  2. ctx.SetState / ctx.GetState / ctx.DeleteState / ctx.ListState —
+//     last-write-wins, session-scoped key-value store backed by a sidecar
+//     file. Use this for snapshot state (current value of X) instead of
+//     ctx.AppendEntry, which is append-only and bloats branch reads.
+//
+//  3. ext.AgentEndEvent.ToolCallCount / .ToolNames / .LLMCallCount /
+//     .InputTokensDelta / .OutputTokensDelta / .CostDelta / .DurationMs —
+//     per-turn aggregates so observer extensions don't need to maintain
+//     parallel bookkeeping.
+//
+// Together these support a simple soft-budget cap: warn when the
+// cumulative cost in this session exceeds a threshold, and print a
+// per-turn report on AgentEnd.
+//
+// Usage: kit -e examples/extensions/usage-budget.go
+func Init(api ext.API) {
+	const warnAtKey = "usage-budget:warn-at-usd"
+
+	// 1. Print per-LLM-call usage with provider, model, and cost.
+	api.OnLLMUsage(func(e ext.LLMUsageEvent, ctx ext.Context) {
+		ctx.Print(fmt.Sprintf(
+			"[usage] step=%d %s/%s tokens=↑%d ↓%d cache=↑%d/↓%d cost=$%.4f (%s)",
+			e.StepNumber, e.Provider, e.Model,
+			e.InputTokens, e.OutputTokens,
+			e.CacheWriteTokens, e.CacheReadTokens,
+			e.Cost, e.FinishReason,
+		))
+
+		// 2. Persist running total in last-write-wins state.
+		current := 0.0
+		if raw, ok := ctx.GetState("usage-budget:total-cost"); ok {
+			current, _ = strconv.ParseFloat(raw, 64)
+		}
+		current += e.Cost
+		ctx.SetState("usage-budget:total-cost", strconv.FormatFloat(current, 'f', 6, 64))
+
+		// Soft warn-at threshold (configurable via state).
+		warnAt := 0.50
+		if raw, ok := ctx.GetState(warnAtKey); ok {
+			if v, err := strconv.ParseFloat(raw, 64); err == nil {
+				warnAt = v
+			}
+		}
+		if current > warnAt {
+			ctx.PrintError(fmt.Sprintf(
+				"[usage] session cost $%.4f exceeds soft cap $%.2f",
+				current, warnAt,
+			))
+		}
+	})
+
+	// 3. Print a per-turn summary using the enriched AgentEndEvent.
+	api.OnAgentEnd(func(e ext.AgentEndEvent, ctx ext.Context) {
+		ctx.Print(fmt.Sprintf(
+			"[turn] stop=%s tools=%d llm-calls=%d tokens=↑%d ↓%d cost=$%.4f duration=%dms",
+			e.StopReason, e.ToolCallCount, e.LLMCallCount,
+			e.InputTokensDelta, e.OutputTokensDelta, e.CostDelta, e.DurationMs,
+		))
+		if len(e.ToolNames) > 0 {
+			ctx.Print(fmt.Sprintf("[turn] tool order: %v", e.ToolNames))
+		}
+	})
+
+	// Bootstrap default soft cap once per session.
+	api.OnSessionStart(func(e ext.SessionStartEvent, ctx ext.Context) {
+		if _, ok := ctx.GetState(warnAtKey); !ok {
+			ctx.SetState(warnAtKey, "0.50")
+		}
+	})
+}

examples/sdk/README.md

@@ -42,4 +42,14 @@ defer host.Close()
 response, err := host.Prompt(ctx, "Hello!")
 ```
 
+Or use the functional-options constructor for quick setups (streaming defaults on):
+
+```go
+host, err := kit.NewAgent(ctx,
+    kit.WithModel("anthropic/claude-sonnet-4-5-20250929"),
+    kit.WithSystemPrompt("You are a helpful assistant."),
+    kit.Ephemeral(),
+)
+```
+
 See the [SDK README](../../pkg/kit/README.md) for the full API reference.

examples/sdk/basic/main.go

@@ -62,7 +62,7 @@ func main() {
 		}
 	})
 	// Subscribe to streaming chunks.
-	host3.OnStreaming(func(e kit.MessageUpdateEvent) {
+	host3.OnMessageUpdate(func(e kit.MessageUpdateEvent) {
 		fmt.Print(e.Chunk)
 	})
 

go.mod

@@ -1,101 +1,105 @@
 module github.com/mark3labs/kit
 
-go 1.26.1
+go 1.26.3
 
 require (
 	charm.land/bubbles/v2 v2.1.0
-	charm.land/bubbletea/v2 v2.0.2
-	charm.land/fantasy v0.17.1
+	charm.land/bubbletea/v2 v2.0.7
+	charm.land/fantasy v0.25.0
 	charm.land/huh/v2 v2.0.3
-	charm.land/lipgloss/v2 v2.0.2
-	github.com/alecthomas/chroma/v2 v2.23.1
+	charm.land/lipgloss/v2 v2.0.3
+	github.com/alecthomas/chroma/v2 v2.26.1
 	github.com/atotto/clipboard v0.1.4
 	github.com/aymanbagabas/go-udiff v0.4.1
+	github.com/charmbracelet/colorprofile v0.4.3
 	github.com/charmbracelet/fang v1.0.0
 	github.com/charmbracelet/log v1.0.0
 	github.com/charmbracelet/openai-go v0.0.0-20260319145158-d0740cc34266
-	github.com/charmbracelet/ultraviolet v0.0.0-20260330092749-0f94982c930b
+	github.com/charmbracelet/ultraviolet v0.0.0-20260601155805-6cf7526a1b3f
+	github.com/charmbracelet/x/editor v0.2.0
 	github.com/clipperhouse/displaywidth v0.11.0
 	github.com/clipperhouse/uax29/v2 v2.7.0
-	github.com/coder/acp-go-sdk v0.6.3
-	github.com/fsnotify/fsnotify v1.9.0
+	github.com/coder/acp-go-sdk v0.13.5
+	github.com/fsnotify/fsnotify v1.10.1
 	github.com/indaco/herald v0.13.0
 	github.com/indaco/herald-md v0.3.0
-	github.com/mark3labs/mcp-go v0.47.0
+	github.com/mark3labs/mcp-go v0.54.1
 	github.com/spf13/cobra v1.10.2
 	github.com/spf13/viper v1.21.0
 	github.com/traefik/yaegi v0.16.1
-	golang.org/x/term v0.41.0
+	golang.org/x/image v0.41.0
+	golang.org/x/term v0.43.0
 	gopkg.in/yaml.v3 v3.0.1
 )
 
 require (
 	cloud.google.com/go v0.123.0 // indirect
-	cloud.google.com/go/auth v0.19.0 // indirect
+	cloud.google.com/go/auth v0.20.0 // indirect
 	cloud.google.com/go/auth/oauth2adapt v0.2.8 // indirect
 	cloud.google.com/go/compute/metadata v0.9.0 // indirect
-	github.com/Azure/azure-sdk-for-go/sdk/azcore v1.21.0 // indirect
+	github.com/Azure/azure-sdk-for-go/sdk/azcore v1.21.1 // indirect
 	github.com/Azure/azure-sdk-for-go/sdk/internal v1.12.0 // indirect
-	github.com/aws/aws-sdk-go-v2 v1.41.5 // indirect
-	github.com/aws/aws-sdk-go-v2/aws/protocol/eventstream v1.7.8 // indirect
-	github.com/aws/aws-sdk-go-v2/config v1.32.14 // indirect
-	github.com/aws/aws-sdk-go-v2/credentials v1.19.14 // indirect
-	github.com/aws/aws-sdk-go-v2/feature/ec2/imds v1.18.21 // indirect
-	github.com/aws/aws-sdk-go-v2/internal/configsources v1.4.21 // indirect
-	github.com/aws/aws-sdk-go-v2/internal/endpoints/v2 v2.7.21 // indirect
-	github.com/aws/aws-sdk-go-v2/internal/ini v1.8.6 // indirect
-	github.com/aws/aws-sdk-go-v2/service/internal/accept-encoding v1.13.7 // indirect
-	github.com/aws/aws-sdk-go-v2/service/internal/presigned-url v1.13.21 // indirect
-	github.com/aws/aws-sdk-go-v2/service/signin v1.0.9 // indirect
-	github.com/aws/aws-sdk-go-v2/service/sso v1.30.15 // indirect
-	github.com/aws/aws-sdk-go-v2/service/ssooidc v1.35.19 // indirect
-	github.com/aws/aws-sdk-go-v2/service/sts v1.41.10 // indirect
-	github.com/aws/smithy-go v1.24.3 // indirect
+	github.com/aws/aws-sdk-go-v2 v1.41.8 // indirect
+	github.com/aws/aws-sdk-go-v2/aws/protocol/eventstream v1.7.10 // indirect
+	github.com/aws/aws-sdk-go-v2/config v1.32.19 // indirect
+	github.com/aws/aws-sdk-go-v2/credentials v1.19.18 // indirect
+	github.com/aws/aws-sdk-go-v2/feature/ec2/imds v1.18.24 // indirect
+	github.com/aws/aws-sdk-go-v2/internal/configsources v1.4.24 // indirect
+	github.com/aws/aws-sdk-go-v2/internal/endpoints/v2 v2.7.24 // indirect
+	github.com/aws/aws-sdk-go-v2/internal/v4a v1.4.25 // indirect
+	github.com/aws/aws-sdk-go-v2/service/internal/accept-encoding v1.13.9 // indirect
+	github.com/aws/aws-sdk-go-v2/service/internal/presigned-url v1.13.24 // indirect
+	github.com/aws/aws-sdk-go-v2/service/signin v1.1.0 // indirect
+	github.com/aws/aws-sdk-go-v2/service/sso v1.30.18 // indirect
+	github.com/aws/aws-sdk-go-v2/service/ssooidc v1.36.1 // indirect
+	github.com/aws/aws-sdk-go-v2/service/sts v1.42.2 // indirect
+	github.com/aws/smithy-go v1.26.0 // indirect
 	github.com/catppuccin/go v0.3.0 // indirect
 	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/charmbracelet/anthropic-sdk-go v0.0.0-20260223140439-63879b0b8dab // indirect
-	github.com/charmbracelet/colorprofile v0.4.3 // indirect
 	github.com/charmbracelet/harmonica v0.2.0 // indirect
 	github.com/charmbracelet/lipgloss v1.1.1-0.20250404203927-76690c660834 // indirect
 	github.com/charmbracelet/x/cellbuf v0.0.15 // indirect
-	github.com/charmbracelet/x/exp/charmtone v0.0.0-20260330094520-2dce04b6f8a4 // indirect
+	github.com/charmbracelet/x/exp/charmtone v0.0.0-20260602025833-85a30b5e440a // indirect
 	github.com/charmbracelet/x/exp/ordered v0.1.0 // indirect
-	github.com/charmbracelet/x/exp/slice v0.0.0-20260330094520-2dce04b6f8a4 // indirect
+	github.com/charmbracelet/x/exp/slice v0.0.0-20260527151214-009e6338d40d // indirect
 	github.com/charmbracelet/x/exp/strings v0.1.0 // indirect
 	github.com/charmbracelet/x/json v0.2.0 // indirect
 	github.com/charmbracelet/x/termios v0.1.1 // indirect
 	github.com/charmbracelet/x/windows v0.2.2 // indirect
-	github.com/dlclark/regexp2 v1.11.5 // indirect
+	github.com/dlclark/regexp2 v1.12.0 // indirect
+	github.com/dlclark/regexp2/v2 v2.1.1 // indirect
 	github.com/dustin/go-humanize v1.0.1 // indirect
 	github.com/felixge/httpsnoop v1.0.4 // indirect
-	github.com/go-json-experiment/json v0.0.0-20260214004413-d219187c3433 // indirect
+	github.com/go-json-experiment/json v0.0.0-20260520185125-572e7c383686 // indirect
 	github.com/go-logfmt/logfmt v0.6.1 // indirect
 	github.com/go-logr/logr v1.4.3 // indirect
 	github.com/go-logr/stdr v1.2.2 // indirect
 	github.com/go-viper/mapstructure/v2 v2.5.0 // indirect
 	github.com/goccy/go-yaml v1.19.2 // indirect
 	github.com/google/go-cmp v0.7.0 // indirect
-	github.com/google/jsonschema-go v0.4.2 // indirect
+	github.com/google/jsonschema-go v0.4.3 // indirect
 	github.com/google/s2a-go v0.1.9 // indirect
 	github.com/google/uuid v1.6.0 // indirect
-	github.com/googleapis/enterprise-certificate-proxy v0.3.14 // indirect
-	github.com/googleapis/gax-go/v2 v2.21.0 // indirect
+	github.com/googleapis/enterprise-certificate-proxy v0.3.16 // indirect
+	github.com/googleapis/gax-go/v2 v2.22.0 // indirect
 	github.com/gorilla/websocket v1.5.3 // indirect
-	github.com/kaptinlin/go-i18n v0.3.0 // indirect
-	github.com/kaptinlin/jsonpointer v0.4.17 // indirect
-	github.com/kaptinlin/jsonschema v0.7.7 // indirect
-	github.com/kaptinlin/messageformat-go v0.4.19 // indirect
+	github.com/kaptinlin/go-i18n v0.4.5 // indirect
+	github.com/kaptinlin/jsonpointer v0.4.25 // indirect
+	github.com/kaptinlin/jsonschema v0.7.13 // indirect
+	github.com/kaptinlin/messageformat-go v0.6.0 // indirect
 	github.com/mitchellh/hashstructure/v2 v2.0.2 // indirect
 	github.com/muesli/mango v0.2.0 // indirect
 	github.com/muesli/mango-cobra v1.3.0 // indirect
 	github.com/muesli/mango-pflag v0.2.0 // indirect
 	github.com/muesli/roff v0.1.0 // indirect
-	github.com/pelletier/go-toml/v2 v2.3.0 // indirect
+	github.com/pelletier/go-toml/v2 v2.3.1 // indirect
 	github.com/sagikazarmark/locafero v0.12.0 // indirect
+	github.com/santhosh-tekuri/jsonschema/v6 v6.0.2 // indirect
 	github.com/spf13/afero v1.15.0 // indirect
 	github.com/spf13/cast v1.10.0 // indirect
 	github.com/subosito/gotenv v1.6.0 // indirect
-	github.com/tidwall/gjson v1.18.0 // indirect
+	github.com/tidwall/gjson v1.19.0 // indirect
 	github.com/tidwall/match v1.2.0 // indirect
 	github.com/tidwall/pretty v1.2.1 // indirect
 	github.com/tidwall/sjson v1.2.5 // indirect
@@ -103,38 +107,38 @@ require (
 	github.com/yosida95/uritemplate/v3 v3.0.2 // indirect
 	github.com/yuin/goldmark v1.8.2 // indirect
 	go.opentelemetry.io/auto/sdk v1.2.1 // indirect
-	go.opentelemetry.io/contrib/instrumentation/google.golang.org/grpc/otelgrpc v0.67.0 // indirect
-	go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp v0.67.0 // indirect
-	go.opentelemetry.io/otel v1.43.0 // indirect
-	go.opentelemetry.io/otel/metric v1.43.0 // indirect
-	go.opentelemetry.io/otel/trace v1.43.0 // indirect
+	go.opentelemetry.io/contrib/instrumentation/google.golang.org/grpc/otelgrpc v0.69.0 // indirect
+	go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp v0.69.0 // indirect
+	go.opentelemetry.io/otel v1.44.0 // indirect
+	go.opentelemetry.io/otel/metric v1.44.0 // indirect
+	go.opentelemetry.io/otel/trace v1.44.0 // indirect
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
-	golang.org/x/crypto v0.49.0 // indirect
-	golang.org/x/exp v0.0.0-20260312153236-7ab1446f8b90 // indirect
-	golang.org/x/net v0.52.0 // indirect
+	golang.org/x/crypto v0.52.0 // indirect
+	golang.org/x/exp v0.0.0-20260603202125-055de637280b // indirect
+	golang.org/x/net v0.55.0 // indirect
 	golang.org/x/oauth2 v0.36.0 // indirect
 	golang.org/x/time v0.15.0 // indirect
-	google.golang.org/api v0.274.0 // indirect
-	google.golang.org/genai v1.52.1 // indirect
-	google.golang.org/genproto/googleapis/rpc v0.0.0-20260401024825-9d38bb4040a9 // indirect
-	google.golang.org/grpc v1.80.0 // indirect
+	google.golang.org/api v0.282.0 // indirect
+	google.golang.org/genai v1.58.0 // indirect
+	google.golang.org/genproto/googleapis/rpc v0.0.0-20260526163538-3dc84a4a5aaa // indirect
+	google.golang.org/grpc v1.81.1 // indirect
 	google.golang.org/protobuf v1.36.11 // indirect
 	gopkg.in/yaml.v2 v2.4.0 // indirect
 )
 
 require (
 	github.com/aymanbagabas/go-osc52/v2 v2.0.1 // indirect
-	github.com/charmbracelet/x/ansi v0.11.6
+	github.com/charmbracelet/x/ansi v0.11.7
 	github.com/charmbracelet/x/term v0.2.2 // indirect
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
 	github.com/lucasb-eyer/go-colorful v1.4.0 // indirect
-	github.com/mattn/go-isatty v0.0.20 // indirect
-	github.com/mattn/go-runewidth v0.0.22 // indirect
+	github.com/mattn/go-isatty v0.0.22 // indirect
+	github.com/mattn/go-runewidth v0.0.24 // indirect
 	github.com/muesli/cancelreader v0.2.2 // indirect
 	github.com/muesli/termenv v0.16.0 // indirect
 	github.com/rivo/uniseg v0.4.7 // indirect
-	github.com/spf13/pflag v1.0.10 // indirect
+	github.com/spf13/pflag v1.0.10
 	golang.org/x/sync v0.20.0 // indirect
-	golang.org/x/sys v0.42.0 // indirect
-	golang.org/x/text v0.35.0
+	golang.org/x/sys v0.45.0 // indirect
+	golang.org/x/text v0.37.0
 )

go.sum

@@ -1,75 +1,98 @@
+cel.dev/expr v0.25.2/go.mod h1:hrXvqGP6G6gyx8UAHSHJ5RGk//1Oj5nXQ2NI02Nrsg4=
 charm.land/bubbles/v2 v2.1.0 h1:YSnNh5cPYlYjPxRrzs5VEn3vwhtEn3jVGRBT3M7/I0g=
 charm.land/bubbles/v2 v2.1.0/go.mod h1:l97h4hym2hvWBVfmJDtrEHHCtkIKeTEb3TTJ4ZOB3wY=
-charm.land/bubbletea/v2 v2.0.2 h1:4CRtRnuZOdFDTWSff9r8QFt/9+z6Emubz3aDMnf/dx0=
-charm.land/bubbletea/v2 v2.0.2/go.mod h1:3LRff2U4WIYXy7MTxfbAQ+AdfM3D8Xuvz2wbsOD9OHQ=
-charm.land/fantasy v0.17.1 h1:SQzfnyJPDuQWt6e//KKmQmEEXdqHMC0IZz10XwkLcEM=
-charm.land/fantasy v0.17.1/go.mod h1:FF5ALCCHETacHJPBqU42CtwMInYQ0ul52fdzIHQMbQk=
+charm.land/bubbletea/v2 v2.0.7 h1:7qw2tTAVar7m7klOPBYfTB0mniv/RuexsYwMRNxSeL0=
+charm.land/bubbletea/v2 v2.0.7/go.mod h1:DGW2q8gvzHnOpMpZTORs0aySVHCox5C+2Svk0fci1qs=
+charm.land/fantasy v0.25.0 h1:oXOWY1ivmTSnhYGzAolscF8zKtavWZyBWv0LHRSwN5Q=
+charm.land/fantasy v0.25.0/go.mod h1:8QrWUzIcKwZQP+aAnC9vLu3iID6hu9/Jt+rPMiieBkc=
 charm.land/huh/v2 v2.0.3 h1:2cJsMqEPwSywGHvdlKsJyQKPtSJLVnFKyFbsYZTlLkU=
 charm.land/huh/v2 v2.0.3/go.mod h1:93eEveeeqn47MwiC3tf+2atZ2l7Is88rAtmZNZ8x9Wc=
-charm.land/lipgloss/v2 v2.0.2 h1:xFolbF8JdpNkM2cEPTfXEcW1p6NRzOWTSamRfYEw8cs=
-charm.land/lipgloss/v2 v2.0.2/go.mod h1:KjPle2Qd3YmvP1KL5OMHiHysGcNwq6u83MUjYkFvEkM=
+charm.land/lipgloss/v2 v2.0.3 h1:yM2zJ4Cf5Y51b7RHIwioil4ApI/aypFXXVHSwlM6RzU=
+charm.land/lipgloss/v2 v2.0.3/go.mod h1:7myLU9iG/3xluAWzpY/fSxYYHCgoKTie7laxk6ATwXA=
+charm.land/x/vcr v0.1.1/go.mod h1:eByq2gqzWvcct/8XE2XO5KznoWEBiXH56+y2gphbltM=
 cloud.google.com/go v0.123.0 h1:2NAUJwPR47q+E35uaJeYoNhuNEM9kM8SjgRgdeOJUSE=
 cloud.google.com/go v0.123.0/go.mod h1:xBoMV08QcqUGuPW65Qfm1o9Y4zKZBpGS+7bImXLTAZU=
-cloud.google.com/go/auth v0.19.0 h1:DGYwtbcsGsT1ywuxsIoWi1u/vlks0moIblQHgSDgQkQ=
-cloud.google.com/go/auth v0.19.0/go.mod h1:2Aph7BT2KnaSFOM0JDPyiYgNh6PL9vGMiP8CUIXZ+IY=
+cloud.google.com/go/auth v0.20.0 h1:kXTssoVb4azsVDoUiF8KvxAqrsQcQtB53DcSgta74CA=
+cloud.google.com/go/auth v0.20.0/go.mod h1:942/yi/itH1SsmpyrbnTMDgGfdy2BUqIKyd0cyYLc5Q=
 cloud.google.com/go/auth/oauth2adapt v0.2.8 h1:keo8NaayQZ6wimpNSmW5OPc283g65QNIiLpZnkHRbnc=
 cloud.google.com/go/auth/oauth2adapt v0.2.8/go.mod h1:XQ9y31RkqZCcwJWNSx2Xvric3RrU88hAYYbjDWYDL+c=
 cloud.google.com/go/compute/metadata v0.9.0 h1:pDUj4QMoPejqq20dK0Pg2N4yG9zIkYGdBtwLoEkH9Zs=
 cloud.google.com/go/compute/metadata v0.9.0/go.mod h1:E0bWwX5wTnLPedCKqk3pJmVgCBSM6qQI1yTBdEb3C10=
-github.com/Azure/azure-sdk-for-go/sdk/azcore v1.21.0 h1:fou+2+WFTib47nS+nz/ozhEBnvU96bKHy6LjRsY4E28=
-github.com/Azure/azure-sdk-for-go/sdk/azcore v1.21.0/go.mod h1:t76Ruy8AHvUAC8GfMWJMa0ElSbuIcO03NLpynfbgsPA=
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@@ -236,6 +292,10 @@ github.com/rogpeppe/go-internal v1.14.1/go.mod h1:MaRKkUm5W0goXpeCfT7UZI6fk/L7L7
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-golang.org/x/exp v0.0.0-20260312153236-7ab1446f8b90/go.mod h1:xE1HEv6b+1SCZ5/uscMRjUBKtIxworgEcEi+/n9NQDQ=
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-golang.org/x/sys v0.42.0 h1:omrd2nAlyT5ESRdCLYdm3+fMfNFE/+Rf4bDIQImRJeo=
-golang.org/x/sys v0.42.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=
-golang.org/x/term v0.41.0 h1:QCgPso/Q3RTJx2Th4bDLqML4W6iJiaXFq2/ftQF13YU=
-golang.org/x/term v0.41.0/go.mod h1:3pfBgksrReYfZ5lvYM0kSO0LIkAl4Yl2bXOkKP7Ec2A=
-golang.org/x/text v0.35.0 h1:JOVx6vVDFokkpaq1AEptVzLTpDe9KGpj5tR4/X+ybL8=
-golang.org/x/text v0.35.0/go.mod h1:khi/HExzZJ2pGnjenulevKNX1W67CUy0AsXcNubPGCA=
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 golang.org/x/time v0.15.0 h1:bbrp8t3bGUeFOx08pvsMYRTCVSMk89u4tKbNOZbp88U=
 golang.org/x/time v0.15.0/go.mod h1:Y4YMaQmXwGQZoFaVFk4YpCt4FLQMYKZe9oeV/f4MSno=
+golang.org/x/tools v0.45.0/go.mod h1:LuUGqqaXcXMEFEruIVJVm5mgDD8vww/z/SR1gQ4uE/0=
+golang.org/x/tools/go/expect v0.1.1-deprecated/go.mod h1:eihoPOH+FgIqa3FpoTwguz/bVUSGBlGQU67vpBeOrBY=
+golang.org/x/tools/go/packages/packagestest v0.1.1-deprecated/go.mod h1:RVAQXBGNv1ib0J382/DPCRS/BPnsGebyM1Gj5VSDpG8=
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-google.golang.org/api v0.274.0 h1:aYhycS5QQCwxHLwfEHRRLf9yNsfvp1JadKKWBE54RFA=
-google.golang.org/api v0.274.0/go.mod h1:JbAt7mF+XVmWu6xNP8/+CTiGH30ofmCmk9nM8d8fHew=
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-google.golang.org/grpc v1.80.0/go.mod h1:ho/dLnxwi3EDJA4Zghp7k2Ec1+c2jqup0bFkw07bwF4=
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internal/acpserver/agent.go

@@ -23,18 +23,6 @@ import (
 // Version is injected at build time; fallback to "dev".
 var Version = "dev"
 
-// thinkingTagOpen and thinkingTagClose are the XML-style tags that some models
-// (Qwen, DeepSeek) wrap reasoning content in. We parse these to extract
-// reasoning/thinking content and send it as ACP thought updates.
-// Also support <think> format used by some models.
-const (
-	thinkingTagOpen    = "<thinking>"
-	thinkingTagClose   = "</thinking>"
-	shortThinkTagOpen  = "<think>"
-	shortThinkTagClose = "</think>"
-)
-
-// Agent implements the acp.Agent interface, delegating to Kit for LLM
 // execution, tool calls, and session management.
 type Agent struct {
 	conn     *acp.AgentSideConnection
@@ -42,10 +30,6 @@ type Agent struct {
 
 	// toolCallCounter provides unique IDs for tool calls within a turn.
 	toolCallCounter atomic.Int64
-
-	// inThinkingTag tracks whether we're currently inside a <thinking> tag
-	// when parsing streaming content from models that wrap reasoning in XML tags.
-	inThinkingTag bool
 }
 
 // NewAgent creates a new ACP agent backed by Kit.
@@ -77,6 +61,12 @@ func (a *Agent) Authenticate(_ context.Context, _ acp.AuthenticateRequest) (acp.
 	return acp.AuthenticateResponse{}, nil
 }
 
+// Logout handles logout requests. Kit doesn't require auth for local stdio
+// usage, so this is a no-op.
+func (a *Agent) Logout(_ context.Context, _ acp.LogoutRequest) (acp.LogoutResponse, error) {
+	return acp.LogoutResponse{}, nil
+}
+
 // Initialize negotiates capabilities with the ACP client.
 func (a *Agent) Initialize(_ context.Context, params acp.InitializeRequest) (acp.InitializeResponse, error) {
 	log.Debug("acp: initialize", "protocol_version", params.ProtocolVersion)
@@ -144,9 +134,6 @@ func (a *Agent) Prompt(ctx context.Context, params acp.PromptRequest) (acp.Promp
 
 	log.Debug("acp: prompt", "session", sessionID, "prompt_len", len(promptText), "files", len(files))
 
-	// Reset thinking tag state for this new prompt turn
-	a.inThinkingTag = false
-
 	// Create a cancellable context for this prompt turn.
 	promptCtx, cancel := context.WithCancel(ctx)
 	sess.setCancel(cancel)
@@ -196,22 +183,75 @@ func (a *Agent) SetSessionMode(_ context.Context, _ acp.SetSessionModeRequest) (
 	return acp.SetSessionModeResponse{}, nil
 }
 
-// SetSessionModel changes the active model for a session.
-func (a *Agent) SetSessionModel(ctx context.Context, params acp.SetSessionModelRequest) (acp.SetSessionModelResponse, error) {
+// ListSessions returns an empty session list. Kit doesn't persist sessions
+// across restarts in ACP mode, so this is effectively a no-op.
+func (a *Agent) ListSessions(_ context.Context, _ acp.ListSessionsRequest) (acp.ListSessionsResponse, error) {
+	return acp.ListSessionsResponse{
+		Sessions: []acp.SessionInfo{},
+	}, nil
+}
+
+// CloseSession cancels any ongoing work for the session and frees its resources.
+func (a *Agent) CloseSession(_ context.Context, params acp.CloseSessionRequest) (acp.CloseSessionResponse, error) {
 	sessionID := string(params.SessionId)
 	sess, ok := a.registry.get(sessionID)
 	if !ok {
-		return acp.SetSessionModelResponse{}, acp.NewInvalidParams(fmt.Sprintf("session not found: %s", sessionID))
+		return acp.CloseSessionResponse{}, nil
 	}
 
-	modelID := string(params.ModelId)
-	log.Debug("acp: set_session_model", "session", sessionID, "model", modelID)
+	log.Debug("acp: close session", "session", sessionID)
+	sess.cancelPrompt()
+	a.registry.remove(sessionID)
+	return acp.CloseSessionResponse{}, nil
+}
 
-	if err := sess.kit.SetModel(ctx, modelID); err != nil {
-		return acp.SetSessionModelResponse{}, fmt.Errorf("set model: %w", err)
+// ResumeSession is not supported — Kit doesn't persist sessions across
+// restarts in ACP mode. Clients should use NewSession instead.
+func (a *Agent) ResumeSession(_ context.Context, _ acp.ResumeSessionRequest) (acp.ResumeSessionResponse, error) {
+	return acp.ResumeSessionResponse{}, fmt.Errorf("resume session not supported")
+}
+
+// SetSessionConfigOption handles session configuration changes. Currently
+// supports the "model" config option to change the active model for a session.
+func (a *Agent) SetSessionConfigOption(ctx context.Context, params acp.SetSessionConfigOptionRequest) (acp.SetSessionConfigOptionResponse, error) {
+	// Extract session ID and config ID from whichever variant is present.
+	var sessionID string
+	var configID string
+	var value string
+
+	switch {
+	case params.ValueId != nil:
+		sessionID = string(params.ValueId.SessionId)
+		configID = string(params.ValueId.ConfigId)
+		value = string(params.ValueId.Value)
+	case params.Boolean != nil:
+		sessionID = string(params.Boolean.SessionId)
+		configID = string(params.Boolean.ConfigId)
+		// Boolean config options are not used for model selection.
+		log.Debug("acp: set_session_config_option (boolean)", "session", sessionID, "config", configID, "value", params.Boolean.Value)
+		return acp.SetSessionConfigOptionResponse{}, nil
+	default:
+		return acp.SetSessionConfigOptionResponse{}, acp.NewInvalidParams("unsupported config option variant")
 	}
 
-	return acp.SetSessionModelResponse{}, nil
+	sess, ok := a.registry.get(sessionID)
+	if !ok {
+		return acp.SetSessionConfigOptionResponse{}, acp.NewInvalidParams(fmt.Sprintf("session not found: %s", sessionID))
+	}
+
+	log.Debug("acp: set_session_config_option", "session", sessionID, "config", configID, "value", value)
+
+	// Handle known config options.
+	switch configID {
+	case "model":
+		if err := sess.kit.SetModel(ctx, value); err != nil {
+			return acp.SetSessionConfigOptionResponse{}, fmt.Errorf("set model: %w", err)
+		}
+	default:
+		log.Debug("acp: unknown config option", "config", configID)
+	}
+
+	return acp.SetSessionConfigOptionResponse{}, nil
 }
 
 // ---------------------------------------------------------------------------
@@ -230,24 +270,8 @@ func (a *Agent) subscribeEvents(ctx context.Context, k *kit.Kit, sessionID acp.S
 		var update *acp.SessionUpdate
 		switch ev := e.(type) {
 		case kit.MessageUpdateEvent:
-			// Handle models that wrap reasoning in <thinking> tags (Qwen, DeepSeek)
-			// Parse the chunk and separate reasoning from regular text
-			reasoning, text := a.parseThinkingTags(ev.Chunk)
-
-			// Send reasoning update if we have reasoning content
-			if reasoning != "" {
-				u := acp.UpdateAgentThoughtText(reasoning)
-				_ = a.conn.SessionUpdate(ctx, acp.SessionNotification{
-					SessionId: sessionID,
-					Update:    u,
-				})
-			}
-
-			// Send text update if we have text content
-			if text != "" {
-				u := acp.UpdateAgentMessageText(text)
-				update = &u
-			}
+			u := acp.UpdateAgentMessageText(ev.Chunk)
+			update = &u
 
 		case kit.ReasoningDeltaEvent:
 			u := acp.UpdateAgentThoughtText(ev.Delta)
@@ -430,81 +454,6 @@ func extractPromptContent(blocks []acp.ContentBlock) (string, []kit.LLMFilePart)
 	return strings.Join(textParts, "\n"), files
 }
 
-// parseThinkingTags parses a text chunk for <thinking> or  tags and separates
-// reasoning content from regular text. This handles models (Qwen, DeepSeek)
-// that wrap reasoning in XML-style tags instead of using proper reasoning events.
-// Returns (reasoningContent, textContent).
-func (a *Agent) parseThinkingTags(chunk string) (reasoning string, text string) {
-	// Handle empty chunk
-	if chunk == "" {
-		return "", ""
-	}
-
-	// Determine which tag format to use (long or short)
-	openTag := thinkingTagOpen
-	closeTag := thinkingTagClose
-
-	if strings.Contains(chunk, shortThinkTagOpen) || strings.Contains(chunk, shortThinkTagClose) {
-		openTag = shortThinkTagOpen
-		closeTag = shortThinkTagClose
-	} else if !strings.Contains(chunk, thinkingTagOpen) && !strings.Contains(chunk, thinkingTagClose) && !a.inThinkingTag {
-		// No tags at all and not in thinking mode - return as text
-		return "", chunk
-	}
-
-	// Check for opening tag
-	if strings.Contains(chunk, openTag) {
-		parts := strings.SplitN(chunk, openTag, 2)
-
-		// Content before the opening tag is regular text
-		if !a.inThinkingTag && parts[0] != "" {
-			text = parts[0]
-		}
-
-		a.inThinkingTag = true
-
-		// Content after the opening tag is reasoning
-		if len(parts) > 1 {
-			// Check if the same chunk contains the closing tag
-			if strings.Contains(parts[1], closeTag) {
-				innerParts := strings.SplitN(parts[1], closeTag, 2)
-				reasoning = innerParts[0]
-				a.inThinkingTag = false
-
-				// Content after closing tag is regular text
-				if len(innerParts) > 1 && innerParts[1] != "" {
-					text += innerParts[1]
-				}
-			} else if parts[1] != "" {
-				// No closing tag yet, all remaining content is reasoning
-				reasoning = parts[1]
-			}
-		}
-		return reasoning, text
-	}
-
-	// Check for closing tag
-	if strings.Contains(chunk, closeTag) {
-		parts := strings.SplitN(chunk, closeTag, 2)
-		a.inThinkingTag = false
-
-		// Content before closing tag is reasoning
-		reasoning = parts[0]
-
-		// Content after closing tag is regular text
-		if len(parts) > 1 && parts[1] != "" {
-			text = parts[1]
-		}
-		return reasoning, text
-	}
-
-	// No tags found - content goes to current mode
-	if a.inThinkingTag {
-		return chunk, ""
-	}
-	return "", chunk
-}
-
 // isTextMimeType returns true if the MIME type indicates text content.
 func isTextMimeType(mimeType string) bool {
 	return strings.HasPrefix(mimeType, "text/") ||

internal/acpserver/session.go

@@ -7,7 +7,9 @@ import (
 	"sync"
 
 	"github.com/charmbracelet/log"
+	"github.com/spf13/viper"
 
+	"github.com/mark3labs/kit/internal/extbridge"
 	"github.com/mark3labs/kit/internal/extensions"
 	kit "github.com/mark3labs/kit/pkg/kit"
 )
@@ -37,10 +39,21 @@ func newSessionRegistry() *sessionRegistry {
 // given working directory. The Kit-generated session ID is used as the ACP
 // session ID so the mapping is 1:1.
 func (r *sessionRegistry) create(ctx context.Context, cwd string) (*acpSession, error) {
+	// Each ACP session gets its own isolated config store (CLI is left nil) so
+	// per-session SetModel / SetThinkingLevel calls cannot race or bleed across
+	// the sessionRegistry. We seed the relevant root-command flag values from
+	// the process-global store (which cobra populated from flags) so launching
+	// `kit acp -m <model> [--thinking-level ...] [--provider-url ...]` is still
+	// honored; .kit.yml and KIT_* env vars are loaded per session by kit.New.
+	streamOn := true
 	kitInstance, err := kit.New(ctx, &kit.Options{
-		SessionDir: cwd,
-		Quiet:      true,
-		Streaming:  true,
+		SessionDir:     cwd,
+		Quiet:          true,
+		Streaming:      &streamOn,
+		Model:          viper.GetString("model"),
+		ThinkingLevel:  viper.GetString("thinking-level"),
+		ProviderURL:    viper.GetString("provider-url"),
+		ProviderAPIKey: viper.GetString("provider-api-key"),
 	})
 	if err != nil {
 		// Provide actionable guidance for provider auth errors, which are
@@ -60,142 +73,70 @@ func (r *sessionRegistry) create(ctx context.Context, cwd string) (*acpSession,
 
 	// Wire extension context with headless implementations so extensions
 	// work in ACP mode. TUI-dependent features (widgets, prompts, editor)
-	// become no-ops or return cancelled; all data/model/tool APIs work
-	// identically to interactive mode.
+	// become no-ops or return cancelled; all data/model/tool APIs come from
+	// extbridge.BaseContext and work identically to interactive mode.
 	if kitInstance.Extensions().HasExtensions() {
-		kitInstance.Extensions().SetContext(extensions.Context{
-			SessionID:   sessionID,
-			CWD:         cwd,
-			Model:       kitInstance.GetModelString(),
-			Interactive: false,
+		// Use a background context for subagent spawns: the create() ctx is
+		// request-scoped and may be cancelled before extensions spawn anything.
+		ec := extbridge.BaseContext(context.Background(), kitInstance)
 
-			// Output — route through structured logger.
-			Print:      func(text string) { log.Debug("extension: print", "text", text) },
-			PrintInfo:  func(text string) { log.Info("extension: info", "text", text) },
-			PrintError: func(text string) { log.Error("extension: error", "text", text) },
-			PrintBlock: func(opts extensions.PrintBlockOpts) {
-				log.Info("extension: block", "subtitle", opts.Subtitle, "text", opts.Text)
-			},
+		ec.SessionID = sessionID
+		ec.CWD = cwd
+		ec.Model = kitInstance.GetModelString()
+		ec.Interactive = false
 
-			// Message injection — no-ops for now; ACP clients drive prompts.
-			SendMessage:   func(string) {},
-			CancelAndSend: func(string) {},
-			Exit:          func() {},
+		// Output — route through structured logger.
+		ec.Print = func(text string) { log.Debug("extension: print", "text", text) }
+		ec.PrintInfo = func(text string) { log.Info("extension: info", "text", text) }
+		ec.PrintError = func(text string) { log.Error("extension: error", "text", text) }
+		ec.PrintBlock = func(opts extensions.PrintBlockOpts) {
+			log.Info("extension: block", "subtitle", opts.Subtitle, "text", opts.Text)
+		}
 
-			// TUI widgets/chrome — silent no-ops (no TUI in ACP).
-			SetWidget:       func(extensions.WidgetConfig) {},
-			RemoveWidget:    func(string) {},
-			SetHeader:       func(extensions.HeaderFooterConfig) {},
-			RemoveHeader:    func() {},
-			SetFooter:       func(extensions.HeaderFooterConfig) {},
-			RemoveFooter:    func() {},
-			SetEditor:       func(extensions.EditorConfig) {},
-			ResetEditor:     func() {},
-			SetEditorText:   func(string) {},
-			SetUIVisibility: func(extensions.UIVisibility) {},
-			SetStatus:       func(string, string, int) {},
-			RemoveStatus:    func(string) {},
+		// Message injection — no-ops for now; ACP clients drive prompts.
+		ec.SendMessage = func(string) {}
+		ec.CancelAndSend = func(string) {}
+		ec.Exit = func() {}
 
-			// Interactive prompts — return cancelled (no user to prompt).
-			PromptSelect: func(extensions.PromptSelectConfig) extensions.PromptSelectResult {
-				return extensions.PromptSelectResult{Cancelled: true}
-			},
-			PromptConfirm: func(extensions.PromptConfirmConfig) extensions.PromptConfirmResult {
-				return extensions.PromptConfirmResult{Cancelled: true}
-			},
-			PromptInput: func(extensions.PromptInputConfig) extensions.PromptInputResult {
-				return extensions.PromptInputResult{Cancelled: true}
-			},
-			ShowOverlay: func(extensions.OverlayConfig) extensions.OverlayResult {
-				return extensions.OverlayResult{Cancelled: true, Index: -1}
-			},
-			SuspendTUI: func(callback func()) error { callback(); return nil },
+		// TUI widgets/chrome — silent no-ops (no TUI in ACP).
+		ec.SetWidget = func(extensions.WidgetConfig) {}
+		ec.RemoveWidget = func(string) {}
+		ec.SetHeader = func(extensions.HeaderFooterConfig) {}
+		ec.RemoveHeader = func() {}
+		ec.SetFooter = func(extensions.HeaderFooterConfig) {}
+		ec.RemoveFooter = func() {}
+		ec.SetEditor = func(extensions.EditorConfig) {}
+		ec.ResetEditor = func() {}
+		ec.SetEditorText = func(string) {}
+		ec.SetUIVisibility = func(extensions.UIVisibility) {}
+		ec.SetStatus = func(string, string, int) {}
+		ec.RemoveStatus = func(string) {}
 
-			// Data access — delegate to Kit instance.
-			GetContextStats: func() extensions.ContextStats {
-				s := kitInstance.GetContextStats()
-				return extensions.ContextStats{
-					EstimatedTokens: s.EstimatedTokens,
-					ContextLimit:    s.ContextLimit,
-					UsagePercent:    s.UsagePercent,
-					MessageCount:    s.MessageCount,
-				}
-			},
-			GetMessages:    func() []extensions.SessionMessage { return kitInstance.Extensions().GetSessionMessages() },
-			GetSessionPath: func() string { return kitInstance.GetSessionPath() },
-			AppendEntry: func(entryType, data string) (string, error) {
-				return kitInstance.Extensions().AppendEntry(entryType, data)
-			},
-			GetEntries: func(entryType string) []extensions.ExtensionEntry {
-				return kitInstance.Extensions().GetEntries(entryType)
-			},
+		// Interactive prompts — return cancelled (no user to prompt).
+		ec.PromptSelect = func(extensions.PromptSelectConfig) extensions.PromptSelectResult {
+			return extensions.PromptSelectResult{Cancelled: true}
+		}
+		ec.PromptConfirm = func(extensions.PromptConfirmConfig) extensions.PromptConfirmResult {
+			return extensions.PromptConfirmResult{Cancelled: true}
+		}
+		ec.PromptInput = func(extensions.PromptInputConfig) extensions.PromptInputResult {
+			return extensions.PromptInputResult{Cancelled: true}
+		}
+		ec.ShowOverlay = func(extensions.OverlayConfig) extensions.OverlayResult {
+			return extensions.OverlayResult{Cancelled: true, Index: -1}
+		}
+		ec.SuspendTUI = func(callback func()) error { callback(); return nil }
 
-			// Options, model, and tool management.
-			GetOption: func(name string) string { return kitInstance.Extensions().GetOption(name) },
-			SetOption: func(name, value string) { kitInstance.Extensions().SetOption(name, value) },
-			SetModel: func(modelString string) error {
-				previousModel := kitInstance.Extensions().GetContext().Model
-				if err := kitInstance.SetModel(context.Background(), modelString); err != nil {
-					return err
-				}
-				kitInstance.Extensions().UpdateContextModel(modelString)
-				kitInstance.Extensions().EmitModelChange(modelString, previousModel, "extension")
-				return nil
-			},
-			GetAvailableModels: func() []extensions.ModelInfoEntry { return kitInstance.GetAvailableModels() },
-			EmitCustomEvent:    func(name, data string) { kitInstance.Extensions().EmitCustomEvent(name, data) },
-			GetAllTools:        func() []extensions.ToolInfo { return kitInstance.Extensions().GetToolInfos() },
-			SetActiveTools:     func(names []string) { kitInstance.Extensions().SetActiveTools(names) },
+		// Render — fall back to logging.
+		ec.RenderMessage = func(name, content string) {
+			renderer := kitInstance.Extensions().GetMessageRenderer(name)
+			if renderer != nil && renderer.Render != nil {
+				content = renderer.Render(content, 80)
+			}
+			log.Info("extension: message", "renderer", name, "content", content)
+		}
 
-			// LLM completions and subagents.
-			Complete: func(req extensions.CompleteRequest) (extensions.CompleteResponse, error) {
-				return kitInstance.ExecuteCompletion(context.Background(), req)
-			},
-			SpawnSubagent: func(config extensions.SubagentConfig) (*extensions.SubagentHandle, *extensions.SubagentResult, error) {
-				sdkCfg := kit.SubagentConfig{
-					Prompt:       config.Prompt,
-					Model:        config.Model,
-					SystemPrompt: config.SystemPrompt,
-					Timeout:      config.Timeout,
-					NoSession:    config.NoSession,
-				}
-				if config.OnEvent != nil {
-					sdkCfg.OnEvent = func(e kit.Event) {
-						se := sdkEventToSubagentEvent(e)
-						if se.Type != "" {
-							config.OnEvent(se)
-						}
-					}
-				}
-				result, err := kitInstance.Subagent(context.Background(), sdkCfg)
-				if result == nil {
-					return nil, &extensions.SubagentResult{Error: err}, err
-				}
-				extResult := &extensions.SubagentResult{
-					Response:  result.Response,
-					Error:     err,
-					SessionID: result.SessionID,
-					Elapsed:   result.Elapsed,
-				}
-				if result.Usage != nil {
-					extResult.Usage = &extensions.SubagentUsage{
-						InputTokens:  result.Usage.InputTokens,
-						OutputTokens: result.Usage.OutputTokens,
-					}
-				}
-				return nil, extResult, err
-			},
-
-			// Render — fall back to logging.
-			RenderMessage: func(name, content string) {
-				renderer := kitInstance.Extensions().GetMessageRenderer(name)
-				if renderer != nil && renderer.Render != nil {
-					content = renderer.Render(content, 80)
-				}
-				log.Info("extension: message", "renderer", name, "content", content)
-			},
-			ReloadExtensions: func() error { return kitInstance.Extensions().Reload() },
-		})
+		kitInstance.Extensions().SetContext(ec)
 		kitInstance.Extensions().EmitSessionStart()
 	}
 
@@ -232,6 +173,20 @@ func (r *sessionRegistry) closeAll() {
 	}
 }
 
+// remove closes and removes a single session by ID.
+func (r *sessionRegistry) remove(sessionID string) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	sess, ok := r.sessions[sessionID]
+	if !ok {
+		return
+	}
+	if sess.kit != nil {
+		_ = sess.kit.Close()
+	}
+	delete(r.sessions, sessionID)
+}
+
 // cancelPrompt cancels the current prompt for a session, if any.
 func (s *acpSession) cancelPrompt() {
 	s.cancelMu.Lock()
@@ -255,40 +210,3 @@ func (s *acpSession) clearCancel() {
 	defer s.cancelMu.Unlock()
 	s.cancelFn = nil
 }
-
-// sdkEventToSubagentEvent converts an SDK event to an extension SubagentEvent.
-func sdkEventToSubagentEvent(e kit.Event) extensions.SubagentEvent {
-	switch ev := e.(type) {
-	case kit.MessageUpdateEvent:
-		return extensions.SubagentEvent{Type: "text", Content: ev.Chunk}
-	case kit.ReasoningDeltaEvent:
-		return extensions.SubagentEvent{Type: "reasoning", Content: ev.Delta}
-	case kit.ToolCallEvent:
-		return extensions.SubagentEvent{
-			Type: "tool_call", ToolCallID: ev.ToolCallID,
-			ToolName: ev.ToolName, ToolKind: ev.ToolKind, ToolArgs: ev.ToolArgs,
-		}
-	case kit.ToolExecutionStartEvent:
-		return extensions.SubagentEvent{
-			Type: "tool_execution_start", ToolCallID: ev.ToolCallID,
-			ToolName: ev.ToolName, ToolKind: ev.ToolKind,
-		}
-	case kit.ToolExecutionEndEvent:
-		return extensions.SubagentEvent{
-			Type: "tool_execution_end", ToolCallID: ev.ToolCallID,
-			ToolName: ev.ToolName, ToolKind: ev.ToolKind,
-		}
-	case kit.ToolResultEvent:
-		return extensions.SubagentEvent{
-			Type: "tool_result", ToolCallID: ev.ToolCallID,
-			ToolName: ev.ToolName, ToolKind: ev.ToolKind,
-			ToolResult: ev.Result, IsError: ev.IsError,
-		}
-	case kit.TurnStartEvent:
-		return extensions.SubagentEvent{Type: "turn_start"}
-	case kit.TurnEndEvent:
-		return extensions.SubagentEvent{Type: "turn_end"}
-	default:
-		return extensions.SubagentEvent{}
-	}
-}

internal/agent/agent_mcp_test.go

@@ -0,0 +1,302 @@
+package agent
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+	"time"
+
+	"charm.land/fantasy"
+
+	"github.com/mark3labs/kit/internal/config"
+)
+
+// mockModel is a minimal LanguageModel that satisfies the interface
+// without making real API calls. Used to test tool management wiring.
+type mockModel struct{}
+
+func (m *mockModel) Generate(_ context.Context, _ fantasy.Call) (*fantasy.Response, error) {
+	return &fantasy.Response{}, nil
+}
+func (m *mockModel) Stream(_ context.Context, _ fantasy.Call) (fantasy.StreamResponse, error) {
+	return nil, nil
+}
+func (m *mockModel) GenerateObject(_ context.Context, _ fantasy.ObjectCall) (*fantasy.ObjectResponse, error) {
+	return &fantasy.ObjectResponse{}, nil
+}
+func (m *mockModel) StreamObject(_ context.Context, _ fantasy.ObjectCall) (fantasy.ObjectStreamResponse, error) {
+	return nil, nil
+}
+func (m *mockModel) Provider() string { return "mock" }
+func (m *mockModel) Model() string    { return "mock-model" }
+
+// testdataDir returns the absolute path to the tools testdata directory.
+func testdataDir(t *testing.T) string {
+	t.Helper()
+	_, file, _, ok := runtime.Caller(0)
+	if !ok {
+		t.Fatal("cannot determine test file path")
+	}
+	return filepath.Join(filepath.Dir(file), "..", "tools", "testdata")
+}
+
+// echoServerConfig returns an MCPServerConfig for the test echo MCP server.
+func echoServerConfig(t *testing.T) config.MCPServerConfig {
+	t.Helper()
+	script := filepath.Join(testdataDir(t), "echo_server.py")
+	if _, err := os.Stat(script); err != nil {
+		t.Skipf("echo_server.py not found: %v", err)
+	}
+	return config.MCPServerConfig{
+		Command: []string{"python3", script},
+	}
+}
+
+// mockAuthHandler is a minimal MCPAuthHandler for testing that auth handler
+// propagation works without requiring a real OAuth server.
+type mockAuthHandler struct {
+	redirectURI string
+}
+
+func (h *mockAuthHandler) RedirectURI() string { return h.redirectURI }
+func (h *mockAuthHandler) HandleAuth(_ context.Context, _ string, _ string) (string, error) {
+	return "", nil
+}
+
+// newTestAgent creates a minimal Agent with a mock model and no core tools,
+// suitable for testing MCP server management without an API key.
+func newTestAgent() *Agent {
+	model := &mockModel{}
+	a := &Agent{
+		model:        model,
+		coreTools:    nil,
+		extraTools:   nil,
+		maxSteps:     10,
+		systemPrompt: "test",
+		fantasyAgent: fantasy.NewAgent(model),
+	}
+	return a
+}
+
+func TestAgent_AddMCPServer(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping integration test in short mode")
+	}
+
+	a := newTestAgent()
+	defer func() { _ = a.Close() }()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	cfg := echoServerConfig(t)
+
+	// Initially no MCP tools.
+	if a.GetMCPToolCount() != 0 {
+		t.Fatalf("Expected 0 MCP tools initially, got %d", a.GetMCPToolCount())
+	}
+
+	// Add a server.
+	count, err := a.AddMCPServer(ctx, "echo", cfg)
+	if err != nil {
+		t.Fatalf("AddMCPServer failed: %v", err)
+	}
+	if count != 2 {
+		t.Errorf("Expected 2 tools, got %d", count)
+	}
+
+	// Verify tools are in the agent's tool list.
+	if a.GetMCPToolCount() != 2 {
+		t.Errorf("Expected 2 MCP tools, got %d", a.GetMCPToolCount())
+	}
+
+	allTools := a.GetTools()
+	toolNames := make(map[string]bool)
+	for _, tool := range allTools {
+		toolNames[tool.Info().Name] = true
+	}
+	if !toolNames["echo__echo"] {
+		t.Error("Expected tool 'echo__echo' in agent tools")
+	}
+	if !toolNames["echo__greet"] {
+		t.Error("Expected tool 'echo__greet' in agent tools")
+	}
+
+	// Verify loaded server names.
+	names := a.GetLoadedServerNames()
+	found := false
+	for _, n := range names {
+		if n == "echo" {
+			found = true
+		}
+	}
+	if !found {
+		t.Errorf("Expected 'echo' in loaded server names: %v", names)
+	}
+}
+
+func TestAgent_RemoveMCPServer(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping integration test in short mode")
+	}
+
+	a := newTestAgent()
+	defer func() { _ = a.Close() }()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	cfg := echoServerConfig(t)
+
+	// Add then remove.
+	_, err := a.AddMCPServer(ctx, "echo", cfg)
+	if err != nil {
+		t.Fatalf("AddMCPServer failed: %v", err)
+	}
+
+	err = a.RemoveMCPServer("echo")
+	if err != nil {
+		t.Fatalf("RemoveMCPServer failed: %v", err)
+	}
+
+	// Verify tools removed.
+	if a.GetMCPToolCount() != 0 {
+		t.Errorf("Expected 0 MCP tools after removal, got %d", a.GetMCPToolCount())
+	}
+
+	// Verify agent's tool list has no MCP tools.
+	for _, tool := range a.GetTools() {
+		if strings.Contains(tool.Info().Name, "echo__") {
+			t.Errorf("Found leftover tool after removal: %s", tool.Info().Name)
+		}
+	}
+}
+
+func TestAgent_RemoveMCPServer_NoToolManager(t *testing.T) {
+	a := newTestAgent()
+	defer func() { _ = a.Close() }()
+
+	err := a.RemoveMCPServer("nonexistent")
+	if err == nil {
+		t.Fatal("Expected error when no tool manager exists")
+	}
+	if !strings.Contains(err.Error(), "no MCP servers loaded") {
+		t.Errorf("Expected 'no MCP servers loaded' error, got: %v", err)
+	}
+}
+
+func TestAgent_AddMCPServer_CreatesToolManager(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping integration test in short mode")
+	}
+
+	a := newTestAgent()
+	defer func() { _ = a.Close() }()
+
+	// Initially no tool manager.
+	if a.GetMCPToolManager() != nil {
+		t.Fatal("Expected nil tool manager initially")
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	cfg := echoServerConfig(t)
+	_, err := a.AddMCPServer(ctx, "echo", cfg)
+	if err != nil {
+		t.Fatalf("AddMCPServer failed: %v", err)
+	}
+
+	// Tool manager should now exist.
+	if a.GetMCPToolManager() == nil {
+		t.Fatal("Expected tool manager to be created by AddMCPServer")
+	}
+}
+
+func TestAgent_AddRemoveAdd_MCP(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping integration test in short mode")
+	}
+
+	a := newTestAgent()
+	defer func() { _ = a.Close() }()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	cfg := echoServerConfig(t)
+
+	// Add → Remove → Add cycle.
+	_, err := a.AddMCPServer(ctx, "echo", cfg)
+	if err != nil {
+		t.Fatalf("First add failed: %v", err)
+	}
+
+	err = a.RemoveMCPServer("echo")
+	if err != nil {
+		t.Fatalf("Remove failed: %v", err)
+	}
+
+	count, err := a.AddMCPServer(ctx, "echo", cfg)
+	if err != nil {
+		t.Fatalf("Re-add failed: %v", err)
+	}
+	if count != 2 {
+		t.Errorf("Expected 2 tools on re-add, got %d", count)
+	}
+	if a.GetMCPToolCount() != 2 {
+		t.Errorf("Expected 2 MCP tools after re-add, got %d", a.GetMCPToolCount())
+	}
+}
+
+// TestAgent_AddMCPServer_InheritsAuthHandler verifies that AddMCPServer()
+// propagates the agent's authHandler and tokenStoreFactory to a newly created
+// MCPToolManager (fix for issue #3).
+func TestAgent_AddMCPServer_InheritsAuthHandler(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping integration test in short mode")
+	}
+
+	handler := &mockAuthHandler{redirectURI: "http://localhost:9999/oauth/callback"}
+
+	model := &mockModel{}
+	a := &Agent{
+		model:             model,
+		coreTools:         nil,
+		extraTools:        nil,
+		maxSteps:          10,
+		systemPrompt:      "test",
+		fantasyAgent:      fantasy.NewAgent(model),
+		authHandler:       handler,
+		tokenStoreFactory: nil, // nil is fine; we just test authHandler propagation
+	}
+	defer func() { _ = a.Close() }()
+
+	// Initially no tool manager.
+	if a.GetMCPToolManager() != nil {
+		t.Fatal("Expected nil tool manager initially")
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	cfg := echoServerConfig(t)
+	_, err := a.AddMCPServer(ctx, "echo", cfg)
+	if err != nil {
+		t.Fatalf("AddMCPServer failed: %v", err)
+	}
+
+	// Tool manager should now exist and have the auth handler set.
+	tm := a.GetMCPToolManager()
+	if tm == nil {
+		t.Fatal("Expected tool manager to be created by AddMCPServer")
+	}
+
+	// Verify the auth handler was propagated by checking the field directly.
+	if tm.GetAuthHandler() == nil {
+		t.Fatal("Expected auth handler to be propagated to tool manager")
+	}
+}

internal/agent/agent_parallel_tool_args_test.go

@@ -0,0 +1,109 @@
+package agent
+
+import (
+	"context"
+	"sync"
+	"testing"
+
+	"charm.land/fantasy"
+)
+
+// fakeParallelAgent simulates a provider that emits two parallel tool_use
+// blocks in a single step. It invokes the streaming callbacks in the order:
+//
+//	OnToolCall(A) -> OnToolCall(B) -> OnToolResult(A) -> OnToolResult(B)
+//
+// Before the fix in #33 the agent-layer wrapper recorded a single
+// `currentToolArgs` variable that was clobbered by the second OnToolCall, so
+// both OnToolResult callbacks received B's args instead of their own.
+type fakeParallelAgent struct {
+	calls   []fantasy.ToolCallContent
+	results []fantasy.ToolResultContent
+}
+
+func (f *fakeParallelAgent) Generate(_ context.Context, _ fantasy.AgentCall) (*fantasy.AgentResult, error) {
+	return &fantasy.AgentResult{}, nil
+}
+
+func (f *fakeParallelAgent) Stream(_ context.Context, opts fantasy.AgentStreamCall) (*fantasy.AgentResult, error) {
+	for _, tc := range f.calls {
+		if opts.OnToolCall != nil {
+			if err := opts.OnToolCall(tc); err != nil {
+				return nil, err
+			}
+		}
+	}
+	for _, tr := range f.results {
+		if opts.OnToolResult != nil {
+			if err := opts.OnToolResult(tr); err != nil {
+				return nil, err
+			}
+		}
+	}
+	return &fantasy.AgentResult{}, nil
+}
+
+// TestGenerateWithCallbacks_ParallelToolArgs is the regression test for #33.
+// It drives the streaming-callback wiring inside GenerateWithCallbacks with a
+// fake fantasy.Agent that emits two parallel tool calls before either result.
+// Each OnToolResult must receive the args of its own tool call (matched by
+// ToolCallID), not the args of the last OnToolCall in the step.
+func TestGenerateWithCallbacks_ParallelToolArgs(t *testing.T) {
+	t.Parallel()
+
+	argsA := `{"name":"scheduled_jobs"}`
+	argsB := `{"name":"gmail_trigger"}`
+
+	fake := &fakeParallelAgent{
+		calls: []fantasy.ToolCallContent{
+			{ToolCallID: "kit-A", ToolName: "load_skill", Input: argsA},
+			{ToolCallID: "kit-B", ToolName: "load_skill", Input: argsB},
+		},
+		results: []fantasy.ToolResultContent{
+			{ToolCallID: "kit-A", ToolName: "load_skill", Result: fantasy.ToolResultOutputContentText{Text: "ok-A"}},
+			{ToolCallID: "kit-B", ToolName: "load_skill", Result: fantasy.ToolResultOutputContentText{Text: "ok-B"}},
+		},
+	}
+
+	a := &Agent{
+		fantasyAgent:     fake,
+		streamingEnabled: false, // exercise the "hasCallbacks" branch
+	}
+
+	var mu sync.Mutex
+	resultArgs := map[string]string{}
+	executionArgs := map[string]string{} // captured when running == false
+
+	cb := GenerateCallbacks{
+		OnToolExecution: func(id, _, args string, running bool) {
+			if running {
+				return
+			}
+			mu.Lock()
+			defer mu.Unlock()
+			executionArgs[id] = args
+		},
+		OnToolResult: func(id, _, args, _, _ string, _ bool) {
+			mu.Lock()
+			defer mu.Unlock()
+			resultArgs[id] = args
+		},
+	}
+
+	if _, err := a.GenerateWithCallbacks(context.Background(), nil, cb); err != nil {
+		t.Fatalf("GenerateWithCallbacks returned error: %v", err)
+	}
+
+	if got, want := resultArgs["kit-A"], argsA; got != want {
+		t.Errorf("OnToolResult for kit-A: args = %q, want %q", got, want)
+	}
+	if got, want := resultArgs["kit-B"], argsB; got != want {
+		t.Errorf("OnToolResult for kit-B: args = %q, want %q", got, want)
+	}
+	if got, want := executionArgs["kit-A"], argsA; got != want {
+		t.Errorf("OnToolExecution(finish) for kit-A: args = %q, want %q", got, want)
+	}
+	if got, want := executionArgs["kit-B"], argsB; got != want {
+		t.Errorf("OnToolExecution(finish) for kit-B: args = %q, want %q", got, want)
+	}
+}

internal/agent/factory.go

@@ -38,13 +38,26 @@ type AgentCreationOptions struct {
 	DebugLogger tools.DebugLogger // Optional debug logger
 	// AuthHandler handles OAuth authorization for remote MCP servers
 	AuthHandler tools.MCPAuthHandler
+	// TokenStoreFactory, if non-nil, creates a custom token store for each
+	// remote MCP server's OAuth tokens. When nil, the default file-based
+	// token store is used.
+	TokenStoreFactory tools.TokenStoreFactory
 	// CoreTools overrides the default core tool set. If empty, core.AllTools()
 	// is used.
 	CoreTools []fantasy.AgentTool
+	// DisableCoreTools, when true, prevents loading any core tools.
+	// If both DisableCoreTools is true and CoreTools is empty, the agent
+	// will have no tools (useful for simple chat completions).
+	DisableCoreTools bool
 	// ToolWrapper wraps the combined tool list before agent creation.
 	ToolWrapper func([]fantasy.AgentTool) []fantasy.AgentTool
 	// ExtraTools are additional tools to include (e.g. from extensions).
 	ExtraTools []fantasy.AgentTool
+	// OnMCPServerLoaded, if non-nil, is called when each MCP server finishes
+	// loading (successfully or with error). Called from the background goroutine.
+	OnMCPServerLoaded func(serverName string, toolCount int, err error)
+	// MCPTaskConfig configures task-augmented tools/call execution.
+	MCPTaskConfig tools.MCPTaskConfig
 }
 
 // CreateAgent creates an agent with optional spinner for Ollama models.
@@ -52,16 +65,20 @@ type AgentCreationOptions struct {
 // Returns the created agent or an error if creation fails.
 func CreateAgent(ctx context.Context, opts *AgentCreationOptions) (*Agent, error) {
 	agentConfig := &AgentConfig{
-		ModelConfig:      opts.ModelConfig,
-		MCPConfig:        opts.MCPConfig,
-		SystemPrompt:     opts.SystemPrompt,
-		MaxSteps:         opts.MaxSteps,
-		StreamingEnabled: opts.StreamingEnabled,
-		DebugLogger:      opts.DebugLogger,
-		AuthHandler:      opts.AuthHandler,
-		CoreTools:        opts.CoreTools,
-		ToolWrapper:      opts.ToolWrapper,
-		ExtraTools:       opts.ExtraTools,
+		ModelConfig:       opts.ModelConfig,
+		MCPConfig:         opts.MCPConfig,
+		SystemPrompt:      opts.SystemPrompt,
+		MaxSteps:          opts.MaxSteps,
+		StreamingEnabled:  opts.StreamingEnabled,
+		DebugLogger:       opts.DebugLogger,
+		AuthHandler:       opts.AuthHandler,
+		TokenStoreFactory: opts.TokenStoreFactory,
+		CoreTools:         opts.CoreTools,
+		DisableCoreTools:  opts.DisableCoreTools,
+		ToolWrapper:       opts.ToolWrapper,
+		ExtraTools:        opts.ExtraTools,
+		OnMCPServerLoaded: opts.OnMCPServerLoaded,
+		MCPTaskConfig:     opts.MCPTaskConfig,
 	}
 
 	var agent *Agent

internal/agent/mcp_adapter.go

@@ -0,0 +1,88 @@
+package agent
+
+import (
+	"context"
+	"fmt"
+
+	"charm.land/fantasy"
+
+	"github.com/mark3labs/kit/internal/tools"
+)
+
+// mcpExecutor is the subset of *tools.MCPToolManager that the adapter
+// actually uses. Extracted as an interface so the adapter is unit-testable
+// without constructing a full manager + connection pool.
+type mcpExecutor interface {
+	ExecuteTool(ctx context.Context, prefixedName, inputJSON string) (*tools.MCPToolResult, error)
+}
+
+// mcpAgentTool adapts an tools.MCPTool to the fantasy.AgentTool interface.
+// This keeps the fantasy dependency confined to the agent layer — the tools
+// package is a pure MCP client library with no LLM framework dependency.
+type mcpAgentTool struct {
+	tool            tools.MCPTool
+	exec            mcpExecutor
+	providerOptions fantasy.ProviderOptions
+}
+
+// Info returns the fantasy tool info including name, description, and parameter schema.
+func (t *mcpAgentTool) Info() fantasy.ToolInfo {
+	return fantasy.ToolInfo{
+		Name:        t.tool.Name,
+		Description: t.tool.Description,
+		Parameters:  t.tool.Parameters,
+		Required:    t.tool.Required,
+	}
+}
+
+// Run executes the MCP tool by delegating to the MCPToolManager.
+//
+// MCP-side failures (JSON-RPC protocol errors, transport failures, schema
+// validation rejections from the server) are surfaced to the model as soft
+// tool errors rather than escalated to a critical agent error. This matches
+// the contract that native Kit tools follow via kit.ErrorResult(...) and
+// lets the model self-correct (e.g. retry with a fixed argument shape) or
+// give up gracefully rather than aborting the turn mid-run.
+//
+// Context cancellation is the one exception: if the caller cancelled the
+// context the turn was aborted intentionally, so we propagate the ctx error
+// to let the agent loop unwind cleanly.
+func (t *mcpAgentTool) Run(ctx context.Context, call fantasy.ToolCall) (fantasy.ToolResponse, error) {
+	result, err := t.exec.ExecuteTool(ctx, t.tool.Name, call.Input)
+	if err != nil {
+		if ctxErr := ctx.Err(); ctxErr != nil {
+			return fantasy.ToolResponse{}, ctxErr
+		}
+		return fantasy.NewTextErrorResponse(
+			fmt.Sprintf("MCP tool %q failed: %s", t.tool.Name, err.Error()),
+		), nil
+	}
+
+	if result.IsError {
+		return fantasy.NewTextErrorResponse(result.Content), nil
+	}
+	return fantasy.NewTextResponse(result.Content), nil
+}
+
+// ProviderOptions returns provider-specific options for this tool.
+func (t *mcpAgentTool) ProviderOptions() fantasy.ProviderOptions {
+	return t.providerOptions
+}
+
+// SetProviderOptions sets provider-specific options for this tool.
+func (t *mcpAgentTool) SetProviderOptions(opts fantasy.ProviderOptions) {
+	t.providerOptions = opts
+}
+
+// mcpToolsToAgentTools converts a slice of MCPTool to fantasy.AgentTool
+// implementations that route execution through the MCPToolManager.
+func mcpToolsToAgentTools(mcpTools []tools.MCPTool, manager *tools.MCPToolManager) []fantasy.AgentTool {
+	agentTools := make([]fantasy.AgentTool, len(mcpTools))
+	for i, t := range mcpTools {
+		agentTools[i] = &mcpAgentTool{
+			tool: t,
+			exec: manager,
+		}
+	}
+	return agentTools
+}

internal/agent/mcp_adapter_test.go

@@ -0,0 +1,158 @@
+package agent
+
+import (
+	"context"
+	"errors"
+	"strings"
+	"testing"
+	"time"
+
+	"charm.land/fantasy"
+
+	"github.com/mark3labs/kit/internal/tools"
+)
+
+// stubExecutor lets each test script the (result, err) pair returned by
+// ExecuteTool. The adapter holds an mcpExecutor interface, so this is the
+// only seam the tests need.
+type stubExecutor struct {
+	result *tools.MCPToolResult
+	err    error
+	// called records the last invocation for assertion.
+	called bool
+	name   string
+	input  string
+}
+
+func (s *stubExecutor) ExecuteTool(_ context.Context, prefixedName, inputJSON string) (*tools.MCPToolResult, error) {
+	s.called = true
+	s.name = prefixedName
+	s.input = inputJSON
+	return s.result, s.err
+}
+
+func newMCPAgentTool(exec mcpExecutor, name string) *mcpAgentTool {
+	return &mcpAgentTool{
+		tool: tools.MCPTool{Name: name},
+		exec: exec,
+	}
+}
+
+// Manager-side Go errors (JSON-RPC protocol errors, transport failures,
+// schema validation rejections from the MCP server) must be surfaced to
+// the model as soft tool errors so the agent loop can keep going. Aborting
+// the turn would discard all prior tool results — see issue #N.
+func TestMCPAgentTool_RPCErrorBecomesSoftError(t *testing.T) {
+	exec := &stubExecutor{
+		err: errors.New("MCP error -32602: Invalid params: missing field \"task\""),
+	}
+	tool := newMCPAgentTool(exec, "pubmed__search")
+
+	resp, err := tool.Run(context.Background(), fantasy.ToolCall{
+		ID:    "call-1",
+		Name:  "pubmed__search",
+		Input: `{"query":"foo"}`,
+	})
+
+	if err != nil {
+		t.Fatalf("expected nil error (soft), got %v", err)
+	}
+	if !resp.IsError {
+		t.Fatalf("expected IsError=true, got false")
+	}
+	if !strings.Contains(resp.Content, "pubmed__search") {
+		t.Errorf("expected tool name in error content, got %q", resp.Content)
+	}
+	if !strings.Contains(resp.Content, "-32602") {
+		t.Errorf("expected underlying error text in content, got %q", resp.Content)
+	}
+}
+
+// Context cancellation is the one error that must remain critical: it
+// means the caller intentionally aborted, and the agent loop needs to
+// unwind cleanly rather than burning more steps.
+func TestMCPAgentTool_CtxCancelStaysCritical(t *testing.T) {
+	exec := &stubExecutor{
+		// Real managers typically return ctx.Err() (or a wrapper) when the
+		// context is cancelled mid-call.
+		err: context.Canceled,
+	}
+	tool := newMCPAgentTool(exec, "slow__tool")
+
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	resp, err := tool.Run(ctx, fantasy.ToolCall{Name: "slow__tool"})
+
+	if !errors.Is(err, context.Canceled) {
+		t.Fatalf("expected context.Canceled, got %v", err)
+	}
+	if resp.IsError || resp.Content != "" {
+		t.Errorf("expected empty response on critical error, got IsError=%v Content=%q", resp.IsError, resp.Content)
+	}
+}
+
+// Deadline-exceeded behaves the same as cancellation: ctx.Err() is
+// non-nil, so the adapter must propagate the critical error rather than
+// converting the executor's error into a soft response.
+func TestMCPAgentTool_CtxDeadlineStaysCritical(t *testing.T) {
+	exec := &stubExecutor{err: context.DeadlineExceeded}
+	tool := newMCPAgentTool(exec, "slow__tool")
+
+	ctx, cancel := context.WithDeadline(context.Background(), time.Now().Add(-time.Second))
+	defer cancel()
+
+	resp, err := tool.Run(ctx, fantasy.ToolCall{Name: "slow__tool"})
+	if !errors.Is(err, context.DeadlineExceeded) {
+		t.Fatalf("expected context.DeadlineExceeded, got %v", err)
+	}
+	if resp.IsError || resp.Content != "" {
+		t.Errorf("expected empty response on critical error, got IsError=%v Content=%q", resp.IsError, resp.Content)
+	}
+}
+
+// Server-side soft errors (CallToolResult{ isError: true }) must continue
+// to flow through as soft errors — this was the existing behavior and
+// must not regress.
+func TestMCPAgentTool_ServerIsErrorRemainsSoftError(t *testing.T) {
+	exec := &stubExecutor{
+		result: &tools.MCPToolResult{
+			IsError: true,
+			Content: "search service is rate limited; try again in 30s",
+		},
+	}
+	tool := newMCPAgentTool(exec, "pubmed__search")
+
+	resp, err := tool.Run(context.Background(), fantasy.ToolCall{Name: "pubmed__search"})
+	if err != nil {
+		t.Fatalf("expected nil error, got %v", err)
+	}
+	if !resp.IsError {
+		t.Fatalf("expected IsError=true, got false")
+	}
+	if resp.Content != "search service is rate limited; try again in 30s" {
+		t.Errorf("expected pass-through content, got %q", resp.Content)
+	}
+}
+
+// Happy path: ordinary successful tool result is passed through unchanged.
+func TestMCPAgentTool_SuccessIsPassthrough(t *testing.T) {
+	exec := &stubExecutor{
+		result: &tools.MCPToolResult{
+			IsError: false,
+			Content: `{"hits":3}`,
+		},
+	}
+	tool := newMCPAgentTool(exec, "pubmed__search")
+
+	resp, err := tool.Run(context.Background(), fantasy.ToolCall{Name: "pubmed__search"})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if resp.IsError {
+		t.Fatalf("expected IsError=false")
+	}
+	if resp.Content != `{"hits":3}` {
+		t.Errorf("expected pass-through content, got %q", resp.Content)
+	}
+}

internal/app/app.go

@@ -13,6 +13,7 @@ import (
 	"charm.land/fantasy"
 
 	"github.com/mark3labs/kit/internal/extensions"
+	"github.com/mark3labs/kit/internal/message"
 	"github.com/mark3labs/kit/internal/session"
 	kit "github.com/mark3labs/kit/pkg/kit"
 )
@@ -70,14 +71,24 @@ type App struct {
 	rootCtx    context.Context
 	rootCancel context.CancelFunc
 
-	// widgetUpdatePending is set to true when a WidgetUpdateEvent has been
-	// sent to the TUI but not yet consumed by its event loop. While the flag
-	// is set, subsequent NotifyWidgetUpdate calls are coalesced (dropped) to
-	// prevent fast extension tickers from flooding the BubbleTea mailbox with
-	// redundant re-render triggers. The flag is cleared after a short debounce
-	// (~1 frame) so new updates are always let through once the TUI has had a
-	// chance to process the pending event.
-	widgetUpdatePending atomic.Bool
+	// widgetUpdatePending is set to true while a WidgetUpdateEvent burst is
+	// being coalesced. The leading edge fires immediately; subsequent calls
+	// within the debounce window set widgetUpdateTrailing so a final event
+	// is delivered with the latest runner state at the end of the window.
+	// Without the trailing send, a rapid SetWidget→RemoveWidget pair (e.g.
+	// SubagentEnd pushing a final frame then removing the widget) would let
+	// the second call get silently dropped, leaving the TUI's layout stuck
+	// on the pre-removal widget height — visible as empty rows below the
+	// status bar after the widget disappears.
+	widgetUpdatePending  atomic.Bool
+	widgetUpdateTrailing atomic.Bool
+
+	// steerDrainFn is the test seam used by releaseBusyAfterCompact to pull
+	// any steer messages that arrived during compaction. In production it is
+	// nil and the helper falls back to a.opts.Kit.DrainSteer(); tests that
+	// need to exercise the steer-drain path without standing up a full
+	// *kit.Kit can set this field directly to inject fake items.
+	steerDrainFn func() []queueItem
 }
 
 // New creates a new App with the provided options and pre-loaded messages.
@@ -333,6 +344,90 @@ func (a *App) SwitchTreeSession(ts *session.TreeManager) {
 	}
 }
 
+// PopLastUserMessage truncates the tree session back to the parent of the
+// most recent user message on the current branch, syncs the in-memory
+// message store, and returns the user prompt text plus any image file
+// parts so the caller can resubmit via Run/RunWithFiles.
+//
+// This is the building block for /retry: the user message and any orphaned
+// assistant/tool entries produced by a failed turn become unreachable on
+// the current branch (they remain in the session file under a different
+// leaf) and are excluded from the next LLM context.
+//
+// Returns an error when:
+//   - the agent is currently working (busy)
+//   - the app has been closed
+//   - no tree session is active (sessions disabled via --no-session)
+//   - no user message exists on the current branch
+//
+// Satisfies ui.AppController.
+func (a *App) PopLastUserMessage() (string, []kit.LLMFilePart, error) {
+	a.mu.Lock()
+	if a.closed {
+		a.mu.Unlock()
+		return "", nil, fmt.Errorf("app is closed")
+	}
+	if a.busy {
+		a.mu.Unlock()
+		return "", nil, fmt.Errorf("cannot retry while the agent is working")
+	}
+	a.mu.Unlock()
+
+	ts := a.opts.TreeSession
+	if ts == nil {
+		return "", nil, fmt.Errorf("no tree session active; /retry requires a session")
+	}
+
+	// Walk the current branch backwards to find the most recent user message.
+	branch := ts.GetBranch("")
+	var target *session.MessageEntry
+	for i := len(branch) - 1; i >= 0; i-- {
+		me, ok := branch[i].(*session.MessageEntry)
+		if !ok {
+			continue
+		}
+		if me.Role == string(message.RoleUser) {
+			target = me
+			break
+		}
+	}
+	if target == nil {
+		return "", nil, fmt.Errorf("no user message to retry")
+	}
+
+	// Extract the prompt text and any image parts from the target entry.
+	msg, err := target.ToMessage()
+	if err != nil {
+		return "", nil, fmt.Errorf("decode user message: %w", err)
+	}
+	prompt := msg.Content()
+	var files []kit.LLMFilePart
+	for _, part := range msg.Parts {
+		if ic, ok := part.(message.ImageContent); ok {
+			files = append(files, kit.LLMFilePart{
+				Data:      ic.Data,
+				MediaType: ic.MediaType,
+			})
+		}
+	}
+
+	// Move the leaf to the parent of the user message. The failed turn's
+	// entries (user message + any partial assistant/tool entries) are still
+	// in the tree file but no longer on the active branch, so they will not
+	// be re-sent to the LLM. runTurn() will append a fresh user message on
+	// the next call.
+	if err := ts.Branch(target.ParentID); err != nil {
+		return "", nil, fmt.Errorf("branch to parent: %w", err)
+	}
+
+	// Sync the in-memory store with the new branch position so subsequent
+	// reads (and ReloadMessagesFromTree() consumers) see the truncated view.
+	a.store.Clear()
+	a.store.Replace(ts.GetLLMMessages())
+
+	return prompt, files, nil
+}
+
 // AddContextMessage adds a user-role message to the conversation history
 // without triggering an LLM response. Used by the ! shell command prefix
 // to inject command output into context so the LLM can reference it in
@@ -356,6 +451,10 @@ func (a *App) AddContextMessage(text string) {
 // tea.Program. customInstructions is optional text appended to the summary
 // prompt (e.g. "Focus on the API design decisions").
 //
+// Any prompts queued via Run/RunWithFiles or steering messages injected via
+// Steer/SteerWithFiles while compaction is running are flushed automatically
+// once compaction completes (see releaseBusyAfterCompact).
+//
 // Satisfies ui.AppController.
 func (a *App) CompactConversation(customInstructions string) error {
 	a.mu.Lock()
@@ -377,11 +476,7 @@ func (a *App) CompactConversation(customInstructions string) error {
 
 	go func() {
 		defer a.wg.Done()
-		defer func() {
-			a.mu.Lock()
-			a.busy = false
-			a.mu.Unlock()
-		}()
+		defer a.releaseBusyAfterCompact()
 
 		// Subscribe to SDK events for streaming compaction summary to the TUI.
 		sendFn := func(msg tea.Msg) {
@@ -420,6 +515,9 @@ func (a *App) CompactConversation(customInstructions string) error {
 // CompactAsync is like CompactConversation but calls onComplete/onError
 // callbacks instead of sending TUI events. Used by the extension API's
 // ctx.Compact() which needs callback-based notification.
+//
+// Like CompactConversation, any prompts/steer messages received during
+// compaction are flushed automatically once compaction finishes.
 func (a *App) CompactAsync(customInstructions string, onComplete func(), onError func(string)) error {
 	a.mu.Lock()
 	if a.closed {
@@ -440,11 +538,7 @@ func (a *App) CompactAsync(customInstructions string, onComplete func(), onError
 
 	go func() {
 		defer a.wg.Done()
-		defer func() {
-			a.mu.Lock()
-			a.busy = false
-			a.mu.Unlock()
-		}()
+		defer a.releaseBusyAfterCompact()
 
 		// Subscribe to SDK events for streaming compaction summary to the TUI.
 		sendFn := func(msg tea.Msg) {
@@ -489,6 +583,81 @@ func (a *App) CompactAsync(customInstructions string, onComplete func(), onError
 	return nil
 }
 
+// releaseBusyAfterCompact is the deferred tail that runs at the end of every
+// compaction goroutine (success, error, or panic-after-recover paths). It
+// flips a.busy back to false, but before doing so it checks whether any
+// prompts piled up while compaction was running:
+//
+//   - Run/RunWithFiles append to a.queue when a.busy is set.
+//   - Steer/SteerWithFiles deposit messages into the SDK steer channel via
+//     Kit.InjectSteerWithFiles when a.busy is set.
+//
+// Without this hand-off the queue would sit idle until the user submits
+// another prompt — see issue #27. If we find anything pending we keep busy
+// set, splice the steer messages to the front of the queue, and start a
+// fresh drainQueue goroutine to deliver them as a single batched turn.
+func (a *App) releaseBusyAfterCompact() {
+	// Pull steer messages outside the app mutex; DrainSteer takes its own
+	// internal lock and we don't want to nest the two. The test seam
+	// (a.steerDrainFn) takes precedence so unit tests can inject fake
+	// steer items without a real *kit.Kit.
+	var steerItems []queueItem
+	switch {
+	case a.steerDrainFn != nil:
+		steerItems = a.steerDrainFn()
+	case a.opts.Kit != nil:
+		if leftover := a.opts.Kit.DrainSteer(); len(leftover) > 0 {
+			steerItems = make([]queueItem, len(leftover))
+			for i, sm := range leftover {
+				steerItems[i] = queueItem{Prompt: sm.Text, Files: sm.Files}
+			}
+		}
+	}
+
+	a.mu.Lock()
+	// If the app was closed while compaction was running, drop everything
+	// and just clear busy. Run/Steer would have rejected new items already
+	// after Close(), but this guards against in-flight items that slipped
+	// in just before closed was set.
+	if a.closed {
+		a.queue = a.queue[:0]
+		a.busy = false
+		a.mu.Unlock()
+		return
+	}
+
+	// Combine steer-channel items (front) with the in-memory queue (back).
+	// Steer messages are placed first so they retain their "act now"
+	// semantics relative to ordinary queued prompts that arrived later.
+	pending := append(steerItems, a.queue...)
+	a.queue = a.queue[:0]
+
+	if len(pending) == 0 {
+		a.busy = false
+		a.mu.Unlock()
+		return
+	}
+
+	// Hand off to drainQueue: it will pick up the first item directly and
+	// scoop the rest from a.queue on its first iteration.
+	first := pending[0]
+	if len(pending) > 1 {
+		a.queue = append(a.queue, pending[1:]...)
+	}
+	// Stay busy across the goroutine swap.
+	a.wg.Add(1)
+	a.mu.Unlock()
+
+	// Notify the UI that steer-channel messages were consumed so the
+	// steering badge can clear; ordinary queued prompts will be reflected
+	// by the QueueUpdatedEvent that drainQueue emits as it picks them up.
+	if len(steerItems) > 0 {
+		a.sendEvent(SteerConsumedEvent{})
+	}
+
+	go a.drainQueue(first)
+}
+
 // --------------------------------------------------------------------------
 // Non-interactive execution
 // --------------------------------------------------------------------------
@@ -497,6 +666,12 @@ func (a *App) CompactAsync(customInstructions string, onComplete func(), onError
 // response text to stdout. No intermediate events are emitted. Blocks until
 // the step completes or ctx is cancelled.
 func (a *App) RunOnce(ctx context.Context, prompt string) error {
+	return a.RunOnceWithFiles(ctx, prompt, nil)
+}
+
+// RunOnceWithFiles executes a single agent step synchronously with optional
+// multimodal file attachments. Prints the response to stdout and returns.
+func (a *App) RunOnceWithFiles(ctx context.Context, prompt string, files []kit.LLMFilePart) error {
 	stepCtx, cancel := context.WithCancel(ctx)
 	defer cancel()
 
@@ -504,7 +679,7 @@ func (a *App) RunOnce(ctx context.Context, prompt string) error {
 	a.cancelStep = cancel
 	a.mu.Unlock()
 
-	result, err := a.executeStep(stepCtx, prompt, nil, nil)
+	result, err := a.executeStep(stepCtx, prompt, nil, files)
 	if err != nil {
 		return err
 	}
@@ -519,6 +694,12 @@ func (a *App) RunOnce(ctx context.Context, prompt string) error {
 // full TurnResult without printing anything. This is used by --json mode to
 // capture structured output for serialization.
 func (a *App) RunOnceResult(ctx context.Context, prompt string) (*kit.TurnResult, error) {
+	return a.RunOnceResultWithFiles(ctx, prompt, nil)
+}
+
+// RunOnceResultWithFiles executes a single agent step synchronously with
+// optional multimodal file attachments and returns the full TurnResult.
+func (a *App) RunOnceResultWithFiles(ctx context.Context, prompt string, files []kit.LLMFilePart) (*kit.TurnResult, error) {
 	stepCtx, cancel := context.WithCancel(ctx)
 	defer cancel()
 
@@ -526,7 +707,7 @@ func (a *App) RunOnceResult(ctx context.Context, prompt string) (*kit.TurnResult
 	a.cancelStep = cancel
 	a.mu.Unlock()
 
-	return a.executeStep(stepCtx, prompt, nil, nil)
+	return a.executeStep(stepCtx, prompt, nil, files)
 }
 
 // RunOnceWithDisplay executes a single agent step synchronously, sending
@@ -540,6 +721,12 @@ func (a *App) RunOnceResult(ctx context.Context, prompt string) (*kit.TurnResult
 //
 // Blocks until the step completes or ctx is cancelled.
 func (a *App) RunOnceWithDisplay(ctx context.Context, prompt string, eventFn func(tea.Msg)) error {
+	return a.RunOnceWithDisplayAndFiles(ctx, prompt, eventFn, nil)
+}
+
+// RunOnceWithDisplayAndFiles executes a single agent step synchronously with
+// optional multimodal file attachments, sending intermediate display events.
+func (a *App) RunOnceWithDisplayAndFiles(ctx context.Context, prompt string, eventFn func(tea.Msg), files []kit.LLMFilePart) error {
 	stepCtx, cancel := context.WithCancel(ctx)
 	defer cancel()
 
@@ -547,7 +734,7 @@ func (a *App) RunOnceWithDisplay(ctx context.Context, prompt string, eventFn fun
 	a.cancelStep = cancel
 	a.mu.Unlock()
 
-	result, err := a.executeStep(stepCtx, prompt, eventFn, nil)
+	result, err := a.executeStep(stepCtx, prompt, eventFn, files)
 	if err != nil {
 		return err
 	}
@@ -870,6 +1057,12 @@ func (a *App) subscribeSDKEvents(sendFn func(tea.Msg), stepUsageSeen *atomic.Boo
 		switch ev := e.(type) {
 		case kit.ToolCallEvent:
 			sendFn(ToolCallStartedEvent{ToolCallID: ev.ToolCallID, ToolName: ev.ToolName, ToolArgs: ev.ToolArgs})
+		case kit.ToolCallStartEvent:
+			sendFn(ToolCallInputStartEvent{ToolCallID: ev.ToolCallID, ToolName: ev.ToolName, ToolKind: ev.ToolKind})
+		case kit.ToolCallDeltaEvent:
+			sendFn(ToolCallInputDeltaEvent{ToolCallID: ev.ToolCallID, Delta: ev.Delta})
+		case kit.ToolCallEndEvent:
+			sendFn(ToolCallInputEndEvent{ToolCallID: ev.ToolCallID})
 		case kit.ToolExecutionStartEvent:
 			sendFn(ToolExecutionEvent{ToolCallID: ev.ToolCallID, ToolName: ev.ToolName, ToolArgs: ev.ToolArgs, IsStarting: true})
 		case kit.ToolExecutionEndEvent:
@@ -899,7 +1092,23 @@ func (a *App) subscribeSDKEvents(sendFn func(tea.Msg), stepUsageSeen *atomic.Boo
 		case kit.SteerConsumedEvent:
 			sendFn(SteerConsumedEvent{})
 		case kit.StepUsageEvent:
-			a.recordStepUsage(ev, stepUsageSeen)
+			a.recordStepUsage(ev, stepUsageSeen, sendFn)
+		case kit.PasswordPromptEvent:
+			// Convert SDK PasswordPromptEvent to app PasswordPromptEvent
+			// The TUI will handle this and send the response back
+			responseCh := make(chan PasswordPromptResponse, 1)
+			sendFn(PasswordPromptEvent{
+				Prompt:     ev.Prompt,
+				ResponseCh: responseCh,
+			})
+			// Wait for TUI response and forward to SDK
+			resp := <-responseCh
+			ev.ResponseCh <- kit.PasswordPromptResponse{
+				Password:  resp.Password,
+				Cancelled: resp.Cancelled,
+			}
+		case kit.TurnEndEvent:
+			a.handleTurnEnd(ev, sendFn)
 		}
 	}))
 
@@ -910,6 +1119,64 @@ func (a *App) subscribeSDKEvents(sendFn func(tea.Msg), stepUsageSeen *atomic.Boo
 	}
 }
 
+// handleTurnEnd inspects a turn's final StopReason and surfaces actionable
+// feedback to the user when the turn ended in a state they can act on.
+//
+// Today the only surfaced case is FinishReasonLength — the model hit its
+// configured max_output_tokens budget and the reply was truncated. Without
+// this banner the TUI used to swallow the truncation silently, leading to
+// "ghost" cut-offs with no indication of why.
+//
+// Separated from subscribeSDKEvents so tests can exercise it directly via a
+// stubbed sendFn without standing up a full Kit.
+func (a *App) handleTurnEnd(ev kit.TurnEndEvent, sendFn func(tea.Msg)) {
+	if sendFn == nil {
+		return
+	}
+	if ev.StopReason != kit.FinishReasonLength {
+		return
+	}
+	sendFn(ExtensionPrintEvent{
+		Level: "info",
+		Text:  a.formatMaxTokensTruncatedMessage(),
+	})
+}
+
+// formatMaxTokensTruncatedMessage builds the user-facing explanation for a
+// truncated turn. It reports the active max_output_tokens budget and, when
+// known, the model's catalog output ceiling so the user can judge how much
+// headroom is available.
+func (a *App) formatMaxTokensTruncatedMessage() string {
+	k := a.opts.Kit
+	if k == nil {
+		// Extremely early / test-stub case: still emit a useful generic hint.
+		return "⚠ Response truncated: the model hit the configured max_output_tokens limit. " +
+			"Raise it with --max-tokens N, KIT_MAX_TOKENS=N, or per-model " +
+			"modelSettings[provider/model].maxTokens in config."
+	}
+	current := k.MaxTokens()
+	ceiling := k.MaxOutputLimit()
+	model := k.GetModelString()
+
+	msg := "⚠ Response truncated: "
+	if model != "" {
+		msg += fmt.Sprintf("%s hit the configured max_output_tokens limit", model)
+	} else {
+		msg += "the model hit the configured max_output_tokens limit"
+	}
+	if current > 0 {
+		msg += fmt.Sprintf(" (%d)", current)
+	}
+	msg += "."
+	if ceiling > 0 && current > 0 && ceiling > current {
+		msg += fmt.Sprintf(" This model supports up to %d output tokens.", ceiling)
+	}
+	msg += "\n\nRaise it with --max-tokens N, KIT_MAX_TOKENS=N, " +
+		"or per-model modelSettings[provider/model].maxTokens in your config. " +
+		"Re-run the last prompt after raising it to get the full response."
+	return msg
+}
+
 // QuitFromExtension triggers a graceful shutdown. In interactive mode it
 // sends a tea.QuitMsg to the program so the TUI exits cleanly. In
 // non-interactive mode it cancels the root context, stopping any in-flight
@@ -930,7 +1197,8 @@ func (a *App) QuitFromExtension() {
 // controls styling: "" for plain text, "info" for a system message block,
 // "error" for an error block. In interactive mode it sends an
 // ExtensionPrintEvent through the program so the TUI can render it with the
-// appropriate renderer. In non-interactive mode it falls back to stdout.
+// appropriate renderer. In non-interactive mode it falls back to stderr with
+// a level prefix so errors are distinguishable from plain output.
 func (a *App) PrintFromExtension(level, text string) {
 	a.mu.Lock()
 	prog := a.program
@@ -939,8 +1207,16 @@ func (a *App) PrintFromExtension(level, text string) {
 		prog.Send(ExtensionPrintEvent{Text: text, Level: level})
 		return
 	}
-	// Non-interactive fallback: write directly to stdout.
-	fmt.Println(text)
+	// Non-interactive fallback: write to stderr with a level prefix so that
+	// errors and info messages are distinguishable from plain output.
+	switch level {
+	case "error":
+		fmt.Fprintf(os.Stderr, "[ERROR] %s\n", text)
+	case "info":
+		fmt.Fprintf(os.Stderr, "[INFO] %s\n", text)
+	default:
+		fmt.Println(text)
+	}
 }
 
 // SetEditorTextFromExtension sends an EditorTextSetEvent to the TUI to
@@ -969,31 +1245,87 @@ func (a *App) NotifyModelChanged(provider, model string) {
 // extension widgets. Called from the extension context's SetWidget/RemoveWidget
 // closures. In non-interactive mode this is a no-op (widgets are TUI-only).
 //
-// Coalescing: if a WidgetUpdateEvent is already queued and not yet consumed
-// by the TUI event loop, additional calls within the same ~16 ms window are
-// dropped. This prevents fast extension tickers from flooding BubbleTea's
-// mailbox with redundant re-render triggers.
+// Coalescing (leading + trailing edge): the first call in an idle period
+// fires immediately for responsiveness. Subsequent calls within a ~16 ms
+// debounce window are batched into a single trailing event delivered at
+// the end of the window. The trailing send is essential for correctness:
+// extensions routinely make tight SetWidget→RemoveWidget pairs (e.g. on
+// SubagentEnd) and silently dropping the second call would leave the TUI's
+// layout stuck on stale widget dimensions until some other event happens
+// to trigger a re-render.
 func (a *App) NotifyWidgetUpdate() {
-	// Coalesce: only one pending update at a time.
 	if !a.widgetUpdatePending.CompareAndSwap(false, true) {
+		// A leading-edge event is already in flight — mark that the runner
+		// state has changed again so the trailing send below picks it up.
+		a.widgetUpdateTrailing.Store(true)
 		return
 	}
 	a.mu.Lock()
 	prog := a.program
 	a.mu.Unlock()
-	if prog != nil {
-		prog.Send(WidgetUpdateEvent{})
-		// Reset the pending flag after a short debounce so subsequent calls
-		// within the same render cycle are also coalesced, but new updates
-		// after the cycle are allowed through.
-		go func() {
-			time.Sleep(16 * time.Millisecond) // ~1 frame at 60 fps
-			a.widgetUpdatePending.Store(false)
-		}()
-	} else {
+	if prog == nil {
 		// No program registered (non-interactive mode); clear the flag so
 		// future calls are never permanently blocked.
 		a.widgetUpdatePending.Store(false)
+		return
+	}
+	prog.Send(WidgetUpdateEvent{})
+	go func() {
+		time.Sleep(16 * time.Millisecond) // ~1 frame at 60 fps
+		// If any extra calls came in during the debounce window, deliver
+		// one trailing event so the TUI sees the latest widget state. We
+		// swap-and-test instead of plain-load so concurrent calls after
+		// the trailing send still race correctly with the pending reset.
+		if a.widgetUpdateTrailing.Swap(false) {
+			a.mu.Lock()
+			p := a.program
+			a.mu.Unlock()
+			if p != nil {
+				p.Send(WidgetUpdateEvent{})
+			}
+		}
+		a.widgetUpdatePending.Store(false)
+	}()
+}
+
+// NotifyContentReload sends a ContentReloadEvent to the TUI so it refreshes
+// prompt templates and skills from their provider callbacks. Called by file
+// watchers when .md/.txt files change in prompt or skill directories.
+// In non-interactive mode this is a no-op.
+func (a *App) NotifyContentReload() {
+	a.mu.Lock()
+	prog := a.program
+	a.mu.Unlock()
+	if prog != nil {
+		prog.Send(ContentReloadEvent{})
+	}
+}
+
+// NotifyMCPToolsReady sends an MCPToolsReadyEvent to the TUI so it refreshes
+// tool names and MCP tool count from provider callbacks. Called when background
+// MCP tool loading completes. In non-interactive mode this is a no-op.
+func (a *App) NotifyMCPToolsReady() {
+	a.mu.Lock()
+	prog := a.program
+	a.mu.Unlock()
+	if prog != nil {
+		prog.Send(MCPToolsReadyEvent{})
+	}
+}
+
+// NotifyMCPServerLoaded sends an MCPServerLoadedEvent to the TUI so it can
+// display a system message when a single MCP server finishes loading. Called
+// per server as background MCP tool loading progresses.
+func (a *App) NotifyMCPServerLoaded(serverName string, toolCount int, err error) {
+	a.mu.Lock()
+	prog := a.program
+	a.mu.Unlock()
+	if prog != nil {
+		prog.Send(MCPServerLoadedEvent{
+			ServerName: serverName,
+			ToolCount:  toolCount,
+			Error:      err,
+		})
 	}
 }
 
@@ -1081,18 +1413,28 @@ func (a *App) PrintBlockFromExtension(opts extensions.PrintBlockOpts) {
 		})
 		return
 	}
-	// Non-interactive fallback.
+	// Non-interactive fallback: render a simple framed block to stderr so
+	// it is visually distinct from plain stdout output.
 	if opts.Subtitle != "" {
-		fmt.Printf("%s\n  — %s\n", opts.Text, opts.Subtitle)
+		fmt.Fprintf(os.Stderr, "--- %s ---\n%s\n", opts.Subtitle, opts.Text)
 	} else {
-		fmt.Println(opts.Text)
+		fmt.Fprintf(os.Stderr, "---\n%s\n---\n", opts.Text)
 	}
 }
 
 // recordStepUsage applies token/cost usage reported for a completed step.
 // Step usage events arrive even when a turn is later cancelled, so this keeps
 // the usage widget accurate on all stop paths.
-func (a *App) recordStepUsage(ev kit.StepUsageEvent, stepUsageSeen *atomic.Bool) {
+//
+// Both session totals (cost, token counts) and the context window fill level
+// are updated here so the status bar reflects progress after every LLM call,
+// not just at the end of the full turn. Context fill monotonically increases
+// across steps because each step re-sends the entire conversation plus any
+// new tool results, so the numbers only go up.
+//
+// sendFn is called with a UsageUpdatedEvent to trigger a TUI re-render so
+// the updated values are visible immediately.
+func (a *App) recordStepUsage(ev kit.StepUsageEvent, stepUsageSeen *atomic.Bool, sendFn func(tea.Msg)) {
 	hasUsage := ev.InputTokens > 0 || ev.OutputTokens > 0 || ev.CacheReadTokens > 0 || ev.CacheWriteTokens > 0
 	if a.opts.Debug {
 		log.Printf("[DEBUG] recordStepUsage: hasUsage=%v input=%d output=%d cacheRead=%d cacheWrite=%d",
@@ -1113,10 +1455,21 @@ func (a *App) recordStepUsage(ev kit.StepUsageEvent, stepUsageSeen *atomic.Bool)
 		int(ev.CacheReadTokens),
 		int(ev.CacheWriteTokens),
 	)
-	// NOTE: We do NOT call SetContextTokens here. Context fill is set once
-	// at turn completion via updateUsageFromTurnResult using FinalUsage.InputTokens,
-	// which reflects the full accumulated context. Per-step context tokens would
-	// cause the display to jump around during multi-step tool calls.
+	// Update context window fill from this step's usage. Each step sends
+	// the full conversation to the LLM, so the reported token counts
+	// represent the actual context utilization at that point.
+	contextFill := int(ev.InputTokens) + int(ev.CacheReadTokens) + int(ev.CacheWriteTokens) + int(ev.OutputTokens)
+	if contextFill > 0 {
+		if a.opts.Debug {
+			log.Printf("[DEBUG] recordStepUsage: SetContextTokens=%d (Input=%d + CacheRead=%d + CacheWrite=%d + Output=%d)",
+				contextFill, ev.InputTokens, ev.CacheReadTokens, ev.CacheWriteTokens, ev.OutputTokens)
+		}
+		a.opts.UsageTracker.SetContextTokens(contextFill)
+	}
+	// Notify the TUI so it re-renders the status bar with updated values.
+	if sendFn != nil {
+		sendFn(UsageUpdatedEvent{})
+	}
 }
 
 // updateUsageFromTurnResult records token usage from an SDK TurnResult into the
@@ -1180,15 +1533,30 @@ func (a *App) updateUsageFromTurnResult(result *kit.TurnResult, userPrompt strin
 	}
 
 	// --- Context window fill (drives the % bar) ---
-	// Use FinalUsage.InputTokens as the context window fill. The API's InputTokens
-	// already includes the full conversation history (system prompt + all previous
-	// messages + current user message). Adding OutputTokens would double-count since
-	// the output becomes part of the input for the next turn.
-	if result.FinalUsage != nil && result.FinalUsage.InputTokens > 0 {
-		if a.opts.Debug {
-			log.Printf("[DEBUG] updateUsageFromTurnResult: calling SetContextTokens=%d (FinalUsage.InputTokens)",
-				result.FinalUsage.InputTokens)
+	// Calculate context fill from the LAST API call's usage. The context
+	// window is filled by everything sent to and received from the model:
+	//
+	//   InputTokens       — non-cached input (may be small with prompt caching)
+	//   CacheReadTokens   — input tokens served from cache
+	//   CacheCreationTokens — input tokens written to cache this call
+	//   OutputTokens      — assistant output (becomes input next turn)
+	//
+	// With Anthropic prompt caching, InputTokens can drop to near-zero while
+	// CacheReadTokens holds the bulk of the context. We must sum all four to
+	// get the true context window utilization.
+	//
+	// We use FinalUsage (last step only), NOT TotalUsage, because TotalUsage
+	// sums across all tool-calling steps — and each step re-sends the full
+	// conversation, so TotalUsage massively overstates the actual window fill.
+	if result.FinalUsage != nil {
+		u := result.FinalUsage
+		contextFill := int(u.InputTokens) + int(u.CacheReadTokens) + int(u.CacheCreationTokens) + int(u.OutputTokens)
+		if contextFill > 0 {
+			if a.opts.Debug {
+				log.Printf("[DEBUG] updateUsageFromTurnResult: SetContextTokens=%d (Input=%d + CacheRead=%d + CacheCreate=%d + Output=%d)",
+					contextFill, u.InputTokens, u.CacheReadTokens, u.CacheCreationTokens, u.OutputTokens)
+			}
+			a.opts.UsageTracker.SetContextTokens(contextFill)
 		}
-		a.opts.UsageTracker.SetContextTokens(int(result.FinalUsage.InputTokens))
 	}
 }

internal/app/app_test.go

@@ -3,11 +3,16 @@ package app
 import (
 	"context"
 	"errors"
+	"strings"
 	"sync"
 	"testing"
 	"time"
 
+	tea "charm.land/bubbletea/v2"
+	"charm.land/fantasy"
 	kit "github.com/mark3labs/kit/pkg/kit"
+
+	"github.com/mark3labs/kit/internal/session"
 )
 
 // --------------------------------------------------------------------------
@@ -532,9 +537,9 @@ func TestQueueLength_reflects(t *testing.T) {
 }
 
 // TestRecordStepUsage_updatesTracker verifies that per-step usage updates are
-// recorded immediately for cost tracking. Context tokens are NOT updated here
-// (only via updateUsageFromTurnResult) to avoid display jumps during multi-step
-// tool calls.
+// recorded immediately for cost tracking. Context tokens are also updated so
+// the status bar reflects context fill after every LLM call in a multi-step
+// turn, not just at the end.
 func TestRecordStepUsage_updatesTracker(t *testing.T) {
 	usage := &usageUpdaterStub{}
 	app := New(Options{UsageTracker: usage}, nil)
@@ -545,7 +550,7 @@ func TestRecordStepUsage_updatesTracker(t *testing.T) {
 		OutputTokens:     45,
 		CacheReadTokens:  5,
 		CacheWriteTokens: 2,
-	}, nil)
+	}, nil, nil)
 
 	usage.mu.Lock()
 	defer usage.mu.Unlock()
@@ -557,9 +562,13 @@ func TestRecordStepUsage_updatesTracker(t *testing.T) {
 		t.Fatalf("unexpected usage update payload: in=%d out=%d cache_read=%d cache_write=%d",
 			usage.lastUpdateInput, usage.lastUpdateOutput, usage.lastUpdateCacheRead, usage.lastUpdateCacheWrite)
 	}
-	// Context tokens should NOT be updated by recordStepUsage (only by updateUsageFromTurnResult)
-	if usage.contextCalls != 0 {
-		t.Fatalf("expected 0 context token updates from recordStepUsage, got %d", usage.contextCalls)
+	// Context tokens should now be updated per-step (Input + CacheRead + CacheWrite + Output).
+	if usage.contextCalls != 1 {
+		t.Fatalf("expected 1 context token update from recordStepUsage, got %d", usage.contextCalls)
+	}
+	expectedContext := 120 + 45 + 5 + 2
+	if usage.lastContextTokens != expectedContext {
+		t.Fatalf("expected context tokens %d, got %d", expectedContext, usage.lastContextTokens)
 	}
 }
 
@@ -630,10 +639,12 @@ func TestUpdateUsageFromTurnResult_recordsWhenInputTokensZero(t *testing.T) {
 	}
 }
 
-// TestUpdateUsageFromTurnResult_contextTokensUsesInputOnly verifies that context
-// window fill uses InputTokens only (not input+output). The API's InputTokens
-// already includes the full conversation history; adding output would double-count.
-func TestUpdateUsageFromTurnResult_contextTokensUsesInputOnly(t *testing.T) {
+// TestUpdateUsageFromTurnResult_contextTokensUsesAllCategories verifies that
+// context window fill uses all token categories from the final API call:
+// InputTokens + CacheReadTokens + CacheCreationTokens + OutputTokens.
+// With Anthropic prompt caching, InputTokens can be near-zero while
+// CacheReadTokens holds the bulk of the context.
+func TestUpdateUsageFromTurnResult_contextTokensUsesAllCategories(t *testing.T) {
 	usage := &usageUpdaterStub{}
 	app := New(Options{UsageTracker: usage}, nil)
 	defer app.Close()
@@ -641,22 +652,466 @@ func TestUpdateUsageFromTurnResult_contextTokensUsesInputOnly(t *testing.T) {
 	app.updateUsageFromTurnResult(&kit.TurnResult{
 		Response: "ok",
 		TotalUsage: &kit.LLMUsage{
-			InputTokens:  1000,
-			OutputTokens: 200,
+			InputTokens:         3,
+			OutputTokens:        5,
+			CacheReadTokens:     0,
+			CacheCreationTokens: 4317,
 		},
 		FinalUsage: &kit.LLMUsage{
-			InputTokens:  1000, // Full context including history
-			OutputTokens: 200,
+			InputTokens:         3,    // Non-cached input (small with caching)
+			OutputTokens:        5,    // Assistant output
+			CacheReadTokens:     0,    // No cache reads on first call
+			CacheCreationTokens: 4317, // System prompt + tools written to cache
 		},
 	}, "prompt", false)
 
 	usage.mu.Lock()
 	defer usage.mu.Unlock()
 
-	// Context tokens should be InputTokens only (1000), not input+output (1200)
-	// because InputTokens already includes the full conversation history
-	if usage.contextCalls != 1 || usage.lastContextTokens != 1000 {
-		t.Fatalf("expected context tokens=1000 (InputTokens only), got calls=%d tokens=%d",
-			usage.contextCalls, usage.lastContextTokens)
+	// Context tokens should be Input + CacheRead + CacheCreate + Output = 4325
+	expected := 3 + 0 + 4317 + 5
+	if usage.contextCalls != 1 || usage.lastContextTokens != expected {
+		t.Fatalf("expected context tokens=%d (all categories), got calls=%d tokens=%d",
+			expected, usage.contextCalls, usage.lastContextTokens)
+	}
+}
+
+// TestHandleTurnEnd_LengthEmitsWarning verifies that when the SDK reports a
+// FinishReasonLength (max_output_tokens hit), the app surfaces a user-visible
+// ExtensionPrintEvent with Level="info" so the TUI can render a banner
+// instead of silently showing a truncated reply.
+func TestHandleTurnEnd_LengthEmitsWarning(t *testing.T) {
+	app := New(Options{}, nil)
+	defer app.Close()
+
+	var mu sync.Mutex
+	var received []tea.Msg
+	sendFn := func(m tea.Msg) {
+		mu.Lock()
+		defer mu.Unlock()
+		received = append(received, m)
+	}
+
+	app.handleTurnEnd(kit.TurnEndEvent{StopReason: kit.FinishReasonLength}, sendFn)
+
+	mu.Lock()
+	defer mu.Unlock()
+	if len(received) != 1 {
+		t.Fatalf("expected 1 event on length stop, got %d", len(received))
+	}
+	ev, ok := received[0].(ExtensionPrintEvent)
+	if !ok {
+		t.Fatalf("expected ExtensionPrintEvent, got %T", received[0])
+	}
+	if ev.Level != "info" {
+		t.Errorf("expected Level=info, got %q", ev.Level)
+	}
+	if ev.Text == "" {
+		t.Error("expected non-empty warning text")
+	}
+	if !strings.Contains(ev.Text, "max_output_tokens") {
+		t.Errorf("warning text should mention max_output_tokens, got: %s", ev.Text)
+	}
+}
+
+// TestHandleTurnEnd_NonLengthIgnored verifies that ordinary stop reasons
+// (stop, tool-calls, error, unknown, "") do not produce a warning banner.
+func TestHandleTurnEnd_NonLengthIgnored(t *testing.T) {
+	app := New(Options{}, nil)
+	defer app.Close()
+
+	reasons := []string{
+		kit.FinishReasonStop,
+		kit.FinishReasonToolCalls,
+		kit.FinishReasonError,
+		kit.FinishReasonContentFilter,
+		kit.FinishReasonOther,
+		kit.FinishReasonUnknown,
+		"",
+	}
+	for _, r := range reasons {
+		var called bool
+		app.handleTurnEnd(kit.TurnEndEvent{StopReason: r}, func(m tea.Msg) {
+			called = true
+		})
+		if called {
+			t.Errorf("stop reason %q unexpectedly emitted a warning", r)
+		}
+	}
+}
+
+// TestHandleTurnEnd_NilSendFn guards against panics when no TUI listener is
+// attached (e.g. early init or headless teardown).
+func TestHandleTurnEnd_NilSendFn(t *testing.T) {
+	app := New(Options{}, nil)
+	defer app.Close()
+
+	// Should not panic with a nil sendFn.
+	app.handleTurnEnd(kit.TurnEndEvent{StopReason: kit.FinishReasonLength}, nil)
+}
+
+// TestFormatMaxTokensTruncatedMessage_NoKit verifies the fallback message
+// when Options.Kit is nil (test/stub path).
+func TestFormatMaxTokensTruncatedMessage_NoKit(t *testing.T) {
+	app := New(Options{}, nil)
+	defer app.Close()
+
+	msg := app.formatMaxTokensTruncatedMessage()
+	if msg == "" {
+		t.Fatal("expected non-empty fallback message")
+	}
+	for _, needle := range []string{"max_output_tokens", "--max-tokens", "KIT_MAX_TOKENS", "modelSettings"} {
+		if !strings.Contains(msg, needle) {
+			t.Errorf("fallback message missing %q:\n%s", needle, msg)
+		}
+	}
+}
+
+// --------------------------------------------------------------------------
+// releaseBusyAfterCompact (issue #27)
+// --------------------------------------------------------------------------
+
+// TestReleaseBusyAfterCompact_flushesQueuedMessages is a regression test for
+// issue #27: messages queued via Run() while /compact is running used to sit
+// in a.queue indefinitely until the user typed another prompt. After the fix
+// the deferred releaseBusyAfterCompact tail picks up any pending items and
+// dispatches drainQueue automatically.
+//
+// We simulate the compaction completion path directly (bypassing the SDK)
+// by toggling busy=true, populating the queue exactly as Run() would have
+// during compaction, and then invoking releaseBusyAfterCompact.
+func TestReleaseBusyAfterCompact_flushesQueuedMessages(t *testing.T) {
+	stub := newStubWithFuncs(
+		func(ctx context.Context) (*kit.TurnResult, error) {
+			return turnResult("compacted then drained"), nil
+		},
+	)
+	app := newTestApp(stub)
+	defer app.Close()
+
+	// Simulate the state at the start of the compaction tail: busy is set
+	// and a couple of prompts have piled up in the queue while we were
+	// summarising. (Run() would have appended them and returned a queue
+	// length > 0 to the caller.)
+	app.mu.Lock()
+	app.busy = true
+	app.queue = append(app.queue,
+		queueItem{Prompt: "queued during compact #1"},
+		queueItem{Prompt: "queued during compact #2"},
+	)
+	app.mu.Unlock()
+
+	// Invoke the deferred tail directly. It should kick off drainQueue.
+	app.releaseBusyAfterCompact()
+
+	// drainQueue runs in a goroutine. Wait for the app to come back to idle.
+	ok := waitForCondition(2*time.Second, func() bool {
+		app.mu.Lock()
+		defer app.mu.Unlock()
+		return !app.busy
+	})
+	if !ok {
+		t.Fatal("app did not become idle after releaseBusyAfterCompact: queue not drained")
+	}
+
+	// Wait for any in-flight goroutine to finish before reading state.
+	app.wg.Wait()
+
+	if got := app.QueueLength(); got != 0 {
+		t.Fatalf("expected empty queue after drain, got %d", got)
+	}
+	if n := stub.callCount(); n == 0 {
+		t.Fatalf("expected stub PromptFunc to fire at least once after compact, got %d calls", n)
+	}
+}
+
+// TestReleaseBusyAfterCompact_idleWhenQueueEmpty verifies that with no
+// pending messages the helper just clears busy and does NOT spawn a
+// drainQueue goroutine (no spurious agent turn).
+func TestReleaseBusyAfterCompact_idleWhenQueueEmpty(t *testing.T) {
+	stub := newStub()
+	app := newTestApp(stub)
+	defer app.Close()
+
+	app.mu.Lock()
+	app.busy = true
+	app.mu.Unlock()
+
+	app.releaseBusyAfterCompact()
+
+	app.mu.Lock()
+	busy := app.busy
+	app.mu.Unlock()
+	if busy {
+		t.Fatal("expected busy=false after releaseBusyAfterCompact with empty queue")
+	}
+
+	// Give any rogue goroutine a moment to (incorrectly) call PromptFunc.
+	time.Sleep(50 * time.Millisecond)
+	if n := stub.callCount(); n != 0 {
+		t.Fatalf("expected 0 PromptFunc calls when queue empty, got %d", n)
+	}
+}
+
+// TestReleaseBusyAfterCompact_splicesSteerAheadOfQueue exercises the SDK
+// steer-drain branch of releaseBusyAfterCompact (issue #27 follow-up).
+//
+// Production wires a.opts.Kit.DrainSteer() to pull messages that arrived via
+// Steer/SteerWithFiles during compaction, but Options.Kit is *kit.Kit (a
+// concrete struct) so unit tests cannot stand up a real instance without a
+// full LLM backend. The test uses the unexported steerDrainFn seam to inject
+// fake steer items, then asserts that:
+//
+//   - Steer items are dispatched ahead of any prompts that piled up in
+//     a.queue (steer retains "act now" priority over ordinary queued
+//     prompts), and
+//   - the helper still hands off to drainQueue so the steer item actually
+//     fires (the previous behaviour left them stranded — see #27).
+func TestReleaseBusyAfterCompact_splicesSteerAheadOfQueue(t *testing.T) {
+	var pmu sync.Mutex
+	var firstPrompt string
+	stub := newStubWithFuncs(
+		func(ctx context.Context) (*kit.TurnResult, error) {
+			return turnResult("steer dispatched"), nil
+		},
+	)
+	// Wrap PromptFunc so we can capture the prompt text the stub receives
+	// (newStubWithFuncs's fns ignore prompt; we need it to verify ordering).
+	capturingPrompt := func(ctx context.Context, prompt string) (*kit.TurnResult, error) {
+		pmu.Lock()
+		if firstPrompt == "" {
+			firstPrompt = prompt
+		}
+		pmu.Unlock()
+		return stub.fn(ctx, prompt)
+	}
+	app := New(Options{PromptFunc: capturingPrompt}, nil)
+	defer app.Close()
+
+	// Inject fake steer items via the test seam. In production the same
+	// items would have been delivered through Kit.InjectSteerWithFiles
+	// during /compact and pulled by DrainSteer here.
+	app.steerDrainFn = func() []queueItem {
+		return []queueItem{
+			{Prompt: "steer-1"},
+			{Prompt: "steer-2"},
+		}
+	}
+
+	// Simulate the state at the end of compaction: busy is set and a couple
+	// of regular Run() prompts have piled up after the steer messages.
+	app.mu.Lock()
+	app.busy = true
+	app.queue = append(app.queue,
+		queueItem{Prompt: "queued-1"},
+		queueItem{Prompt: "queued-2"},
+	)
+	app.mu.Unlock()
+
+	app.releaseBusyAfterCompact()
+
+	// Wait for the dispatched batch to complete.
+	ok := waitForCondition(2*time.Second, func() bool {
+		app.mu.Lock()
+		defer app.mu.Unlock()
+		return !app.busy
+	})
+	if !ok {
+		t.Fatal("app did not become idle after steer-spliced releaseBusyAfterCompact")
+	}
+	app.wg.Wait()
+
+	// drainQueue picks up `first` directly and batches the rest. With
+	// PromptFunc set, executeBatch invokes us with items[0] only — that
+	// item must be the first steer message, proving steer items were
+	// spliced ahead of the previously queued prompts.
+	pmu.Lock()
+	got := firstPrompt
+	pmu.Unlock()
+	if got != "steer-1" {
+		t.Fatalf("expected first dispatched prompt to be steer item %q (steer items must come before queued prompts), got %q",
+			"steer-1", got)
+	}
+
+	// Queue should be fully drained and PromptFunc must have actually fired.
+	if n := app.QueueLength(); n != 0 {
+		t.Fatalf("expected empty queue after drain, got %d entries", n)
+	}
+	if n := stub.callCount(); n == 0 {
+		t.Fatal("expected stub PromptFunc to fire at least once after splice")
+	}
+}
+
+// TestReleaseBusyAfterCompact_dropsQueueWhenClosed verifies that if the app
+// was closed during compaction the helper discards any pending items rather
+// than spawning drainQueue against a torn-down App.
+func TestReleaseBusyAfterCompact_dropsQueueWhenClosed(t *testing.T) {
+	stub := newStub()
+	app := newTestApp(stub)
+
+	app.mu.Lock()
+	app.busy = true
+	app.queue = append(app.queue, queueItem{Prompt: "would have run"})
+	app.closed = true
+	app.mu.Unlock()
+
+	app.releaseBusyAfterCompact()
+
+	app.mu.Lock()
+	busy := app.busy
+	qLen := len(app.queue)
+	app.mu.Unlock()
+	if busy {
+		t.Fatal("expected busy=false even when closed")
+	}
+	if qLen != 0 {
+		t.Fatalf("expected queue cleared on closed app, got %d entries", qLen)
+	}
+	time.Sleep(20 * time.Millisecond)
+	if n := stub.callCount(); n != 0 {
+		t.Fatalf("expected 0 PromptFunc calls on closed app, got %d", n)
+	}
+}
+
+// --------------------------------------------------------------------------
+// PopLastUserMessage (/retry building block)
+// --------------------------------------------------------------------------
+
+// TestPopLastUserMessage_NoTreeSession verifies that PopLastUserMessage
+// returns an error when no tree session is active.
+func TestPopLastUserMessage_NoTreeSession(t *testing.T) {
+	app := newTestApp(newStub())
+	defer app.Close()
+
+	prompt, files, err := app.PopLastUserMessage()
+	if err == nil {
+		t.Fatal("expected error when no tree session is active")
+	}
+	if prompt != "" || files != nil {
+		t.Fatalf("expected zero values on error, got prompt=%q files=%v", prompt, files)
+	}
+}
+
+// TestPopLastUserMessage_WhileBusy verifies that PopLastUserMessage
+// refuses to truncate while the agent is busy (would race with executeBatch).
+func TestPopLastUserMessage_WhileBusy(t *testing.T) {
+	app := newTestApp(newStub())
+	defer app.Close()
+
+	app.mu.Lock()
+	app.busy = true
+	app.mu.Unlock()
+
+	_, _, err := app.PopLastUserMessage()
+	if err == nil {
+		t.Fatal("expected error when agent is busy")
+	}
+	if !strings.Contains(err.Error(), "working") {
+		t.Fatalf("expected error mentioning busy/working, got %q", err.Error())
+	}
+}
+
+// TestPopLastUserMessage_WhenClosed verifies that PopLastUserMessage
+// returns an error after Close().
+func TestPopLastUserMessage_WhenClosed(t *testing.T) {
+	app := newTestApp(newStub())
+	app.Close()
+
+	_, _, err := app.PopLastUserMessage()
+	if err == nil {
+		t.Fatal("expected error on closed app")
+	}
+}
+
+// TestPopLastUserMessage_TruncatesAndReturnsPrompt verifies the happy path:
+// a real tree session with user→assistant→user→assistant entries is
+// truncated back to before the most recent user message, and that user's
+// text is returned.
+func TestPopLastUserMessage_TruncatesAndReturnsPrompt(t *testing.T) {
+	dir := t.TempDir()
+	ts, err := session.CreateTreeSession(dir)
+	if err != nil {
+		t.Fatalf("create tree session: %v", err)
+	}
+	defer func() { _ = ts.Close() }()
+
+	// Build history: user "first" → assistant "ack 1" → user "second" → assistant "ack 2".
+	if _, err := ts.AppendLLMMessage(fantasy.NewUserMessage("first")); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := ts.AppendLLMMessage(fantasy.Message{
+		Role:    fantasy.MessageRoleAssistant,
+		Content: []fantasy.MessagePart{fantasy.TextPart{Text: "ack 1"}},
+	}); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := ts.AppendLLMMessage(fantasy.NewUserMessage("second")); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := ts.AppendLLMMessage(fantasy.Message{
+		Role:    fantasy.MessageRoleAssistant,
+		Content: []fantasy.MessagePart{fantasy.TextPart{Text: "ack 2"}},
+	}); err != nil {
+		t.Fatal(err)
+	}
+
+	app := New(Options{TreeSession: ts, PromptFunc: newStub().fn}, nil)
+	defer app.Close()
+
+	prompt, files, err := app.PopLastUserMessage()
+	if err != nil {
+		t.Fatalf("PopLastUserMessage: %v", err)
+	}
+	if prompt != "second" {
+		t.Fatalf("expected prompt=%q, got %q", "second", prompt)
+	}
+	if files != nil {
+		t.Fatalf("expected no files, got %v", files)
+	}
+
+	// After truncation the branch should only contain the first user
+	// message and its assistant response (the "second" turn is orphaned).
+	msgs := ts.GetLLMMessages()
+	if len(msgs) != 2 {
+		t.Fatalf("expected 2 messages on truncated branch, got %d", len(msgs))
+	}
+	if got := messageText(msgs[0]); got != "first" {
+		t.Fatalf("expected first message %q, got %q", "first", got)
+	}
+	if got := messageText(msgs[1]); got != "ack 1" {
+		t.Fatalf("expected second message %q, got %q", "ack 1", got)
+	}
+}
+
+// messageText extracts concatenated TextPart content from a fantasy.Message.
+func messageText(m fantasy.Message) string {
+	var out strings.Builder
+	for _, p := range m.Content {
+		if tp, ok := p.(fantasy.TextPart); ok {
+			out.WriteString(tp.Text)
+		}
+	}
+	return out.String()
+}
+
+// TestPopLastUserMessage_NoUserOnBranch verifies that an empty tree (no
+// user messages at all) returns a friendly error rather than panicking.
+func TestPopLastUserMessage_NoUserOnBranch(t *testing.T) {
+	dir := t.TempDir()
+	ts, err := session.CreateTreeSession(dir)
+	if err != nil {
+		t.Fatalf("create tree session: %v", err)
+	}
+	defer func() { _ = ts.Close() }()
+
+	app := New(Options{TreeSession: ts, PromptFunc: newStub().fn}, nil)
+	defer app.Close()
+
+	_, _, err = app.PopLastUserMessage()
+	if err == nil {
+		t.Fatal("expected error when no user message exists on branch")
+	}
+	if !strings.Contains(err.Error(), "no user message") {
+		t.Fatalf("expected error mentioning missing user message, got %q", err.Error())
 	}
 }

internal/app/events.go

@@ -32,6 +32,36 @@ type ToolCallStartedEvent struct {
 	ToolArgs string
 }
 
+// ToolCallInputStartEvent is sent when the LLM begins generating tool call
+// arguments. The tool name is known but the full argument JSON is still being
+// streamed. UIs can use this to show a "running" indicator immediately instead
+// of waiting for the full argument JSON to finish streaming.
+type ToolCallInputStartEvent struct {
+	// ToolCallID is the stable identifier for correlating tool lifecycle events.
+	ToolCallID string
+	// ToolName is the name of the tool being called.
+	ToolName string
+	// ToolKind classifies the tool: "execute", "edit", "read", "search", "agent".
+	ToolKind string
+}
+
+// ToolCallInputDeltaEvent is sent for each streamed fragment of tool call
+// arguments as they arrive from the LLM. Useful for live-previewing content
+// or showing a progress indicator with byte count.
+type ToolCallInputDeltaEvent struct {
+	// ToolCallID is the stable identifier for correlating tool lifecycle events.
+	ToolCallID string
+	// Delta is a JSON fragment of tool call arguments.
+	Delta string
+}
+
+// ToolCallInputEndEvent is sent when tool argument streaming is complete,
+// before the tool call is parsed and execution begins.
+type ToolCallInputEndEvent struct {
+	// ToolCallID is the stable identifier for correlating tool lifecycle events.
+	ToolCallID string
+}
+
 // ToolExecutionEvent is sent when a tool starts or finishes executing.
 // The IsStarting flag distinguishes between the start and end of execution.
 type ToolExecutionEvent struct {
@@ -79,6 +109,24 @@ type ToolCallContentEvent struct {
 	Content string
 }
 
+// PasswordPromptEvent is sent when a sudo command needs a password.
+// The TUI should display a password prompt overlay and send the result back.
+type PasswordPromptEvent struct {
+	// Prompt is the message to display to the user.
+	Prompt string
+	// ResponseCh receives the password from the TUI.
+	// The TUI must send exactly one value.
+	ResponseCh chan<- PasswordPromptResponse
+}
+
+// PasswordPromptResponse carries the user's password input.
+type PasswordPromptResponse struct {
+	// Password is the entered password.
+	Password string
+	// Cancelled is true if the user cancelled the prompt.
+	Cancelled bool
+}
+
 // ResponseCompleteEvent is sent when the LLM produces a final (non-streaming) response.
 // In streaming mode, this may be empty if all content was delivered via StreamChunkEvents.
 type ResponseCompleteEvent struct {
@@ -162,11 +210,36 @@ type ModelChangedEvent struct {
 	ModelName string
 }
 
+// UsageUpdatedEvent is sent after each completed LLM step to notify the TUI
+// that token counts and costs have changed. The UsageTracker is updated
+// in-place before this event is sent; the TUI just needs to re-render to
+// reflect the new values in the status bar.
+type UsageUpdatedEvent struct{}
+
 // WidgetUpdateEvent is sent when an extension adds, updates, or removes a
 // widget via ctx.SetWidget or ctx.RemoveWidget. The TUI re-reads widget state
 // from its WidgetProvider on the next render cycle.
 type WidgetUpdateEvent struct{}
 
+// ContentReloadEvent is sent when prompt templates or skills are reloaded
+// from disk (e.g. by a file watcher detecting changes). The TUI refreshes
+// its autocomplete entries and internal state from the provider callbacks.
+type ContentReloadEvent struct{}
+
+// MCPToolsReadyEvent is sent when background MCP tool loading completes.
+// The TUI refreshes its tool names and MCP tool count from provider callbacks
+// so that /tools and the startup info bar reflect the loaded MCP tools.
+type MCPToolsReadyEvent struct{}
+
+// MCPServerLoadedEvent is sent when a single MCP server finishes loading
+// (successfully or with error). The TUI displays a system message so users
+// see real-time progress as each server initializes.
+type MCPServerLoadedEvent struct {
+	ServerName string
+	ToolCount  int
+	Error      error // nil on success
+}
+
 // EditorTextSetEvent is sent when an extension calls ctx.SetEditorText to
 // pre-fill the input editor with text. The TUI handles this by setting the
 // textarea content and moving the cursor to the end.

internal/app/messages.go

@@ -13,11 +13,6 @@ type MessageStore struct {
 	messages []kit.LLMMessage
 }
 
-// NewMessageStore creates an empty MessageStore.
-func NewMessageStore() *MessageStore {
-	return &MessageStore{}
-}
-
 // NewMessageStoreWithMessages creates a MessageStore pre-populated with the
 // given messages. This is used when loading an existing session at startup.
 func NewMessageStoreWithMessages(msgs []kit.LLMMessage) *MessageStore {

internal/app/messages_test.go

@@ -3,24 +3,21 @@ package app
 import (
 	"testing"
 
-	"charm.land/fantasy"
-
 	kit "github.com/mark3labs/kit/pkg/kit"
 )
 
-// makeTextMsg builds a minimal kit.LLMMessage using fantasy.NewUserMessage
-// or constructing with the given role.
+// makeTextMsg builds a minimal kit.LLMMessage with the given role and text.
 func makeTextMsg(role, text string) kit.LLMMessage {
 	return kit.LLMMessage{
 		Role:    kit.LLMMessageRole(role),
-		Content: []fantasy.MessagePart{fantasy.TextPart{Text: text}},
+		Content: []kit.LLMMessagePart{kit.LLMTextPart{Text: text}},
 	}
 }
 
 // textOf extracts the plain text from an LLMMessage for assertions.
 func textOf(msg kit.LLMMessage) string {
 	for _, part := range msg.Content {
-		if tp, ok := part.(fantasy.TextPart); ok {
+		if tp, ok := part.(kit.LLMTextPart); ok {
 			return tp.Text
 		}
 	}
@@ -32,7 +29,7 @@ func textOf(msg kit.LLMMessage) string {
 // --------------------------------------------------------------------------
 
 func TestNewMessageStore_empty(t *testing.T) {
-	s := NewMessageStore()
+	s := NewMessageStoreWithMessages(nil)
 	if s == nil {
 		t.Fatal("expected non-nil store")
 	}
@@ -75,7 +72,7 @@ func TestNewMessageStoreWithMessages_isolatesInput(t *testing.T) {
 // --------------------------------------------------------------------------
 
 func TestAdd_appendsMessage(t *testing.T) {
-	s := NewMessageStore()
+	s := NewMessageStoreWithMessages(nil)
 	s.Add(makeTextMsg("user", "first"))
 	s.Add(makeTextMsg("assistant", "second"))
 
@@ -85,7 +82,7 @@ func TestAdd_appendsMessage(t *testing.T) {
 }
 
 func TestAdd_preservesOrder(t *testing.T) {
-	s := NewMessageStore()
+	s := NewMessageStoreWithMessages(nil)
 	texts := []string{"a", "b", "c"}
 	for _, t2 := range texts {
 		s.Add(makeTextMsg("user", t2))
@@ -103,7 +100,7 @@ func TestAdd_preservesOrder(t *testing.T) {
 // --------------------------------------------------------------------------
 
 func TestReplace_swapsHistory(t *testing.T) {
-	s := NewMessageStore()
+	s := NewMessageStoreWithMessages(nil)
 	s.Add(makeTextMsg("user", "old"))
 
 	replacement := []kit.LLMMessage{
@@ -123,7 +120,7 @@ func TestReplace_swapsHistory(t *testing.T) {
 
 // Replace must deep-copy the incoming slice.
 func TestReplace_isolatesInput(t *testing.T) {
-	s := NewMessageStore()
+	s := NewMessageStoreWithMessages(nil)
 	replacement := []kit.LLMMessage{makeTextMsg("user", "original")}
 	s.Replace(replacement)
 
@@ -140,7 +137,7 @@ func TestReplace_isolatesInput(t *testing.T) {
 // --------------------------------------------------------------------------
 
 func TestGetAll_returnsCopy(t *testing.T) {
-	s := NewMessageStore()
+	s := NewMessageStoreWithMessages(nil)
 	s.Add(makeTextMsg("user", "hello"))
 
 	got := s.GetAll()
@@ -154,7 +151,7 @@ func TestGetAll_returnsCopy(t *testing.T) {
 }
 
 func TestGetAll_emptyStore(t *testing.T) {
-	s := NewMessageStore()
+	s := NewMessageStoreWithMessages(nil)
 	got := s.GetAll()
 	if len(got) != 0 {
 		t.Fatalf("expected empty slice, got %d elements", len(got))
@@ -166,7 +163,7 @@ func TestGetAll_emptyStore(t *testing.T) {
 // --------------------------------------------------------------------------
 
 func TestClear_removesAllMessages(t *testing.T) {
-	s := NewMessageStore()
+	s := NewMessageStoreWithMessages(nil)
 	s.Add(makeTextMsg("user", "a"))
 	s.Add(makeTextMsg("user", "b"))
 	s.Clear()
@@ -177,7 +174,7 @@ func TestClear_removesAllMessages(t *testing.T) {
 }
 
 func TestClear_allowsSubsequentAdds(t *testing.T) {
-	s := NewMessageStore()
+	s := NewMessageStoreWithMessages(nil)
 	s.Add(makeTextMsg("user", "before"))
 	s.Clear()
 	s.Add(makeTextMsg("user", "after"))
@@ -196,7 +193,7 @@ func TestClear_allowsSubsequentAdds(t *testing.T) {
 // --------------------------------------------------------------------------
 
 func TestConcurrentAccess(t *testing.T) {
-	s := NewMessageStore()
+	s := NewMessageStoreWithMessages(nil)
 	done := make(chan struct{})
 
 	// Writer goroutine.

internal/app/options.go

@@ -21,8 +21,10 @@ type UsageUpdater interface {
 	// the provider does not return exact counts.
 	EstimateAndUpdateUsage(inputText, outputText string)
 	// SetContextTokens records the approximate current context window fill
-	// level. This should be the final API call's input+output tokens (from
-	// FinalResponse.Usage), NOT the aggregate TotalUsage.
+	// level. This should be the sum of ALL token categories from the last
+	// API call: InputTokens + CacheReadTokens + CacheCreationTokens +
+	// OutputTokens. With Anthropic prompt caching, InputTokens can be
+	// near-zero while CacheReadTokens holds the bulk of the context.
 	SetContextTokens(tokens int)
 }
 

internal/auth/credentials.go

@@ -1,6 +1,7 @@
 package auth
 
 import (
+	"context"
 	"encoding/json"
 	"fmt"
 	"os"
@@ -9,11 +10,11 @@ import (
 	"time"
 )
 
-// CredentialStore holds all stored credentials for various providers.
-// Currently supports Anthropic and OpenAI credentials with both OAuth and API key authentication methods.
+// CredentialStore holds stored credentials for Anthropic, OpenAI, and GitHub Copilot.
 type CredentialStore struct {
 	Anthropic *AnthropicCredentials `json:"anthropic,omitempty"`
 	OpenAI    *OpenAICredentials    `json:"openai,omitempty"`
+	Copilot   *CopilotCredentials   `json:"copilot,omitempty"`
 }
 
 // AnthropicCredentials holds Anthropic API credentials supporting both OAuth
@@ -43,6 +44,16 @@ type OpenAICredentials struct {
 	CreatedAt    time.Time `json:"created_at"`
 }
 
+// CopilotCredentials holds GitHub OAuth credentials and the short-lived
+// GitHub Copilot API token derived from them.
+type CopilotCredentials struct {
+	Type               string    `json:"type"`                           // "oauth"
+	GitHubToken        string    `json:"github_token,omitempty"`         // GitHub device-flow OAuth token
+	CopilotAccessToken string    `json:"copilot_access_token,omitempty"` // Short-lived Copilot API token
+	ExpiresAt          int64     `json:"expires_at,omitempty"`           // Copilot token expiry
+	CreatedAt          time.Time `json:"created_at"`
+}
+
 // oauthTokenExpired reports whether an OAuth token with the given type and
 // expiry unix timestamp is past its expiry. Returns false for API key
 // credentials or when no expiry is set.
@@ -91,6 +102,16 @@ func (c *OpenAICredentials) NeedsRefresh() bool {
 	return oauthTokenNeedsRefresh(c.Type, c.ExpiresAt)
 }
 
+// IsExpired checks if the Copilot API token is expired.
+func (c *CopilotCredentials) IsExpired() bool {
+	return oauthTokenExpired(c.Type, c.ExpiresAt)
+}
+
+// NeedsRefresh reports whether the Copilot API token should be renewed.
+func (c *CopilotCredentials) NeedsRefresh() bool {
+	return oauthTokenNeedsRefresh(c.Type, c.ExpiresAt)
+}
+
 // CredentialManager handles secure storage and retrieval of authentication credentials.
 // It manages a JSON file stored in the user's config directory with appropriate
 // file permissions for security.
@@ -222,7 +243,7 @@ func (cm *CredentialManager) RemoveAnthropicCredentials() error {
 	store.Anthropic = nil
 
 	// If store is empty, remove the file entirely
-	if store.Anthropic == nil {
+	if store.Anthropic == nil && store.OpenAI == nil && store.Copilot == nil {
 		if err := os.Remove(cm.credentialsPath); err != nil && !os.IsNotExist(err) {
 			return fmt.Errorf("failed to remove credentials file: %w", err)
 		}
@@ -255,29 +276,6 @@ func (cm *CredentialManager) HasAnthropicCredentials() (bool, error) {
 	}
 }
 
-// SetOpenAICredentials stores OpenAI API key credentials. It validates the
-// API key format before storing. The API key must start with "sk-" and be
-// at least 20 characters long. Returns an error if the API key is invalid or
-// if storage fails.
-func (cm *CredentialManager) SetOpenAICredentials(apiKey string) error {
-	if err := validateOpenAIAPIKey(apiKey); err != nil {
-		return err
-	}
-
-	store, err := cm.LoadCredentials()
-	if err != nil {
-		return err
-	}
-
-	store.OpenAI = &OpenAICredentials{
-		Type:      "api_key",
-		APIKey:    apiKey,
-		CreatedAt: time.Now(),
-	}
-
-	return cm.SaveCredentials(store)
-}
-
 // GetOpenAICredentials retrieves stored OpenAI credentials. Returns nil if
 // no credentials are stored. The returned credentials may be either OAuth or API
 // key type, check the Type field to determine which.
@@ -302,7 +300,7 @@ func (cm *CredentialManager) RemoveOpenAICredentials() error {
 	store.OpenAI = nil
 
 	// If store is empty, remove the file entirely
-	if store.Anthropic == nil && store.OpenAI == nil {
+	if store.Anthropic == nil && store.OpenAI == nil && store.Copilot == nil {
 		if err := os.Remove(cm.credentialsPath); err != nil && !os.IsNotExist(err) {
 			return fmt.Errorf("failed to remove credentials file: %w", err)
 		}
@@ -312,6 +310,104 @@ func (cm *CredentialManager) RemoveOpenAICredentials() error {
 	return cm.SaveCredentials(store)
 }
 
+// GetCopilotCredentials retrieves stored GitHub Copilot credentials.
+func (cm *CredentialManager) GetCopilotCredentials() (*CopilotCredentials, error) {
+	store, err := cm.LoadCredentials()
+	if err != nil {
+		return nil, err
+	}
+
+	return store.Copilot, nil
+}
+
+// RemoveCopilotCredentials removes stored GitHub Copilot credentials.
+func (cm *CredentialManager) RemoveCopilotCredentials() error {
+	store, err := cm.LoadCredentials()
+	if err != nil {
+		return err
+	}
+
+	store.Copilot = nil
+
+	if store.Anthropic == nil && store.OpenAI == nil && store.Copilot == nil {
+		if err := os.Remove(cm.credentialsPath); err != nil && !os.IsNotExist(err) {
+			return fmt.Errorf("failed to remove credentials file: %w", err)
+		}
+		return nil
+	}
+
+	return cm.SaveCredentials(store)
+}
+
+// HasCopilotCredentials checks if valid GitHub Copilot credentials are stored.
+func (cm *CredentialManager) HasCopilotCredentials() (bool, error) {
+	creds, err := cm.GetCopilotCredentials()
+	if err != nil {
+		return false, err
+	}
+	if creds == nil {
+		return false, nil
+	}
+
+	return creds.Type == "oauth" && creds.GitHubToken != "", nil
+}
+
+// SetCopilotOAuthCredentials stores GitHub Copilot OAuth credentials.
+func (cm *CredentialManager) SetCopilotOAuthCredentials(creds *CopilotCredentials) error {
+	store, err := cm.LoadCredentials()
+	if err != nil {
+		return err
+	}
+
+	store.Copilot = creds
+	return cm.SaveCredentials(store)
+}
+
+// GetValidCopilotAccessToken returns a fresh Copilot API token, renewing it
+// with the stored GitHub OAuth token when needed.
+func (cm *CredentialManager) GetValidCopilotAccessToken() (string, error) {
+	return cm.GetValidCopilotAccessTokenContext(context.Background())
+}
+
+// GetValidCopilotAccessTokenContext returns a fresh Copilot API token, renewing
+// it with the stored GitHub OAuth token when needed.
+func (cm *CredentialManager) GetValidCopilotAccessTokenContext(ctx context.Context) (string, error) {
+	if ctx == nil {
+		ctx = context.Background()
+	}
+
+	creds, err := cm.GetCopilotCredentials()
+	if err != nil {
+		return "", err
+	}
+	if creds == nil {
+		return "", fmt.Errorf("no Copilot credentials found")
+	}
+	if creds.Type != "oauth" {
+		return "", fmt.Errorf("unknown credential type: %s", creds.Type)
+	}
+	if creds.GitHubToken == "" {
+		return "", fmt.Errorf("GitHub OAuth token missing from Copilot credentials")
+	}
+
+	if creds.CopilotAccessToken == "" || creds.NeedsRefresh() {
+		client := NewCopilotOAuthClient()
+		newCreds, err := client.RefreshCopilotToken(ctx, creds.GitHubToken)
+		if err != nil {
+			return "", fmt.Errorf("failed to refresh Copilot token: %w", err)
+		}
+		newCreds.CreatedAt = creds.CreatedAt
+
+		if err := cm.SetCopilotOAuthCredentials(newCreds); err != nil {
+			return "", fmt.Errorf("failed to save refreshed Copilot token: %w", err)
+		}
+
+		return newCreds.CopilotAccessToken, nil
+	}
+
+	return creds.CopilotAccessToken, nil
+}
+
 // HasOpenAICredentials checks if valid OpenAI credentials are stored.
 // Returns true if either a non-empty OAuth access token or API key is present,
 // false otherwise. Returns an error if credentials cannot be loaded.
@@ -417,24 +513,18 @@ func validateAnthropicAPIKey(apiKey string) error {
 	return nil
 }
 
-// validateOpenAIAPIKey validates the format of an OpenAI API key
-func validateOpenAIAPIKey(apiKey string) error {
-	apiKey = strings.TrimSpace(apiKey)
+// CredentialSourceOAuth is the source description returned by
+// GetAnthropicAPIKey when the key resolves to stored OAuth credentials.
+// Consumers should compare against this constant (or use IsAnthropicOAuth)
+// rather than matching the string literal.
+const CredentialSourceOAuth = "stored OAuth credentials"
 
-	if apiKey == "" {
-		return fmt.Errorf("API key cannot be empty")
-	}
-
-	// OpenAI API keys typically start with "sk-" and are quite long
-	if !strings.HasPrefix(apiKey, "sk-") {
-		return fmt.Errorf("invalid OpenAI API key format (should start with 'sk-')")
-	}
-
-	if len(apiKey) < 20 {
-		return fmt.Errorf("API key appears to be too short")
-	}
-
-	return nil
+// IsAnthropicOAuth reports whether the active Anthropic credential resolves
+// to a stored OAuth token (in which case the user is not billed per-token).
+// flagValue is the --provider-api-key flag value (may be empty).
+func IsAnthropicOAuth(flagValue string) bool {
+	_, source, err := GetAnthropicAPIKey(flagValue)
+	return err == nil && source == CredentialSourceOAuth
 }
 
 // GetAnthropicAPIKey retrieves an Anthropic API key from multiple sources in priority order:
@@ -459,7 +549,7 @@ func GetAnthropicAPIKey(flagValue string) (string, string, error) {
 				if err != nil {
 					return "", "", fmt.Errorf("failed to get valid OAuth token: %w", err)
 				}
-				return token, "stored OAuth credentials", nil
+				return token, CredentialSourceOAuth, nil
 			} else if creds.Type == "api_key" && creds.APIKey != "" {
 				return creds.APIKey, "stored API key", nil
 			}
@@ -471,5 +561,13 @@ func GetAnthropicAPIKey(flagValue string) (string, string, error) {
 		return envKey, "ANTHROPIC_API_KEY environment variable", nil
 	}
 
+	// Check if OpenAI credentials exist to provide a helpful suggestion
+	if cm != nil {
+		hasOpenAI, _ := cm.HasOpenAICredentials()
+		if hasOpenAI {
+			return "", "", fmt.Errorf("no Anthropic API key found. Use 'kit auth login anthropic', set ANTHROPIC_API_KEY environment variable, or use --provider-api-key flag\n\nNote: OpenAI credentials were detected. To use OpenAI, run with --model openai/gpt-5.4 or set it as default:\n  kit auth login openai --set-default")
+		}
+	}
+
 	return "", "", fmt.Errorf("no Anthropic API key found. Use 'kit auth login anthropic', set ANTHROPIC_API_KEY environment variable, or use --provider-api-key flag")
 }

internal/auth/credentials_test.go

@@ -4,6 +4,7 @@ import (
 	"os"
 	"path/filepath"
 	"testing"
+	"time"
 )
 
 func TestCredentialManager(t *testing.T) {
@@ -215,6 +216,7 @@ func TestCredentialStorePersistence(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Failed to create temp dir: %v", err)
 	}
+
 	defer func() { _ = os.RemoveAll(tempDir) }()
 
 	credentialsPath := filepath.Join(tempDir, "credentials.json")
@@ -252,3 +254,98 @@ func TestCredentialStorePersistence(t *testing.T) {
 		t.Errorf("Expected file permissions 0600, got %v", info.Mode().Perm())
 	}
 }
+
+func TestCopilotCredentials(t *testing.T) {
+	tempDir, err := os.MkdirTemp("", "kit-auth-test")
+	if err != nil {
+		t.Fatalf("Failed to create temp dir: %v", err)
+	}
+	defer func() { _ = os.RemoveAll(tempDir) }()
+
+	cm := &CredentialManager{
+		credentialsPath: filepath.Join(tempDir, "credentials.json"),
+	}
+
+	creds := &CopilotCredentials{
+		Type:               "oauth",
+		GitHubToken:        "github-token",
+		CopilotAccessToken: "copilot-token",
+		ExpiresAt:          time.Now().Add(time.Hour).Unix(),
+		CreatedAt:          time.Now(),
+	}
+
+	if err := cm.SetCopilotOAuthCredentials(creds); err != nil {
+		t.Fatalf("SetCopilotOAuthCredentials failed: %v", err)
+	}
+
+	hasAuth, err := cm.HasCopilotCredentials()
+	if err != nil {
+		t.Fatalf("HasCopilotCredentials failed: %v", err)
+	}
+	if !hasAuth {
+		t.Fatal("Expected Copilot credentials")
+	}
+
+	token, err := cm.GetValidCopilotAccessToken()
+	if err != nil {
+		t.Fatalf("GetValidCopilotAccessToken failed: %v", err)
+	}
+	if token != creds.CopilotAccessToken {
+		t.Fatalf("Expected Copilot token %q, got %q", creds.CopilotAccessToken, token)
+	}
+
+	if err := cm.RemoveCopilotCredentials(); err != nil {
+		t.Fatalf("RemoveCopilotCredentials failed: %v", err)
+	}
+	hasAuth, err = cm.HasCopilotCredentials()
+	if err != nil {
+		t.Fatalf("HasCopilotCredentials after removal failed: %v", err)
+	}
+	if hasAuth {
+		t.Fatal("Expected no Copilot credentials after removal")
+	}
+}
+
+func TestRemoveCredentialsPreservesOtherProviders(t *testing.T) {
+	tempDir, err := os.MkdirTemp("", "kit-auth-test")
+	if err != nil {
+		t.Fatalf("Failed to create temp dir: %v", err)
+	}
+	defer func() { _ = os.RemoveAll(tempDir) }()
+
+	cm := &CredentialManager{
+		credentialsPath: filepath.Join(tempDir, "credentials.json"),
+	}
+
+	if err := cm.SetOpenAIOAuthCredentials(&OpenAICredentials{
+		Type:         "oauth",
+		AccessToken:  "openai-token",
+		RefreshToken: "refresh-token",
+		ExpiresAt:    time.Now().Add(time.Hour).Unix(),
+		AccountID:    "account",
+		CreatedAt:    time.Now(),
+	}); err != nil {
+		t.Fatalf("SetOpenAIOAuthCredentials failed: %v", err)
+	}
+	if err := cm.SetCopilotOAuthCredentials(&CopilotCredentials{
+		Type:               "oauth",
+		GitHubToken:        "github-token",
+		CopilotAccessToken: "copilot-token",
+		ExpiresAt:          time.Now().Add(time.Hour).Unix(),
+		CreatedAt:          time.Now(),
+	}); err != nil {
+		t.Fatalf("SetCopilotOAuthCredentials failed: %v", err)
+	}
+
+	if err := cm.RemoveCopilotCredentials(); err != nil {
+		t.Fatalf("RemoveCopilotCredentials failed: %v", err)
+	}
+
+	hasOpenAI, err := cm.HasOpenAICredentials()
+	if err != nil {
+		t.Fatalf("HasOpenAICredentials failed: %v", err)
+	}
+	if !hasOpenAI {
+		t.Fatal("Expected OpenAI credentials to remain after removing Copilot credentials")
+	}
+}

internal/auth/oauth.go

@@ -10,6 +10,7 @@ import (
 	"io"
 	"net/http"
 	"net/url"
+	"strconv"
 	"strings"
 	"time"
 )
@@ -211,6 +212,262 @@ type OpenAIOAuthClient struct {
 	Scopes       string
 }
 
+// CopilotOAuthClient handles GitHub device-flow OAuth and exchanges the
+// GitHub token for a short-lived GitHub Copilot API token.
+//
+// The GitHub token comes from GitHub's OAuth device flow. It is then presented
+// to GitHub's internal Copilot token endpoint, which returns the bearer token
+// used by api.githubcopilot.com.
+type CopilotOAuthClient struct {
+	ClientID      string
+	DeviceURL     string
+	TokenURL      string
+	CopilotURL    string
+	Scopes        string
+	PollTimeout   time.Duration
+	ClientTimeout time.Duration
+}
+
+// CopilotDeviceCode contains data returned by GitHub's device-code endpoint.
+type CopilotDeviceCode struct {
+	DeviceCode      string `json:"device_code"`
+	UserCode        string `json:"user_code"`
+	VerificationURI string `json:"verification_uri"`
+	ExpiresIn       int    `json:"expires_in"`
+	Interval        int    `json:"interval"`
+}
+
+// NewCopilotOAuthClient creates a GitHub Copilot OAuth client.
+func NewCopilotOAuthClient() *CopilotOAuthClient {
+	return &CopilotOAuthClient{
+		ClientID:      "Iv1.b507a08c87ecfe98",
+		DeviceURL:     "https://github.com/login/device/code",
+		TokenURL:      "https://github.com/login/oauth/access_token",
+		CopilotURL:    "https://api.github.com/copilot_internal/v2/token",
+		Scopes:        "read:user",
+		PollTimeout:   15 * time.Minute,
+		ClientTimeout: 30 * time.Second,
+	}
+}
+
+// StartDeviceFlow requests a GitHub device code for browser login.
+//
+// The returned user code and verification URI are displayed by loginCopilot.
+// GitHub's response may omit interval, so this method normalizes it to the
+// documented five-second default.
+func (c *CopilotOAuthClient) StartDeviceFlow(ctx context.Context) (*CopilotDeviceCode, error) {
+	if ctx == nil {
+		ctx = context.Background()
+	}
+
+	data := url.Values{
+		"client_id": {c.ClientID},
+		"scope":     {c.Scopes},
+	}
+
+	req, err := http.NewRequestWithContext(ctx, "POST", c.DeviceURL, strings.NewReader(data.Encode()))
+	if err != nil {
+		return nil, fmt.Errorf("failed to create device-code request: %w", err)
+	}
+	req.Header.Set("Accept", "application/json")
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+
+	resp, err := (&http.Client{Timeout: c.ClientTimeout}).Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("failed to request device code: %w", err)
+	}
+	defer func() { _ = resp.Body.Close() }()
+
+	if resp.StatusCode != http.StatusOK {
+		body, _ := io.ReadAll(resp.Body)
+		return nil, fmt.Errorf("device-code request failed with status %d: %s", resp.StatusCode, string(body))
+	}
+
+	var code CopilotDeviceCode
+	if err := json.NewDecoder(resp.Body).Decode(&code); err != nil {
+		return nil, fmt.Errorf("failed to decode device-code response: %w", err)
+	}
+	if code.DeviceCode == "" || code.UserCode == "" || code.VerificationURI == "" {
+		return nil, fmt.Errorf("device-code response missing required fields")
+	}
+	if code.Interval <= 0 {
+		code.Interval = 5
+	}
+	return &code, nil
+}
+
+// PollDeviceToken waits until the user authorizes the device code and returns
+// the resulting GitHub OAuth token.
+//
+// It follows GitHub's device-flow polling contract: authorization_pending keeps
+// polling, slow_down increases the interval, and polling stops at the earlier of
+// the client timeout or the device-code expiry.
+func (c *CopilotOAuthClient) PollDeviceToken(ctx context.Context, deviceCode *CopilotDeviceCode) (string, error) {
+	if ctx == nil {
+		ctx = context.Background()
+	}
+
+	if deviceCode == nil || deviceCode.DeviceCode == "" {
+		return "", fmt.Errorf("device code missing")
+	}
+
+	deadline := time.Now().Add(c.PollTimeout)
+	if deviceCode.ExpiresIn > 0 {
+		expiresAt := time.Now().Add(time.Duration(deviceCode.ExpiresIn) * time.Second)
+		if expiresAt.Before(deadline) {
+			deadline = expiresAt
+		}
+	}
+
+	interval := time.Duration(deviceCode.Interval) * time.Second
+	if interval <= 0 {
+		interval = 5 * time.Second
+	}
+
+	for time.Now().Before(deadline) {
+		wait := interval
+		if remaining := time.Until(deadline); remaining < wait {
+			wait = remaining
+		}
+		select {
+		case <-ctx.Done():
+			return "", ctx.Err()
+		case <-time.After(wait):
+		}
+
+		data := url.Values{
+			"client_id":   {c.ClientID},
+			"device_code": {deviceCode.DeviceCode},
+			"grant_type":  {"urn:ietf:params:oauth:grant-type:device_code"},
+		}
+
+		req, err := http.NewRequestWithContext(ctx, "POST", c.TokenURL, strings.NewReader(data.Encode()))
+		if err != nil {
+			return "", fmt.Errorf("failed to create device-token request: %w", err)
+		}
+		req.Header.Set("Accept", "application/json")
+		req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+
+		resp, err := (&http.Client{Timeout: c.ClientTimeout}).Do(req)
+		if err != nil {
+			return "", fmt.Errorf("failed to poll device token: %w", err)
+		}
+
+		var tokenResp struct {
+			AccessToken string `json:"access_token"`
+			Error       string `json:"error"`
+			Description string `json:"error_description"`
+		}
+		decodeErr := json.NewDecoder(resp.Body).Decode(&tokenResp)
+		_ = resp.Body.Close()
+		if decodeErr != nil {
+			return "", fmt.Errorf("failed to decode device-token response: %w", decodeErr)
+		}
+
+		if tokenResp.AccessToken != "" {
+			return tokenResp.AccessToken, nil
+		}
+
+		switch tokenResp.Error {
+		case "authorization_pending":
+			continue
+		case "slow_down":
+			interval += 5 * time.Second
+			continue
+		case "expired_token":
+			return "", fmt.Errorf("device code expired; restart login")
+		case "access_denied":
+			return "", fmt.Errorf("github login denied")
+		case "":
+			return "", fmt.Errorf("device-token request failed with status %d", resp.StatusCode)
+		default:
+			if tokenResp.Description != "" {
+				return "", fmt.Errorf("device-token request failed: %s: %s", tokenResp.Error, tokenResp.Description)
+			}
+			return "", fmt.Errorf("device-token request failed: %s", tokenResp.Error)
+		}
+	}
+
+	return "", fmt.Errorf("timed out waiting for github device authorization")
+}
+
+// ExchangeGitHubToken converts a GitHub OAuth token into a Copilot API token.
+// It is a semantic wrapper over RefreshCopilotToken used by the login flow.
+func (c *CopilotOAuthClient) ExchangeGitHubToken(ctx context.Context, githubToken string) (*CopilotCredentials, error) {
+	return c.RefreshCopilotToken(ctx, githubToken)
+}
+
+// RefreshCopilotToken obtains a fresh short-lived Copilot token from GitHub.
+//
+// GitHub may return expires_at as either a Unix timestamp or RFC3339 string.
+// parseCopilotExpiry handles both forms and falls back to a conservative
+// 20-minute lifetime when the field is absent or unrecognized.
+func (c *CopilotOAuthClient) RefreshCopilotToken(ctx context.Context, githubToken string) (*CopilotCredentials, error) {
+	if ctx == nil {
+		ctx = context.Background()
+	}
+
+	req, err := http.NewRequestWithContext(ctx, "GET", c.CopilotURL, nil)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create copilot token request: %w", err)
+	}
+	req.Header.Set("Authorization", "token "+githubToken)
+	req.Header.Set("Accept", "application/json")
+	req.Header.Set("User-Agent", "kit")
+	req.Header.Set("X-GitHub-Api-Version", "2022-11-28")
+
+	resp, err := (&http.Client{Timeout: c.ClientTimeout}).Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("failed to request copilot token: %w", err)
+	}
+	defer func() { _ = resp.Body.Close() }()
+
+	if resp.StatusCode != http.StatusOK {
+		body, _ := io.ReadAll(resp.Body)
+		return nil, fmt.Errorf("copilot token request failed with status %d: %s", resp.StatusCode, string(body))
+	}
+
+	var tokenResp struct {
+		Token     string `json:"token"`
+		ExpiresAt any    `json:"expires_at"`
+	}
+	if err := json.NewDecoder(resp.Body).Decode(&tokenResp); err != nil {
+		return nil, fmt.Errorf("failed to decode copilot token response: %w", err)
+	}
+	if tokenResp.Token == "" {
+		return nil, fmt.Errorf("copilot token response missing token")
+	}
+
+	expiresAt := parseCopilotExpiry(tokenResp.ExpiresAt)
+	if expiresAt == 0 {
+		expiresAt = time.Now().Add(20 * time.Minute).Unix()
+	}
+
+	return &CopilotCredentials{
+		Type:               "oauth",
+		GitHubToken:        githubToken,
+		CopilotAccessToken: tokenResp.Token,
+		ExpiresAt:          expiresAt,
+		CreatedAt:          time.Now(),
+	}, nil
+}
+
+// parseCopilotExpiry normalizes GitHub's expires_at variants to a Unix second.
+func parseCopilotExpiry(value any) int64 {
+	switch v := value.(type) {
+	case float64:
+		return int64(v)
+	case string:
+		if parsed, err := strconv.ParseInt(v, 10, 64); err == nil {
+			return parsed
+		}
+		if parsed, err := time.Parse(time.RFC3339, v); err == nil {
+			return parsed.Unix()
+		}
+	}
+	return 0
+}
+
 // NewOpenAIOAuthClient creates a new OAuth client configured for OpenAI Codex OAuth.
 // This uses the public client ID for CLI applications with PKCE for security.
 func NewOpenAIOAuthClient() *OpenAIOAuthClient {

internal/auth/oauth_test.go

@@ -0,0 +1,124 @@
+package auth
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+)
+
+func TestCopilotStartDeviceFlow(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Method != http.MethodPost {
+			t.Fatalf("expected POST, got %s", r.Method)
+		}
+		if err := r.ParseForm(); err != nil {
+			t.Fatalf("ParseForm failed: %v", err)
+		}
+		if r.Form.Get("client_id") != "client-id" {
+			t.Fatalf("expected client id, got %q", r.Form.Get("client_id"))
+		}
+		if r.Form.Get("scope") != "read:user" {
+			t.Fatalf("expected scope, got %q", r.Form.Get("scope"))
+		}
+		_ = json.NewEncoder(w).Encode(map[string]any{
+			"device_code":      "device-code",
+			"user_code":        "USER-CODE",
+			"verification_uri": "https://github.com/login/device",
+			"expires_in":       600,
+			"interval":         1,
+		})
+	}))
+	defer server.Close()
+
+	client := NewCopilotOAuthClient()
+	client.ClientID = "client-id"
+	client.DeviceURL = server.URL
+
+	code, err := client.StartDeviceFlow(context.Background())
+	if err != nil {
+		t.Fatalf("StartDeviceFlow failed: %v", err)
+	}
+	if code.DeviceCode != "device-code" || code.UserCode != "USER-CODE" || code.Interval != 1 {
+		t.Fatalf("unexpected device code: %#v", code)
+	}
+}
+
+func TestCopilotPollDeviceToken(t *testing.T) {
+	polls := 0
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		polls++
+		if r.Method != http.MethodPost {
+			t.Fatalf("expected POST, got %s", r.Method)
+		}
+		if err := r.ParseForm(); err != nil {
+			t.Fatalf("ParseForm failed: %v", err)
+		}
+		if r.Form.Get("grant_type") != "urn:ietf:params:oauth:grant-type:device_code" {
+			t.Fatalf("unexpected grant type: %q", r.Form.Get("grant_type"))
+		}
+		if polls == 1 {
+			_ = json.NewEncoder(w).Encode(map[string]any{"error": "authorization_pending"})
+			return
+		}
+		_ = json.NewEncoder(w).Encode(map[string]any{"access_token": "github-token"})
+	}))
+	defer server.Close()
+
+	client := NewCopilotOAuthClient()
+	client.ClientID = "client-id"
+	client.TokenURL = server.URL
+	client.PollTimeout = 5 * time.Second
+	client.ClientTimeout = time.Second
+
+	token, err := client.PollDeviceToken(context.Background(), &CopilotDeviceCode{
+		DeviceCode: "device-code",
+		ExpiresIn:  10,
+		Interval:   1,
+	})
+	if err != nil {
+		t.Fatalf("PollDeviceToken failed: %v", err)
+	}
+	if token != "github-token" {
+		t.Fatalf("expected github-token, got %q", token)
+	}
+	if polls != 2 {
+		t.Fatalf("expected 2 polls, got %d", polls)
+	}
+}
+
+func TestCopilotRefreshToken(t *testing.T) {
+	expiresAt := time.Now().Add(time.Hour).Unix()
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Method != http.MethodGet {
+			t.Fatalf("expected GET, got %s", r.Method)
+		}
+		if r.Header.Get("Authorization") != "token github-token" {
+			t.Fatalf("unexpected authorization header: %q", r.Header.Get("Authorization"))
+		}
+		if r.Header.Get("User-Agent") != "kit" {
+			t.Fatalf("unexpected user agent: %q", r.Header.Get("User-Agent"))
+		}
+		_ = json.NewEncoder(w).Encode(map[string]any{
+			"token":      "copilot-token",
+			"expires_at": expiresAt,
+		})
+	}))
+	defer server.Close()
+
+	client := NewCopilotOAuthClient()
+	client.CopilotURL = server.URL
+
+	creds, err := client.RefreshCopilotToken(context.Background(), "github-token")
+	if err != nil {
+		t.Fatalf("RefreshCopilotToken failed: %v", err)
+	}
+	if creds.GitHubToken != "github-token" || creds.CopilotAccessToken != "copilot-token" {
+		t.Fatalf("unexpected credentials: %#v", creds)
+	}
+	if creds.ExpiresAt != expiresAt {
+		t.Fatalf("expected expires_at %d, got %d", expiresAt, creds.ExpiresAt)
+	}
+}

internal/config/config.go

@@ -22,6 +22,45 @@ type MCPServerConfig struct {
 	AllowedTools  []string          `json:"allowedTools,omitempty" yaml:"allowedTools,omitempty"`
 	ExcludedTools []string          `json:"excludedTools,omitempty" yaml:"excludedTools,omitempty"`
 
+	// OAuth configuration for remote servers that don't support dynamic
+	// client registration (e.g. GitHub). When OAuthClientID is set, it is
+	// passed directly to the transport's OAuthConfig instead of relying on
+	// dynamic registration.
+	OAuthClientID     string   `json:"oauthClientId,omitempty" yaml:"oauthClientId,omitempty"`
+	OAuthClientSecret string   `json:"oauthClientSecret,omitempty" yaml:"oauthClientSecret,omitempty"`
+	OAuthScopes       []string `json:"oauthScopes,omitempty" yaml:"oauthScopes,omitempty"`
+
+	// NoOAuth disables OAuth transport configuration for this server, even
+	// when the connection pool has an auth handler. Use this for public MCP
+	// servers (e.g. PubMed) that don't require authentication. Without this
+	// flag, the pool would attach OAuth transport to every remote server,
+	// causing proactive dynamic-client-registration attempts that fail on
+	// servers that don't support it.
+	NoOAuth bool `json:"noOAuth,omitempty" yaml:"noOAuth,omitempty"`
+
+	// TasksMode controls when this server's tools/call requests are augmented
+	// with MCP task metadata (turning a synchronous call into an asynchronous,
+	// pollable job — see https://modelcontextprotocol.io/specification/2025-11-25/basic/utilities/tasks).
+	//
+	// Valid values:
+	//   - "" or "auto": (default) augment requests with task metadata only
+	//     when the server advertises tasks/toolCalls capability during initialize.
+	//   - "never":      never augment — every tool call is synchronous, regardless
+	//     of server capability.
+	//   - "always":     always augment, even when the server didn't advertise
+	//     task support. The server may still respond synchronously; this just
+	//     opts in unconditionally on the client side.
+	//
+	// In all modes, when the server returns a CreateTaskResult the client polls
+	// tasks/get / tasks/result until the task reaches a terminal state.
+	TasksMode string `json:"tasksMode,omitempty" yaml:"tasksMode,omitempty"`
+
+	// InProcessServer holds a live *server.MCPServer for in-process transport.
+	// When set (and Type is "inprocess"), the connection pool creates an
+	// in-process client instead of spawning a subprocess or making HTTP calls.
+	// This field is never serialized — it is only used programmatically via the SDK.
+	InProcessServer any `json:"-" yaml:"-"`
+
 	// Legacy fields for backward compatibility
 	Transport string         `json:"transport,omitempty"`
 	Args      []string       `json:"args,omitempty"`
@@ -35,13 +74,18 @@ type MCPServerConfig struct {
 func (s *MCPServerConfig) UnmarshalJSON(data []byte) error {
 	// First try to unmarshal as the new format
 	type newFormat struct {
-		Type          string            `json:"type"`
-		Command       []string          `json:"command,omitempty"`
-		Environment   map[string]string `json:"environment,omitempty"`
-		URL           string            `json:"url,omitempty"`
-		Headers       []string          `json:"headers,omitempty"`
-		AllowedTools  []string          `json:"allowedTools,omitempty" yaml:"allowedTools,omitempty"`
-		ExcludedTools []string          `json:"excludedTools,omitempty" yaml:"excludedTools,omitempty"`
+		Type              string            `json:"type"`
+		Command           []string          `json:"command,omitempty"`
+		Environment       map[string]string `json:"environment,omitempty"`
+		URL               string            `json:"url,omitempty"`
+		Headers           []string          `json:"headers,omitempty"`
+		AllowedTools      []string          `json:"allowedTools,omitempty" yaml:"allowedTools,omitempty"`
+		ExcludedTools     []string          `json:"excludedTools,omitempty" yaml:"excludedTools,omitempty"`
+		OAuthClientID     string            `json:"oauthClientId,omitempty" yaml:"oauthClientId,omitempty"`
+		OAuthClientSecret string            `json:"oauthClientSecret,omitempty" yaml:"oauthClientSecret,omitempty"`
+		OAuthScopes       []string          `json:"oauthScopes,omitempty" yaml:"oauthScopes,omitempty"`
+		NoOAuth           bool              `json:"noOAuth,omitempty" yaml:"noOAuth,omitempty"`
+		TasksMode         string            `json:"tasksMode,omitempty" yaml:"tasksMode,omitempty"`
 	}
 
 	// Also try legacy format
@@ -54,6 +98,7 @@ func (s *MCPServerConfig) UnmarshalJSON(data []byte) error {
 		Headers       []string       `json:"headers,omitempty"`
 		AllowedTools  []string       `json:"allowedTools,omitempty" yaml:"allowedTools,omitempty"`
 		ExcludedTools []string       `json:"excludedTools,omitempty" yaml:"excludedTools,omitempty"`
+		TasksMode     string         `json:"tasksMode,omitempty" yaml:"tasksMode,omitempty"`
 	}
 
 	// Try new format first
@@ -66,6 +111,11 @@ func (s *MCPServerConfig) UnmarshalJSON(data []byte) error {
 		s.Headers = newConfig.Headers
 		s.AllowedTools = newConfig.AllowedTools
 		s.ExcludedTools = newConfig.ExcludedTools
+		s.OAuthClientID = newConfig.OAuthClientID
+		s.OAuthClientSecret = newConfig.OAuthClientSecret
+		s.OAuthScopes = newConfig.OAuthScopes
+		s.NoOAuth = newConfig.NoOAuth
+		s.TasksMode = newConfig.TasksMode
 		return nil
 	}
 
@@ -86,6 +136,7 @@ func (s *MCPServerConfig) UnmarshalJSON(data []byte) error {
 	s.Headers = legacyConfig.Headers
 	s.AllowedTools = legacyConfig.AllowedTools
 	s.ExcludedTools = legacyConfig.ExcludedTools
+	s.TasksMode = legacyConfig.TasksMode
 
 	// Infer type from legacy format for better compatibility
 	// Only set Type when it doesn't change existing transport behavior
@@ -157,6 +208,21 @@ type Theme struct {
 	Markdown MarkdownThemeConfig `json:"markdown,omitzero" yaml:"markdown,omitempty"`
 }
 
+// GenerationParams defines generation parameter defaults that can be attached
+// to individual models. These act as model-level defaults — CLI flags and
+// global config values take precedence when explicitly set.
+type GenerationParams struct {
+	MaxTokens        *int     `json:"maxTokens,omitempty" yaml:"maxTokens,omitempty"`
+	Temperature      *float32 `json:"temperature,omitempty" yaml:"temperature,omitempty"`
+	TopP             *float32 `json:"topP,omitempty" yaml:"topP,omitempty"`
+	TopK             *int32   `json:"topK,omitempty" yaml:"topK,omitempty"`
+	FrequencyPenalty *float32 `json:"frequencyPenalty,omitempty" yaml:"frequencyPenalty,omitempty"`
+	PresencePenalty  *float32 `json:"presencePenalty,omitempty" yaml:"presencePenalty,omitempty"`
+	StopSequences    []string `json:"stopSequences,omitempty" yaml:"stopSequences,omitempty"`
+	ThinkingLevel    string   `json:"thinkingLevel,omitempty" yaml:"thinkingLevel,omitempty"`
+	SystemPrompt     string   `json:"systemPrompt,omitempty" yaml:"systemPrompt,omitempty"`
+}
+
 // CustomModelConfig defines a custom model that can be used with custom/custom
 // or other custom/ prefixed models. These models are loaded from the config file
 // and merged into the custom provider in the model registry.
@@ -171,6 +237,11 @@ type CustomModelConfig struct {
 	Knowledge   string      `json:"knowledge,omitempty" yaml:"knowledge,omitempty"`
 	Cost        CostConfig  `json:"cost" yaml:"cost"`
 	Limit       LimitConfig `json:"limit" yaml:"limit"`
+
+	// Generation parameter defaults for this model.
+	// These are applied when the user hasn't explicitly set the corresponding
+	// CLI flag or global config value.
+	Params GenerationParams `json:"params,omitzero" yaml:"params,omitempty"`
 }
 
 // CostConfig defines the pricing for a custom model.
@@ -199,11 +270,13 @@ type Config struct {
 	Stream         *bool                      `json:"stream,omitempty" yaml:"stream,omitempty"`
 	Theme          any                        `json:"theme" yaml:"theme"`
 	// Model generation parameters
-	MaxTokens     int      `json:"max-tokens,omitempty" yaml:"max-tokens,omitempty"`
-	Temperature   *float32 `json:"temperature,omitempty" yaml:"temperature,omitempty"`
-	TopP          *float32 `json:"top-p,omitempty" yaml:"top-p,omitempty"`
-	TopK          *int32   `json:"top-k,omitempty" yaml:"top-k,omitempty"`
-	StopSequences []string `json:"stop-sequences,omitempty" yaml:"stop-sequences,omitempty"`
+	MaxTokens        int      `json:"max-tokens,omitempty" yaml:"max-tokens,omitempty"`
+	Temperature      *float32 `json:"temperature,omitempty" yaml:"temperature,omitempty"`
+	TopP             *float32 `json:"top-p,omitempty" yaml:"top-p,omitempty"`
+	TopK             *int32   `json:"top-k,omitempty" yaml:"top-k,omitempty"`
+	FrequencyPenalty *float32 `json:"frequency-penalty,omitempty" yaml:"frequency-penalty,omitempty"`
+	PresencePenalty  *float32 `json:"presence-penalty,omitempty" yaml:"presence-penalty,omitempty"`
+	StopSequences    []string `json:"stop-sequences,omitempty" yaml:"stop-sequences,omitempty"`
 
 	// Thinking / extended reasoning
 	ThinkingLevel string `json:"thinking-level,omitempty" yaml:"thinking-level,omitempty"`
@@ -217,6 +290,12 @@ type Config struct {
 
 	// Custom model definitions (under custom/ provider)
 	CustomModels map[string]CustomModelConfig `json:"customModels,omitempty" yaml:"customModels,omitempty"`
+
+	// Per-model generation parameter overrides. Keys are "provider/model" strings
+	// (e.g. "anthropic/claude-sonnet-4-5-20250929", "openai/gpt-4o"). These
+	// settings act as model-level defaults — CLI flags and global config values
+	// take precedence when explicitly set.
+	ModelSettings map[string]GenerationParams `json:"modelSettings,omitempty" yaml:"modelSettings,omitempty"`
 }
 
 // GetTransportType returns the transport type for the server config, mapping
@@ -235,11 +314,18 @@ func (s *MCPServerConfig) GetTransportType() string {
 			return "stdio"
 		case "remote":
 			return "streamable"
+		case "inprocess":
+			return "inprocess"
 		default:
 			return s.Type
 		}
 	}
 
+	// Programmatic in-process server detection.
+	if s.InProcessServer != nil {
+		return "inprocess"
+	}
+
 	// Backward compatibility: infer transport type
 	if len(s.Command) > 0 {
 		return "stdio"
@@ -259,6 +345,17 @@ func (c *Config) Validate() error {
 			return fmt.Errorf("server %s: allowedTools and excludedTools are mutually exclusive", serverName)
 		}
 
+		// Reject unknown tasksMode values up front so a typo (e.g. "alwasy")
+		// fails loud here instead of being silently downgraded to "auto" by
+		// the runtime parser. Comparison is case-insensitive to match
+		// tools.ParseTaskMode.
+		switch strings.ToLower(strings.TrimSpace(serverConfig.TasksMode)) {
+		case "", "auto", "never", "always":
+			// ok
+		default:
+			return fmt.Errorf("server %s: invalid tasksMode %q (expected one of: auto, never, always)", serverName, serverConfig.TasksMode)
+		}
+
 		transport := serverConfig.GetTransportType()
 		switch transport {
 		case "stdio":
@@ -270,8 +367,12 @@ func (c *Config) Validate() error {
 			if serverConfig.URL == "" {
 				return fmt.Errorf("server %s: url is required for %s transport", serverName, transport)
 			}
+		case "inprocess":
+			if serverConfig.InProcessServer == nil {
+				return fmt.Errorf("server %s: InProcessServer is required for inprocess transport", serverName)
+			}
 		default:
-			return fmt.Errorf("server %s: unsupported transport type '%s'. Supported types: stdio, sse, streamable", serverName, transport)
+			return fmt.Errorf("server %s: unsupported transport type '%s'. Supported types: stdio, sse, streamable, inprocess", serverName, transport)
 		}
 	}
 	return nil
@@ -365,16 +466,61 @@ mcpServers:
 # debug: false                                 # Enable debug logging
 # system-prompt: "/path/to/system-prompt.txt" # System prompt text file
 
-# Model generation parameters (all optional)
+# Model generation parameters (all optional, apply globally to all models)
 # max-tokens: 4096                             # Maximum tokens in response
 # temperature: 0.7                             # Randomness (0.0-1.0)
 # top-p: 0.95                                  # Nucleus sampling (0.0-1.0)
 # top-k: 40                                    # Top K sampling
+# frequency-penalty: 0.0                        # Penalize frequent tokens (0.0-2.0)
+# presence-penalty: 0.0                         # Penalize present tokens (0.0-2.0)
 # stop-sequences: ["Human:", "Assistant:"]     # Custom stop sequences
 
+# Per-model generation parameter overrides (apply to specific models)
+# These act as model-level defaults — CLI flags and global settings above take precedence.
+# Keys are "provider/model" strings matching the model you use.
+# modelSettings:
+#   anthropic/claude-sonnet-4-5-20250929:
+#     temperature: 0.3
+#     maxTokens: 8192
+#   openai/gpt-4o:
+#     temperature: 0.7
+#     topP: 0.95
+#     topK: 40
+#     frequencyPenalty: 0.1
+#     presencePenalty: 0.1
+#   anthropic/claude-opus-4-6:
+#     thinkingLevel: "high"
+#     maxTokens: 16384
+#     systemPrompt: "You are a deep reasoning assistant."  # or a file path
+
+# Skills configuration (all optional)
+# no-skills: false                          # Set to true to disable all skill loading
+# skill:                                    # Explicit skill files/dirs (disables auto-discovery)
+#   - "/path/to/skill.md"
+# skills-dir: "/path/to/skills"            # Override project-local directory for auto-discovery
+
 # API Configuration (can also use environment variables)
 # provider-api-key: "your-api-key"         # API key for OpenAI, Anthropic, or Google
 # provider-url: "https://api.openai.com/v1" # Base URL for OpenAI, Anthropic, or Ollama
+
+# Custom model definitions (under custom/ provider)
+# customModels:
+#   my-local-llama:
+#     name: "Local Llama 3"
+#     baseUrl: "http://localhost:8080/v1"
+#     family: "llama"
+#     temperature: true
+#     cost:
+#       input: 0.0
+#       output: 0.0
+#     limit:
+#       context: 131072
+#       output: 8192
+#     params:                              # Generation parameter defaults for this model
+#       temperature: 0.8
+#       topP: 0.95
+#       topK: 40
+#       systemPrompt: "You are a helpful local assistant."
 `
 
 	_, err = file.WriteString(content)

internal/config/config_test.go

@@ -6,6 +6,8 @@ import (
 	"path/filepath"
 	"strings"
 	"testing"
+
+	"gopkg.in/yaml.v3"
 )
 
 func TestMCPServerConfig_NewFormat(t *testing.T) {
@@ -203,6 +205,9 @@ func TestEnsureConfigExists(t *testing.T) {
 		"type: \"local\"",
 		"type: \"remote\"",
 		"Core tools",
+		"# Skills configuration",
+		"no-skills:",
+		"skills-dir:",
 	}
 
 	for _, expected := range expectedSections {
@@ -542,3 +547,175 @@ func TestEnsureConfigExistsWhenFileExists(t *testing.T) {
 		t.Error("Existing config file was modified when it shouldn't have been")
 	}
 }
+
+func TestMCPServerConfig_OAuthFields_JSON(t *testing.T) {
+	jsonData := `{
+		"type": "remote",
+		"url": "https://api.githubcopilot.com/mcp/",
+		"oauthClientId": "Ov23liXXXXXXXXXXXXXX",
+		"oauthClientSecret": "secret123",
+		"oauthScopes": ["read:user", "repo"]
+	}`
+
+	var cfg MCPServerConfig
+	err := json.Unmarshal([]byte(jsonData), &cfg)
+	if err != nil {
+		t.Fatalf("Failed to unmarshal: %v", err)
+	}
+
+	if cfg.Type != "remote" {
+		t.Errorf("Expected type 'remote', got %q", cfg.Type)
+	}
+	if cfg.URL != "https://api.githubcopilot.com/mcp/" {
+		t.Errorf("Expected URL, got %q", cfg.URL)
+	}
+	if cfg.OAuthClientID != "Ov23liXXXXXXXXXXXXXX" {
+		t.Errorf("Expected OAuthClientID 'Ov23liXXXXXXXXXXXXXX', got %q", cfg.OAuthClientID)
+	}
+	if cfg.OAuthClientSecret != "secret123" {
+		t.Errorf("Expected OAuthClientSecret 'secret123', got %q", cfg.OAuthClientSecret)
+	}
+	if len(cfg.OAuthScopes) != 2 || cfg.OAuthScopes[0] != "read:user" || cfg.OAuthScopes[1] != "repo" {
+		t.Errorf("Expected OAuthScopes [read:user, repo], got %v", cfg.OAuthScopes)
+	}
+}
+
+func TestMCPServerConfig_OAuthFields_YAML(t *testing.T) {
+	yamlData := `
+type: remote
+url: https://api.githubcopilot.com/mcp/
+oauthClientId: "Ov23liXXXXXXXXXXXXXX"
+oauthScopes:
+  - read:user
+  - repo
+`
+
+	var cfg MCPServerConfig
+	err := yaml.Unmarshal([]byte(yamlData), &cfg)
+	if err != nil {
+		t.Fatalf("Failed to unmarshal YAML: %v", err)
+	}
+
+	if cfg.Type != "remote" {
+		t.Errorf("Expected type 'remote', got %q", cfg.Type)
+	}
+	if cfg.OAuthClientID != "Ov23liXXXXXXXXXXXXXX" {
+		t.Errorf("Expected OAuthClientID 'Ov23liXXXXXXXXXXXXXX', got %q", cfg.OAuthClientID)
+	}
+	if len(cfg.OAuthScopes) != 2 || cfg.OAuthScopes[0] != "read:user" || cfg.OAuthScopes[1] != "repo" {
+		t.Errorf("Expected OAuthScopes [read:user, repo], got %v", cfg.OAuthScopes)
+	}
+}
+
+func TestMCPServerConfig_OAuthFields_Omitted(t *testing.T) {
+	// Verify that omitting OAuth fields still works (backward compat).
+	jsonData := `{
+		"type": "remote",
+		"url": "https://example.com/mcp"
+	}`
+
+	var cfg MCPServerConfig
+	err := json.Unmarshal([]byte(jsonData), &cfg)
+	if err != nil {
+		t.Fatalf("Failed to unmarshal: %v", err)
+	}
+
+	if cfg.OAuthClientID != "" {
+		t.Errorf("Expected empty OAuthClientID, got %q", cfg.OAuthClientID)
+	}
+	if cfg.OAuthClientSecret != "" {
+		t.Errorf("Expected empty OAuthClientSecret, got %q", cfg.OAuthClientSecret)
+	}
+	if len(cfg.OAuthScopes) != 0 {
+		t.Errorf("Expected empty OAuthScopes, got %v", cfg.OAuthScopes)
+	}
+}
+
+func TestMCPServerConfig_TasksMode_NewFormat(t *testing.T) {
+	jsonData := `{
+		"type": "remote",
+		"url": "https://my-mcp-server.com",
+		"tasksMode": "always"
+	}`
+	var cfg MCPServerConfig
+	if err := json.Unmarshal([]byte(jsonData), &cfg); err != nil {
+		t.Fatalf("Failed to unmarshal: %v", err)
+	}
+	if cfg.TasksMode != "always" {
+		t.Errorf("expected TasksMode 'always', got %q", cfg.TasksMode)
+	}
+}
+
+func TestMCPServerConfig_TasksMode_LegacyFormat(t *testing.T) {
+	// tasksMode also recognised in the legacy unmarshal path so users on
+	// the older command/args shape can opt in without migrating.
+	jsonData := `{
+		"command": "npx",
+		"args": ["@modelcontextprotocol/server-filesystem", "/path"],
+		"tasksMode": "never"
+	}`
+	var cfg MCPServerConfig
+	if err := json.Unmarshal([]byte(jsonData), &cfg); err != nil {
+		t.Fatalf("Failed to unmarshal: %v", err)
+	}
+	if cfg.TasksMode != "never" {
+		t.Errorf("expected TasksMode 'never', got %q", cfg.TasksMode)
+	}
+}
+
+func TestMCPServerConfig_TasksMode_DefaultEmpty(t *testing.T) {
+	// When tasksMode is not set the field stays empty, which downstream
+	// resolves to "auto" via tools.ParseTaskMode.
+	jsonData := `{"type":"remote","url":"https://x.example"}`
+	var cfg MCPServerConfig
+	if err := json.Unmarshal([]byte(jsonData), &cfg); err != nil {
+		t.Fatalf("Failed to unmarshal: %v", err)
+	}
+	if cfg.TasksMode != "" {
+		t.Errorf("expected default TasksMode to be empty, got %q", cfg.TasksMode)
+	}
+}
+
+func TestConfig_Validate_TasksMode(t *testing.T) {
+	t.Run("empty is valid", func(t *testing.T) {
+		cfg := &Config{
+			MCPServers: map[string]MCPServerConfig{
+				"a": {Type: "remote", URL: "https://x.example"},
+			},
+		}
+		if err := cfg.Validate(); err != nil {
+			t.Errorf("empty TasksMode should validate, got %v", err)
+		}
+	})
+
+	t.Run("known values are valid", func(t *testing.T) {
+		for _, mode := range []string{"auto", "never", "always", "AUTO", " always "} {
+			cfg := &Config{
+				MCPServers: map[string]MCPServerConfig{
+					"a": {Type: "remote", URL: "https://x.example", TasksMode: mode},
+				},
+			}
+			if err := cfg.Validate(); err != nil {
+				t.Errorf("TasksMode=%q should validate, got %v", mode, err)
+			}
+		}
+	})
+
+	t.Run("typo is rejected with a clear error", func(t *testing.T) {
+		cfg := &Config{
+			MCPServers: map[string]MCPServerConfig{
+				"buildbot": {Type: "remote", URL: "https://x.example", TasksMode: "alwasy"},
+			},
+		}
+		err := cfg.Validate()
+		if err == nil {
+			t.Fatal("expected validation error for invalid TasksMode")
+		}
+		// Error must mention the server name AND the bad value so the
+		// user knows where to look.
+		msg := err.Error()
+		if !strings.Contains(msg, "buildbot") || !strings.Contains(msg, `"alwasy"`) {
+			t.Errorf("error %q should mention both server name and bad value", msg)
+		}
+	})
+}

internal/config/merger.go

@@ -7,32 +7,48 @@ import (
 	"github.com/spf13/viper"
 )
 
-// LoadAndValidateConfig loads configuration from viper, fixes environment variable
-// casing issues, and validates the configuration. Returns an error if loading or
-// validation fails.
+// LoadAndValidateConfig loads configuration from the process-global viper
+// store, fixes environment variable casing issues, and validates the
+// configuration. Returns an error if loading or validation fails.
+//
+// This is a convenience wrapper around [LoadAndValidateConfigFrom] using the
+// shared global store; it is retained for the CLI and other callers that rely
+// on viper's process-global state.
 func LoadAndValidateConfig() (*Config, error) {
+	return LoadAndValidateConfigFrom(viper.GetViper())
+}
+
+// LoadAndValidateConfigFrom loads configuration from the supplied per-instance
+// store, fixes environment variable casing issues, and validates the
+// configuration. When v is nil, the process-global store is used. Threading an
+// explicit store lets each Kit instance own an isolated configuration without
+// clobbering other instances in the same process.
+func LoadAndValidateConfigFrom(v *viper.Viper) (*Config, error) {
+	if v == nil {
+		v = viper.GetViper()
+	}
 	config := &Config{
 		MCPServers: make(map[string]MCPServerConfig),
 	}
-	if err := viper.Unmarshal(config); err != nil {
-		return nil, fmt.Errorf("failed to unmarshal config: %v", err)
+	if err := v.Unmarshal(config); err != nil {
+		return nil, fmt.Errorf("failed to unmarshal config: %w", err)
 	}
 
 	// Fix environment variable case sensitivity issue
 	// Viper lowercases all keys, but we need to preserve the original case for environment variables
-	fixEnvironmentCase(config)
+	fixEnvironmentCase(v, config)
 
 	if err := config.Validate(); err != nil {
-		return nil, fmt.Errorf("invalid config: %v", err)
+		return nil, fmt.Errorf("invalid config: %w", err)
 	}
 
 	return config, nil
 }
 
 // fixEnvironmentCase fixes the case of environment variable keys that were lowercased by Viper
-func fixEnvironmentCase(config *Config) {
+func fixEnvironmentCase(v *viper.Viper, config *Config) {
 	// Get the raw config data from viper
-	rawConfig := viper.AllSettings()
+	rawConfig := v.AllSettings()
 
 	// Check if we have mcpServers in the raw config
 	if mcpServersRaw, ok := rawConfig["mcpservers"]; ok {

internal/config/substitution.go

@@ -56,9 +56,3 @@ func (e *EnvSubstituter) SubstituteEnvVars(content string) (string, error) {
 
 	return result, nil
 }
-
-// HasEnvVars checks if content contains environment variable patterns (${env://...}).
-// This is useful for determining if substitution is needed before processing.
-func HasEnvVars(content string) bool {
-	return envVarPattern.MatchString(content)
-}

internal/config/substitution_test.go

@@ -187,41 +187,3 @@ func TestEnvSubstituter_SubstituteEnvVars(t *testing.T) {
 		})
 	}
 }
-
-func TestHasEnvVars(t *testing.T) {
-	tests := []struct {
-		name     string
-		content  string
-		expected bool
-	}{
-		{
-			name:     "has env vars",
-			content:  `{"token": "${env://GITHUB_TOKEN}"}`,
-			expected: true,
-		},
-		{
-			name:     "has env vars with default",
-			content:  `{"debug": "${env://DEBUG:-false}"}`,
-			expected: true,
-		},
-		{
-			name:     "no env vars",
-			content:  `{"name": "${username}", "normal": "value"}`,
-			expected: false,
-		},
-		{
-			name:     "empty content",
-			content:  "",
-			expected: false,
-		},
-	}
-
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			result := HasEnvVars(tt.content)
-			if result != tt.expected {
-				t.Errorf("Expected %v, got %v", tt.expected, result)
-			}
-		})
-	}
-}

internal/core/bash.go

@@ -19,10 +19,18 @@ import (
 // It receives tool call ID, tool name, output chunk, and whether it's stderr.
 type ToolOutputCallback func(toolCallID, toolName, chunk string, isStderr bool)
 
+// PasswordPromptCallback is the signature for password prompts.
+// It receives a prompt message and returns the password and whether it was cancelled.
+type PasswordPromptCallback func(prompt string) (password string, cancelled bool)
+
 // contextKey is a custom type for context keys to avoid collisions.
 type contextKey string
 
-const toolOutputCallbackKey contextKey = "toolOutputCallback"
+const (
+	toolOutputCallbackKey contextKey = "toolOutputCallback"
+	sudoPasswordKey       contextKey = "sudoPassword"
+	passwordPromptKey     contextKey = "passwordPrompt"
+)
 
 // ContextWithToolOutputCallback returns a new context with the tool output callback set.
 func ContextWithToolOutputCallback(ctx context.Context, callback ToolOutputCallback) context.Context {
@@ -37,6 +45,28 @@ func toolOutputCallbackFromContext(ctx context.Context) ToolOutputCallback {
 	return nil
 }
 
+// ContextWithPasswordPrompt returns a new context with the password prompt callback set.
+// This allows the TUI to show a modal password prompt when sudo needs a password.
+func ContextWithPasswordPrompt(ctx context.Context, callback PasswordPromptCallback) context.Context {
+	return context.WithValue(ctx, passwordPromptKey, callback)
+}
+
+// passwordPromptFromContext retrieves the password prompt callback from context.
+func passwordPromptFromContext(ctx context.Context) PasswordPromptCallback {
+	if cb, ok := ctx.Value(passwordPromptKey).(PasswordPromptCallback); ok {
+		return cb
+	}
+	return nil
+}
+
+// sudoPasswordFromContext retrieves the sudo password from context.
+func sudoPasswordFromContext(ctx context.Context) string {
+	if pw, ok := ctx.Value(sudoPasswordKey).(string); ok {
+		return pw
+	}
+	return ""
+}
+
 const defaultBashTimeout = 120 * time.Second
 const maxBashTimeout = 600 * time.Second
 
@@ -73,6 +103,57 @@ func NewBashTool(opts ...ToolOption) fantasy.AgentTool {
 	}
 }
 
+// sudoCommandRe matches sudo commands that need to be rewritten for -S mode.
+// It matches "sudo" as a word boundary, optionally preceded by environment variables.
+var sudoCommandRe = regexp.MustCompile(`(?i)(^|[&|;|]|\|\||&&)\s*(\w+=\S+\s+)?\bsudo\b`)
+
+// truncateCommand truncates a long command for display.
+func truncateCommand(cmd string, maxLen int) string {
+	if len(cmd) <= maxLen {
+		return cmd
+	}
+	return cmd[:maxLen-3] + "..."
+}
+
+// rewriteSudoForStdin rewrites sudo commands to use -S -p ” for stdin password input.
+// It transforms: sudo cmd → sudo -S -p ” cmd
+func rewriteSudoForStdin(command string) string {
+	// Find all matches and their positions
+	matches := sudoCommandRe.FindAllStringIndex(command, -1)
+	if matches == nil {
+		return command
+	}
+
+	// Build result from end to start to preserve indices
+	result := command
+	for i := len(matches) - 1; i >= 0; i-- {
+		match := matches[i]
+		start, end := match[0], match[1]
+		matchedText := result[start:end]
+
+		// Extract just the "sudo" part (after any prefix)
+		sudoIdx := strings.Index(strings.ToLower(matchedText), "sudo")
+		if sudoIdx == -1 {
+			continue
+		}
+		prefix := matchedText[:sudoIdx]
+		sudoPart := matchedText[sudoIdx:]
+
+		// Check if the text immediately after "sudo" in the result contains -S
+		afterSudo := result[end:]
+		if strings.HasPrefix(strings.TrimLeft(afterSudo, " \t"), "-S") {
+			// Already has -S flag, skip
+			continue
+		}
+
+		// Insert -S -p '' after "sudo"
+		newSudo := strings.Replace(sudoPart, "sudo", "sudo -S -p ''", 1)
+		result = result[:start] + prefix + newSudo + result[end:]
+	}
+
+	return result
+}
+
 func executeBash(ctx context.Context, call fantasy.ToolCall, workDir string) (fantasy.ToolResponse, error) {
 	var args bashArgs
 	if err := parseArgs(call.Input, &args); err != nil {
@@ -97,7 +178,47 @@ func executeBash(ctx context.Context, call fantasy.ToolCall, workDir string) (fa
 	cmdCtx, cancel := context.WithTimeout(ctx, timeout)
 	defer cancel()
 
-	cmd := exec.CommandContext(cmdCtx, "bash", "-c", args.Command)
+	// Check for sudo password in context or environment
+	sudoPassword := sudoPasswordFromContext(ctx)
+	if sudoPassword == "" {
+		sudoPassword = os.Getenv("SUDO_PASSWORD")
+	}
+	command := args.Command
+
+	// If command contains sudo and we don't have a password, check if sudo needs one
+	if sudoPassword == "" && sudoCommandRe.MatchString(command) {
+		// Check if sudo credentials are cached using sudo -n (non-interactive)
+		testCmd := exec.CommandContext(cmdCtx, "sudo", "-n", "true")
+		testCmd.Dir = workDir
+		if err := testCmd.Run(); err != nil {
+			// Sudo needs a password - try to prompt via callback
+			if promptCallback := passwordPromptFromContext(ctx); promptCallback != nil {
+				pw, cancelled := promptCallback("Sudo password required for: " + truncateCommand(args.Command, 60))
+				if cancelled {
+					return fantasy.NewTextErrorResponse("sudo password prompt cancelled"), nil
+				}
+				if pw == "" {
+					return fantasy.NewTextErrorResponse("no sudo password provided"), nil
+				}
+				sudoPassword = pw
+				command = rewriteSudoForStdin(command)
+			} else {
+				// No callback available - return error with helpful message
+				return fantasy.NewTextErrorResponse(
+					"This command requires sudo access. " +
+						"Please run 'sudo -v' in your terminal first to cache credentials, " +
+						"or set the SUDO_PASSWORD environment variable."), nil
+			}
+		}
+		// Credentials are cached or password was provided, proceed
+	}
+
+	// If we have a sudo password, rewrite the command to use sudo -S
+	if sudoPassword != "" && sudoCommandRe.MatchString(command) {
+		command = rewriteSudoForStdin(command)
+	}
+
+	cmd := exec.CommandContext(cmdCtx, "bash", "-c", command)
 	if workDir != "" {
 		cmd.Dir = workDir
 	}
@@ -115,44 +236,94 @@ func executeBash(ctx context.Context, call fantasy.ToolCall, workDir string) (fa
 
 	if outputCallback != nil {
 		// Streaming mode: use pipes to capture output as it arrives
-		return executeBashStreaming(cmdCtx, call, cmd, outputCallback)
+		return executeBashStreaming(cmdCtx, call, cmd, outputCallback, sudoPassword)
 	}
 
 	// Non-streaming mode: collect all output at once (original behavior)
-	return executeBashBuffered(cmdCtx, call, cmd)
+	return executeBashBuffered(cmdCtx, call, cmd, sudoPassword)
+}
+
+// setupBashPipes opens stdout/stderr pipes (plus an optional sudo stdin),
+// starts the command, and asynchronously writes the sudo password if any.
+// Returns the readers ready for the caller to consume. If setup fails,
+// errResp is non-nil and the readers must not be used; the caller should
+// return the response directly.
+func setupBashPipes(cmd *exec.Cmd, sudoPassword string) (stdout, stderr io.Reader, errResp *fantasy.ToolResponse) {
+	stdoutPipe, err := cmd.StdoutPipe()
+	if err != nil {
+		r := fantasy.NewTextErrorResponse("failed to create stdout pipe")
+		return nil, nil, &r
+	}
+	stderrPipe, err := cmd.StderrPipe()
+	if err != nil {
+		r := fantasy.NewTextErrorResponse("failed to create stderr pipe")
+		return nil, nil, &r
+	}
+
+	var stdinPipe io.WriteCloser
+	if sudoPassword != "" {
+		stdinPipe, err = cmd.StdinPipe()
+		if err != nil {
+			r := fantasy.NewTextErrorResponse("failed to create stdin pipe")
+			return nil, nil, &r
+		}
+	}
+
+	if err := cmd.Start(); err != nil {
+		r := fantasy.NewTextErrorResponse(fmt.Sprintf("failed to start command: %v", err))
+		return nil, nil, &r
+	}
+
+	if sudoPassword != "" && stdinPipe != nil {
+		go func() {
+			defer func() { _ = stdinPipe.Close() }()
+			_, _ = io.WriteString(stdinPipe, sudoPassword+"\n")
+		}()
+	}
+
+	return stdoutPipe, stderrPipe, nil
+}
+
+// interpretBashExit decodes cmd.Wait()'s error into an exit code, mapping
+// context-deadline-exceeded to a friendly "command timed out" response.
+// errResp is non-nil only when the caller should short-circuit and return
+// it directly (e.g. timeout).
+func interpretBashExit(waitErr error, cmdCtx context.Context) (exitCode int, errResp *fantasy.ToolResponse) {
+	if waitErr == nil {
+		return 0, nil
+	}
+	if exitErr, ok := waitErr.(*exec.ExitError); ok {
+		return exitErr.ExitCode(), nil
+	}
+	if cmdCtx.Err() == context.DeadlineExceeded {
+		r := fantasy.NewTextErrorResponse("command timed out")
+		return 0, &r
+	}
+	return 0, nil
 }
 
 // executeBashBuffered collects all output before returning (original behavior).
 // It uses explicit pipes (not cmd.Stdout) so that cmd.WaitDelay can forcibly
 // close them when grandchild processes hold pipe handles open after the
 // direct child exits.
-func executeBashBuffered(cmdCtx context.Context, call fantasy.ToolCall, cmd *exec.Cmd) (fantasy.ToolResponse, error) {
-	stdoutPipe, err := cmd.StdoutPipe()
-	if err != nil {
-		return fantasy.NewTextErrorResponse("failed to create stdout pipe"), nil
-	}
-	stderrPipe, err := cmd.StderrPipe()
-	if err != nil {
-		return fantasy.NewTextErrorResponse("failed to create stderr pipe"), nil
-	}
-
-	if err := cmd.Start(); err != nil {
-		return fantasy.NewTextErrorResponse(fmt.Sprintf("failed to start command: %v", err)), nil
+func executeBashBuffered(cmdCtx context.Context, _ fantasy.ToolCall, cmd *exec.Cmd, sudoPassword string) (fantasy.ToolResponse, error) {
+	stdoutPipe, stderrPipe, errResp := setupBashPipes(cmd, sudoPassword)
+	if errResp != nil {
+		return *errResp, nil
 	}
 
 	// Read pipes concurrently
 	var wg sync.WaitGroup
 	var stdout, stderr strings.Builder
-	var stdoutErr, stderrErr error
 
 	wg.Add(2)
 	go func() {
 		defer wg.Done()
-		_, stdoutErr = io.Copy(&stdout, stdoutPipe)
+		_, _ = io.Copy(&stdout, stdoutPipe)
 	}()
 	go func() {
 		defer wg.Done()
-		_, stderrErr = io.Copy(&stderr, stderrPipe)
+		_, _ = io.Copy(&stderr, stderrPipe)
 	}()
 
 	// Wait for the process to exit first. cmd.WaitDelay ensures that if
@@ -163,37 +334,19 @@ func executeBashBuffered(cmdCtx context.Context, call fantasy.ToolCall, cmd *exe
 	// Wait for pipe readers to finish draining.
 	wg.Wait()
 
-	// Ignore pipe read errors caused by WaitDelay force-closing —
-	// we still have whatever was read before the close.
-	_ = stdoutErr
-	_ = stderrErr
-
-	exitCode := 0
-	if waitErr != nil {
-		if exitErr, ok := waitErr.(*exec.ExitError); ok {
-			exitCode = exitErr.ExitCode()
-		} else if cmdCtx.Err() == context.DeadlineExceeded {
-			return fantasy.NewTextErrorResponse("command timed out"), nil
-		}
+	exitCode, errResp := interpretBashExit(waitErr, cmdCtx)
+	if errResp != nil {
+		return *errResp, nil
 	}
 
 	return buildBashResponse(stdout.String(), stderr.String(), exitCode)
 }
 
 // executeBashStreaming streams output as it arrives via the callback.
-func executeBashStreaming(cmdCtx context.Context, call fantasy.ToolCall, cmd *exec.Cmd, outputCallback ToolOutputCallback) (fantasy.ToolResponse, error) {
-	stdoutPipe, err := cmd.StdoutPipe()
-	if err != nil {
-		return fantasy.NewTextErrorResponse("failed to create stdout pipe"), nil
-	}
-	stderrPipe, err := cmd.StderrPipe()
-	if err != nil {
-		return fantasy.NewTextErrorResponse("failed to create stderr pipe"), nil
-	}
-
-	// Start command execution
-	if err := cmd.Start(); err != nil {
-		return fantasy.NewTextErrorResponse(fmt.Sprintf("failed to start command: %v", err)), nil
+func executeBashStreaming(cmdCtx context.Context, call fantasy.ToolCall, cmd *exec.Cmd, outputCallback ToolOutputCallback, sudoPassword string) (fantasy.ToolResponse, error) {
+	stdoutPipe, stderrPipe, errResp := setupBashPipes(cmd, sudoPassword)
+	if errResp != nil {
+		return *errResp, nil
 	}
 
 	// Stream stdout and stderr concurrently
@@ -230,20 +383,16 @@ func executeBashStreaming(cmdCtx context.Context, call fantasy.ToolCall, cmd *ex
 	// Wait for the process to exit. cmd.WaitDelay ensures that if pipes
 	// remain open (held by grandchild processes), they'll be forcibly closed
 	// after the grace period, which unblocks the scanners above.
-	err = cmd.Wait()
+	waitErr := cmd.Wait()
 
 	// Wait for the pipe readers to finish draining. This will complete
 	// quickly since cmd.Wait() (with WaitDelay) has already ensured
 	// the pipes are closed.
 	wg.Wait()
 
-	exitCode := 0
-	if err != nil {
-		if exitErr, ok := err.(*exec.ExitError); ok {
-			exitCode = exitErr.ExitCode()
-		} else if cmdCtx.Err() == context.DeadlineExceeded {
-			return fantasy.NewTextErrorResponse("command timed out"), nil
-		}
+	exitCode, errResp := interpretBashExit(waitErr, cmdCtx)
+	if errResp != nil {
+		return *errResp, nil
 	}
 
 	return buildBashResponse(strings.Join(stdoutChunks, "\n"), strings.Join(stderrChunks, "\n"), exitCode)

internal/core/bash_test.go

@@ -127,3 +127,72 @@ func TestBash_EmptyCommand(t *testing.T) {
 		t.Fatal("expected error for empty command")
 	}
 }
+
+func TestRewriteSudoForStdin(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    string
+		expected string
+	}{
+		{
+			name:     "simple sudo",
+			input:    "sudo apt update",
+			expected: "sudo -S -p '' apt update",
+		},
+		{
+			name:     "sudo with env var",
+			input:    "DEBIAN_FRONTEND=noninteractive sudo apt update",
+			expected: "DEBIAN_FRONTEND=noninteractive sudo -S -p '' apt update",
+		},
+		{
+			name:     "sudo in pipeline",
+			input:    "echo test | sudo tee /etc/test.conf",
+			expected: "echo test | sudo -S -p '' tee /etc/test.conf",
+		},
+		{
+			name:     "sudo after &&",
+			input:    "apt update && sudo apt upgrade",
+			expected: "apt update && sudo -S -p '' apt upgrade",
+		},
+		{
+			name:     "already has -S flag",
+			input:    "sudo -S apt update",
+			expected: "sudo -S apt update",
+		},
+		{
+			name:     "no sudo",
+			input:    "apt update && apt upgrade",
+			expected: "apt update && apt upgrade",
+		},
+		{
+			name:     "sudo in string (should not match)",
+			input:    "echo 'use sudo carefully'",
+			expected: "echo 'use sudo carefully'",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := rewriteSudoForStdin(tt.input)
+			if result != tt.expected {
+				t.Errorf("rewriteSudoForStdin(%q) = %q, want %q", tt.input, result, tt.expected)
+			}
+		})
+	}
+}
+
+func TestSudoPasswordFromContext(t *testing.T) {
+	// Test with password in context
+	ctx := context.WithValue(context.Background(), sudoPasswordKey, "secret123")
+	pw := sudoPasswordFromContext(ctx)
+	if pw != "secret123" {
+		t.Errorf("expected password 'secret123', got %q", pw)
+	}
+
+	// Test without password
+	ctx = context.Background()
+	pw = sudoPasswordFromContext(ctx)
+	if pw != "" {
+		t.Errorf("expected empty password, got %q", pw)
+	}
+}

internal/core/edit.go

@@ -21,12 +21,9 @@ type Edit struct {
 }
 
 // editArgs holds the arguments for the edit tool.
-// Supports both single-edit mode (old_text/new_text) and multi-edit mode (edits array).
 type editArgs struct {
-	Path    string `json:"path"`
-	OldText string `json:"old_text"` // Single-edit mode
-	NewText string `json:"new_text"` // Single-edit mode
-	Edits   []Edit `json:"edits"`    // Multi-edit mode
+	Path  string `json:"path"`
+	Edits []Edit `json:"edits"`
 }
 
 // replacement represents a normalized edit ready for processing.
@@ -52,20 +49,12 @@ func NewEditTool(opts ...ToolOption) fantasy.AgentTool {
 	return &coreTool{
 		info: fantasy.ToolInfo{
 			Name:        "edit",
-			Description: "Edit a file by replacing exact text. Supports single edit via old_text/new_text, or multiple edits via the edits array. All edits in the array are matched against the original file content (non-incremental) and must be non-overlapping.",
+			Description: "Edit a file by replacing exact text. All edits in the array are matched against the original file content (non-incremental) and must be non-overlapping.",
 			Parameters: map[string]any{
 				"path": map[string]any{
 					"type":        "string",
 					"description": "Path to the file to edit (relative or absolute)",
 				},
-				"old_text": map[string]any{
-					"type":        "string",
-					"description": "Exact text to find and replace (single-edit mode). Must not be used with 'edits' array.",
-				},
-				"new_text": map[string]any{
-					"type":        "string",
-					"description": "New text to replace the old text with (single-edit mode). Must not be used with 'edits' array.",
-				},
 				"edits": map[string]any{
 					"type":        "array",
 					"description": "Array of edits for multi-region replacement. Each edit must have unique, non-overlapping old_text. All matches are against the original file content.",
@@ -85,7 +74,7 @@ func NewEditTool(opts ...ToolOption) fantasy.AgentTool {
 					},
 				},
 			},
-			Required: []string{"path"},
+			Required: []string{"path", "edits"},
 		},
 		handler: func(ctx context.Context, call fantasy.ToolCall) (fantasy.ToolResponse, error) {
 			return executeEdit(ctx, call, cfg.WorkDir)
@@ -94,6 +83,9 @@ func NewEditTool(opts ...ToolOption) fantasy.AgentTool {
 }
 
 func executeEdit(ctx context.Context, call fantasy.ToolCall, workDir string) (fantasy.ToolResponse, error) {
+	if err := ctx.Err(); err != nil {
+		return fantasy.ToolResponse{}, err
+	}
 	var args editArgs
 	if err := parseArgs(call.Input, &args); err != nil {
 		return fantasy.NewTextErrorResponse("failed to parse arguments: " + err.Error()), nil
@@ -163,36 +155,11 @@ func executeEdit(ctx context.Context, call fantasy.ToolCall, workDir string) (fa
 }
 
 // normalizeEditInput validates and normalizes the edit input.
-// Returns error if both single-edit and multi-edit modes are used.
 func normalizeEditInput(args editArgs) ([]replacement, error) {
-	singleMode := args.OldText != "" || args.NewText != ""
-	multiMode := len(args.Edits) > 0
-
-	if singleMode && multiMode {
-		return nil, fmt.Errorf("cannot use old_text/new_text together with edits array")
+	if len(args.Edits) == 0 {
+		return nil, fmt.Errorf("edits array is required and must not be empty")
 	}
 
-	if !singleMode && !multiMode {
-		return nil, fmt.Errorf("must provide either old_text/new_text or edits array")
-	}
-
-	if singleMode {
-		if args.OldText == "" {
-			return nil, fmt.Errorf("old_text is required when using single-edit mode")
-		}
-		if args.NewText == "" {
-			return nil, fmt.Errorf("new_text is required when using single-edit mode")
-		}
-		return []replacement{{
-			oldText:     strings.ReplaceAll(args.OldText, "\r\n", "\n"),
-			newText:     strings.ReplaceAll(args.NewText, "\r\n", "\n"),
-			originalOld: args.OldText,
-			originalNew: args.NewText,
-			index:       0,
-		}}, nil
-	}
-
-	// Multi-edit mode
 	var reps []replacement
 	for i, edit := range args.Edits {
 		if edit.OldText == "" {

internal/core/edit_test.go

@@ -389,9 +389,11 @@ func TestExecuteEdit_ExactMatch(t *testing.T) {
 	writeFileOrFail(t, path, original)
 
 	input, _ := json.Marshal(editArgs{
-		Path:    path,
-		OldText: "fmt.Println(\"hello\")",
-		NewText: "fmt.Println(\"world\")",
+		Path: path,
+		Edits: []Edit{{
+			OldText: "fmt.Println(\"hello\")",
+			NewText: "fmt.Println(\"world\")",
+		}},
 	})
 
 	resp, err := executeEdit(t.Context(), fantasy.ToolCall{Input: string(input)}, dir)
@@ -426,9 +428,11 @@ func TestExecuteEdit_ExactMatch_DoesNotCorruptRest(t *testing.T) {
 	target := lines[49]
 	replacement := "REPLACED_LINE_50"
 	input, _ := json.Marshal(editArgs{
-		Path:    path,
-		OldText: target,
-		NewText: replacement,
+		Path: path,
+		Edits: []Edit{{
+			OldText: target,
+			NewText: replacement,
+		}},
 	})
 
 	resp, err := executeEdit(t.Context(), fantasy.ToolCall{Input: string(input)}, dir)
@@ -470,9 +474,11 @@ func TestExecuteEdit_FuzzyMatch_TrailingWhitespace(t *testing.T) {
 
 	// Search without trailing whitespace (common LLM behavior)
 	input, _ := json.Marshal(editArgs{
-		Path:    path,
-		OldText: "func foo() {\n\treturn 1\n}",
-		NewText: "func foo() {\n\treturn 2\n}",
+		Path: path,
+		Edits: []Edit{{
+			OldText: "func foo() {\n\treturn 1\n}",
+			NewText: "func foo() {\n\treturn 2\n}",
+		}},
 	})
 
 	resp, err := executeEdit(t.Context(), fantasy.ToolCall{Input: string(input)}, dir)
@@ -519,9 +525,11 @@ func TestExecuteEdit_FuzzyMatch_DoesNotCorruptRest(t *testing.T) {
 	search := strings.Repeat("x", 10) + "\n" + strings.Repeat("x", 10)
 	// But this matches lines 1-2, 2-3, etc. — should fail due to ambiguity.
 	input, _ := json.Marshal(editArgs{
-		Path:    path,
-		OldText: search,
-		NewText: "REPLACED",
+		Path: path,
+		Edits: []Edit{{
+			OldText: search,
+			NewText: "REPLACED",
+		}},
 	})
 
 	resp, err := executeEdit(t.Context(), fantasy.ToolCall{Input: string(input)}, dir)
@@ -546,9 +554,11 @@ func TestExecuteEdit_MultipleMatches_Fails(t *testing.T) {
 	writeFileOrFail(t, path, "hello\nworld\nhello\n")
 
 	input, _ := json.Marshal(editArgs{
-		Path:    path,
-		OldText: "hello",
-		NewText: "goodbye",
+		Path: path,
+		Edits: []Edit{{
+			OldText: "hello",
+			NewText: "goodbye",
+		}},
 	})
 
 	resp, err := executeEdit(t.Context(), fantasy.ToolCall{Input: string(input)}, dir)
@@ -575,9 +585,11 @@ func TestExecuteEdit_NoMatch_Fails(t *testing.T) {
 	writeFileOrFail(t, path, "hello world\n")
 
 	input, _ := json.Marshal(editArgs{
-		Path:    path,
-		OldText: "nonexistent text",
-		NewText: "replacement",
+		Path: path,
+		Edits: []Edit{{
+			OldText: "nonexistent text",
+			NewText: "replacement",
+		}},
 	})
 
 	resp, err := executeEdit(t.Context(), fantasy.ToolCall{Input: string(input)}, dir)
@@ -601,9 +613,11 @@ func TestExecuteEdit_CRLFNormalization(t *testing.T) {
 	writeFileOrFail(t, path, "line1\r\nline2\r\nline3\r\n")
 
 	input, _ := json.Marshal(editArgs{
-		Path:    path,
-		OldText: "line2",
-		NewText: "LINE2",
+		Path: path,
+		Edits: []Edit{{
+			OldText: "line2",
+			NewText: "LINE2",
+		}},
 	})
 
 	resp, err := executeEdit(t.Context(), fantasy.ToolCall{Input: string(input)}, dir)
@@ -622,8 +636,10 @@ func TestExecuteEdit_CRLFNormalization(t *testing.T) {
 
 func TestExecuteEdit_MissingPath(t *testing.T) {
 	input, _ := json.Marshal(editArgs{
-		OldText: "x",
-		NewText: "y",
+		Edits: []Edit{{
+			OldText: "x",
+			NewText: "y",
+		}},
 	})
 	resp, err := executeEdit(t.Context(), fantasy.ToolCall{Input: string(input)}, "")
 	if err != nil {
@@ -636,9 +652,11 @@ func TestExecuteEdit_MissingPath(t *testing.T) {
 
 func TestExecuteEdit_NonexistentFile(t *testing.T) {
 	input, _ := json.Marshal(editArgs{
-		Path:    "/tmp/nonexistent_edit_test_file_12345.go",
-		OldText: "x",
-		NewText: "y",
+		Path: "/tmp/nonexistent_edit_test_file_12345.go",
+		Edits: []Edit{{
+			OldText: "x",
+			NewText: "y",
+		}},
 	})
 	resp, err := executeEdit(t.Context(), fantasy.ToolCall{Input: string(input)}, "")
 	if err != nil {
@@ -661,9 +679,11 @@ func TestExecuteEdit_DiffContainsHunkHeader(t *testing.T) {
 	writeFileOrFail(t, path, strings.Join(lines, "\n")+"\n")
 
 	input, _ := json.Marshal(editArgs{
-		Path:    path,
-		OldText: "line_10_content",
-		NewText: "REPLACED",
+		Path: path,
+		Edits: []Edit{{
+			OldText: "line_10_content",
+			NewText: "REPLACED",
+		}},
 	})
 
 	resp, err := executeEdit(t.Context(), fantasy.ToolCall{Input: string(input)}, dir)
@@ -684,9 +704,11 @@ func TestExecuteEdit_MetadataContainsFileDiffs(t *testing.T) {
 	writeFileOrFail(t, path, "old content\n")
 
 	input, _ := json.Marshal(editArgs{
-		Path:    path,
-		OldText: "old content",
-		NewText: "new content",
+		Path: path,
+		Edits: []Edit{{
+			OldText: "old content",
+			NewText: "new content",
+		}},
 	})
 
 	resp, err := executeEdit(t.Context(), fantasy.ToolCall{Input: string(input)}, dir)
@@ -905,18 +927,14 @@ func TestExecuteEdit_MultiEdit_EmptyArray(t *testing.T) {
 	}
 }
 
-func TestExecuteEdit_MultiEdit_MixedWithSingleMode(t *testing.T) {
+func TestExecuteEdit_EmptyEditsArray_Fails(t *testing.T) {
 	dir := t.TempDir()
-	path := filepath.Join(dir, "mixed.txt")
+	path := filepath.Join(dir, "empty.txt")
 	writeFileOrFail(t, path, "hello\n")
 
-	input, _ := json.Marshal(map[string]any{
-		"path":     path,
-		"old_text": "hello",
-		"new_text": "HELLO",
-		"edits": []Edit{
-			{OldText: "hello", NewText: "HI"},
-		},
+	input, _ := json.Marshal(editArgs{
+		Path:  path,
+		Edits: []Edit{},
 	})
 
 	resp, err := executeEdit(t.Context(), fantasy.ToolCall{Input: string(input)}, dir)
@@ -924,10 +942,10 @@ func TestExecuteEdit_MultiEdit_MixedWithSingleMode(t *testing.T) {
 		t.Fatalf("executeEdit error: %v", err)
 	}
 	if !resp.IsError {
-		t.Error("expected error when mixing single and multi-edit modes")
+		t.Error("expected error for empty edits array")
 	}
-	if !strings.Contains(resp.Content, "cannot use") {
-		t.Errorf("expected 'cannot use' in error, got: %s", resp.Content)
+	if !strings.Contains(resp.Content, "required") {
+		t.Errorf("expected 'required' in error, got: %s", resp.Content)
 	}
 }
 

internal/core/ls.go

@@ -42,6 +42,9 @@ func NewLsTool(opts ...ToolOption) fantasy.AgentTool {
 }
 
 func executeLs(ctx context.Context, call fantasy.ToolCall, workDir string) (fantasy.ToolResponse, error) {
+	if err := ctx.Err(); err != nil {
+		return fantasy.ToolResponse{}, err
+	}
 	var args lsArgs
 	_ = parseArgs(call.Input, &args) // optional args
 

internal/core/read.go

@@ -47,6 +47,9 @@ func NewReadTool(opts ...ToolOption) fantasy.AgentTool {
 }
 
 func executeRead(ctx context.Context, call fantasy.ToolCall, workDir string) (fantasy.ToolResponse, error) {
+	if err := ctx.Err(); err != nil {
+		return fantasy.ToolResponse{}, err
+	}
 	var args readArgs
 	if err := parseArgs(call.Input, &args); err != nil {
 		return fantasy.NewTextErrorResponse("path parameter is required"), nil

internal/core/subagent.go

@@ -86,7 +86,7 @@ Example use cases:
 				},
 				"model": map[string]any{
 					"type":        "string",
-					"description": "Optional model override (e.g. 'anthropic/claude-haiku-3-5-20241022' for faster/cheaper tasks)",
+					"description": "Optional model override. Empty string uses the current model.",
 				},
 				"system_prompt": map[string]any{
 					"type":        "string",
@@ -94,7 +94,7 @@ Example use cases:
 				},
 				"timeout_seconds": map[string]any{
 					"type":        "number",
-					"description": "Maximum execution time in seconds (default: 300, max: 1800)",
+					"description": "Maximum execution time in seconds (default: 300, max: 1800, minimum recommended: 240)",
 				},
 			},
 			Required: []string{"task"},

internal/core/write.go

@@ -41,6 +41,9 @@ func NewWriteTool(opts ...ToolOption) fantasy.AgentTool {
 }
 
 func executeWrite(ctx context.Context, call fantasy.ToolCall, workDir string) (fantasy.ToolResponse, error) {
+	if err := ctx.Err(); err != nil {
+		return fantasy.ToolResponse{}, err
+	}
 	var args writeArgs
 	if err := parseArgs(call.Input, &args); err != nil {
 		return fantasy.NewTextErrorResponse("path and content parameters are required"), nil

internal/extbridge/context.go

@@ -0,0 +1,234 @@
+package extbridge
+
+import (
+	"context"
+
+	"github.com/mark3labs/kit/internal/extensions"
+	kit "github.com/mark3labs/kit/pkg/kit"
+)
+
+// BaseContext returns an extensions.Context populated with the headless,
+// TUI-independent delegation fields: data access, state, options,
+// model/tool management, completions, subagents, tree navigation, skills,
+// template parsing, and model resolution.
+//
+// Callers overlay their UI-specific fields (print routes, widgets, prompts,
+// editor, TUI-aware SetModel/ReloadExtensions, etc.) on the returned value:
+// cmd/extension_context.go for the interactive TUI and
+// internal/acpserver/session.go for headless ACP mode. Keeping the shared
+// half here means a new data-access Context field only has to be wired once.
+//
+// ctx is used for subagent spawns; pass a long-lived context (not a
+// per-request one) so later spawns aren't cancelled prematurely.
+func BaseContext(ctx context.Context, kitInstance *kit.Kit) extensions.Context {
+	return extensions.Context{
+		// -------------------------------------------------------------------
+		// Data access
+		// -------------------------------------------------------------------
+		GetContextStats: func() extensions.ContextStats {
+			s := kitInstance.GetContextStats()
+			return extensions.ContextStats{
+				EstimatedTokens: s.EstimatedTokens,
+				ContextLimit:    s.ContextLimit,
+				UsagePercent:    s.UsagePercent,
+				MessageCount:    s.MessageCount,
+			}
+		},
+		GetMessages: func() []extensions.SessionMessage {
+			return kitInstance.Extensions().GetSessionMessages()
+		},
+		GetSessionPath: func() string {
+			return kitInstance.GetSessionPath()
+		},
+		AppendEntry: func(entryType string, data string) (string, error) {
+			return kitInstance.Extensions().AppendEntry(entryType, data)
+		},
+		GetEntries: func(entryType string) []extensions.ExtensionEntry {
+			return kitInstance.Extensions().GetEntries(entryType)
+		},
+
+		// -------------------------------------------------------------------
+		// Extension state
+		// -------------------------------------------------------------------
+		SetState: func(key string, value string) {
+			kitInstance.Extensions().SetState(key, value)
+		},
+		GetState: func(key string) (string, bool) {
+			return kitInstance.Extensions().GetState(key)
+		},
+		DeleteState: func(key string) {
+			kitInstance.Extensions().DeleteState(key)
+		},
+		ListState: func() []string {
+			return kitInstance.Extensions().ListState()
+		},
+
+		// -------------------------------------------------------------------
+		// Options, model, and tool management
+		// -------------------------------------------------------------------
+		GetOption: func(name string) string {
+			return kitInstance.Extensions().GetOption(name)
+		},
+		SetOption: func(name string, value string) {
+			kitInstance.Extensions().SetOption(name, value)
+		},
+		// Headless model switch. The interactive TUI overrides this with a
+		// version that also notifies the TUI and refreshes the usage tracker.
+		SetModel: func(modelString string) error {
+			previousModel := kitInstance.Extensions().GetContext().Model
+			if err := kitInstance.SetModel(context.Background(), modelString); err != nil {
+				return err
+			}
+			kitInstance.Extensions().UpdateContextModel(modelString)
+			kitInstance.Extensions().EmitModelChange(modelString, previousModel, "extension")
+			return nil
+		},
+		GetAvailableModels: func() []extensions.ModelInfoEntry {
+			return kitInstance.GetAvailableModels()
+		},
+		EmitCustomEvent: func(name string, data string) {
+			kitInstance.Extensions().EmitCustomEvent(name, data)
+		},
+		GetAllTools: func() []extensions.ToolInfo {
+			return kitInstance.Extensions().GetToolInfos()
+		},
+		SetActiveTools: func(names []string) {
+			kitInstance.Extensions().SetActiveTools(names)
+		},
+		// Headless reload. The interactive TUI overrides this to also
+		// refresh widgets/status/commands.
+		ReloadExtensions: func() error {
+			return kitInstance.Extensions().Reload()
+		},
+
+		// -------------------------------------------------------------------
+		// LLM completions and subagents
+		// -------------------------------------------------------------------
+		Complete: func(req extensions.CompleteRequest) (extensions.CompleteResponse, error) {
+			return kitInstance.ExecuteCompletion(context.Background(), req)
+		},
+		SpawnSubagent: func(config extensions.SubagentConfig) (*extensions.SubagentHandle, *extensions.SubagentResult, error) {
+			return SpawnSubagent(ctx, kitInstance, config)
+		},
+
+		// -------------------------------------------------------------------
+		// Tree Navigation API
+		// -------------------------------------------------------------------
+		GetTreeNode: func(entryID string) *extensions.TreeNode {
+			node := kitInstance.GetTreeNode(entryID)
+			if node == nil {
+				return nil
+			}
+			return &extensions.TreeNode{
+				ID:        node.ID,
+				ParentID:  node.ParentID,
+				Type:      node.Type,
+				Role:      node.Role,
+				Content:   node.Content,
+				Model:     node.Model,
+				Provider:  node.Provider,
+				Timestamp: node.Timestamp,
+				Children:  node.Children,
+			}
+		},
+		GetCurrentBranch: func() []extensions.TreeNode {
+			nodes := kitInstance.GetCurrentBranch()
+			result := make([]extensions.TreeNode, len(nodes))
+			for i, n := range nodes {
+				result[i] = extensions.TreeNode{
+					ID:        n.ID,
+					ParentID:  n.ParentID,
+					Type:      n.Type,
+					Role:      n.Role,
+					Content:   n.Content,
+					Model:     n.Model,
+					Provider:  n.Provider,
+					Timestamp: n.Timestamp,
+					Children:  n.Children,
+				}
+			}
+			return result
+		},
+		GetChildren: func(parentID string) []string {
+			return kitInstance.GetChildren(parentID)
+		},
+		NavigateTo: func(entryID string) extensions.TreeNavigationResult {
+			err := kitInstance.NavigateTo(entryID)
+			if err != nil {
+				return extensions.TreeNavigationResult{Success: false, Error: err.Error()}
+			}
+			return extensions.TreeNavigationResult{Success: true}
+		},
+		SummarizeBranch: func(fromID, toID string) string {
+			summary, _ := kitInstance.SummarizeBranch(fromID, toID)
+			return summary
+		},
+		CollapseBranch: func(fromID, toID, summary string) extensions.TreeNavigationResult {
+			err := kitInstance.CollapseBranch(fromID, toID, summary)
+			if err != nil {
+				return extensions.TreeNavigationResult{Success: false, Error: err.Error()}
+			}
+			return extensions.TreeNavigationResult{Success: true}
+		},
+
+		// -------------------------------------------------------------------
+		// Skill Loading API (context-injection variants are TUI-specific and
+		// wired by the interactive overlay)
+		// -------------------------------------------------------------------
+		LoadSkill: func(path string) (*extensions.Skill, string) {
+			s, err := kitInstance.LoadSkillForExtension(path)
+			return s, err
+		},
+		LoadSkillsFromDir: func(dir string) extensions.SkillLoadResult {
+			return kitInstance.LoadSkillsFromDirForExtension(dir)
+		},
+		DiscoverSkills: func() extensions.SkillLoadResult {
+			skills := kitInstance.DiscoverSkillsForExtension()
+			return extensions.SkillLoadResult{Skills: skills}
+		},
+		GetAvailableSkills: func() []extensions.Skill {
+			return kitInstance.DiscoverSkillsForExtension()
+		},
+
+		// -------------------------------------------------------------------
+		// Template Parsing API
+		// -------------------------------------------------------------------
+		ParseTemplate: func(name, content string) extensions.PromptTemplate {
+			return kit.ParseTemplate(name, content)
+		},
+		RenderTemplate: func(tpl extensions.PromptTemplate, vars map[string]string) string {
+			return kit.RenderTemplate(tpl, vars)
+		},
+		ParseArguments: func(input string, pattern extensions.ArgumentPattern) extensions.ParseResult {
+			return kit.ParseArguments(input, pattern)
+		},
+		SimpleParseArguments: func(input string, count int) []string {
+			return kit.SimpleParseArguments(input, count)
+		},
+		EvaluateModelConditional: func(condition string) bool {
+			return kit.EvaluateModelConditional(kitInstance.Extensions().GetContext().Model, condition)
+		},
+		RenderWithModelConditionals: func(content string) string {
+			return kit.RenderWithModelConditionals(content, kitInstance.Extensions().GetContext().Model)
+		},
+
+		// -------------------------------------------------------------------
+		// Model Resolution API
+		// -------------------------------------------------------------------
+		ResolveModelChain: func(preferences []string) extensions.ModelResolutionResult {
+			return kit.ResolveModelChain(preferences)
+		},
+		GetModelCapabilities: func(model string) (extensions.ModelCapabilities, string) {
+			return kit.GetModelCapabilities(model)
+		},
+		CheckModelAvailable: func(model string) bool {
+			return kit.CheckModelAvailable(model)
+		},
+		GetCurrentProvider: func() string {
+			return kit.GetCurrentProvider(kitInstance.Extensions().GetContext().Model)
+		},
+		GetCurrentModelID: func() string {
+			return kit.GetCurrentModelID(kitInstance.Extensions().GetContext().Model)
+		},
+	}
+}

internal/extbridge/extbridge.go

@@ -0,0 +1,98 @@
+// Package extbridge wires the public Kit SDK to the internal extensions
+// package. It exists so that cmd/ and internal/acpserver/ don't both
+// reimplement the same SDK→extension event/subagent conversions.
+package extbridge
+
+import (
+	"context"
+
+	"github.com/mark3labs/kit/internal/extensions"
+	kit "github.com/mark3labs/kit/pkg/kit"
+)
+
+// SDKEventToSubagentEvent converts an SDK [kit.Event] into the
+// extension-facing [extensions.SubagentEvent]. Returns a zero-value event
+// (Type=="") for events that don't map to anything useful — callers should
+// drop those.
+func SDKEventToSubagentEvent(e kit.Event) extensions.SubagentEvent {
+	switch ev := e.(type) {
+	case kit.MessageUpdateEvent:
+		return extensions.SubagentEvent{Type: "text", Content: ev.Chunk}
+	case kit.ReasoningDeltaEvent:
+		return extensions.SubagentEvent{Type: "reasoning", Content: ev.Delta}
+	case kit.ToolCallEvent:
+		return extensions.SubagentEvent{
+			Type: "tool_call", ToolCallID: ev.ToolCallID,
+			ToolName: ev.ToolName, ToolKind: ev.ToolKind, ToolArgs: ev.ToolArgs,
+		}
+	case kit.ToolExecutionStartEvent:
+		return extensions.SubagentEvent{
+			Type: "tool_execution_start", ToolCallID: ev.ToolCallID,
+			ToolName: ev.ToolName, ToolKind: ev.ToolKind,
+		}
+	case kit.ToolExecutionEndEvent:
+		return extensions.SubagentEvent{
+			Type: "tool_execution_end", ToolCallID: ev.ToolCallID,
+			ToolName: ev.ToolName, ToolKind: ev.ToolKind,
+		}
+	case kit.ToolResultEvent:
+		return extensions.SubagentEvent{
+			Type: "tool_result", ToolCallID: ev.ToolCallID,
+			ToolName: ev.ToolName, ToolKind: ev.ToolKind,
+			ToolResult: ev.Result, IsError: ev.IsError,
+		}
+	case kit.TurnStartEvent:
+		return extensions.SubagentEvent{Type: "turn_start"}
+	case kit.TurnEndEvent:
+		return extensions.SubagentEvent{Type: "turn_end"}
+	default:
+		return extensions.SubagentEvent{}
+	}
+}
+
+// SpawnSubagent runs a subagent in-process via the Kit SDK and translates
+// the result/events back into the extension-facing types. The returned
+// handle is always nil — the SDK path runs synchronously and does not
+// expose a separate process handle. Callers that need non-blocking
+// behaviour should run this in their own goroutine.
+//
+// This function consolidates the previously-duplicated wiring in
+// cmd/root.go (interactive + runtime contexts) and
+// internal/acpserver/session.go.
+func SpawnSubagent(ctx context.Context, k *kit.Kit, cfg extensions.SubagentConfig) (*extensions.SubagentHandle, *extensions.SubagentResult, error) {
+	sdkCfg := kit.SubagentConfig{
+		Prompt:       cfg.Prompt,
+		Model:        cfg.Model,
+		SystemPrompt: cfg.SystemPrompt,
+		Timeout:      cfg.Timeout,
+		NoSession:    cfg.NoSession,
+		Tools:        k.GetToolsForSubagent(),
+	}
+	if cfg.OnEvent != nil {
+		sdkCfg.OnEvent = func(e kit.Event) {
+			se := SDKEventToSubagentEvent(e)
+			if se.Type != "" {
+				cfg.OnEvent(se)
+			}
+		}
+	}
+
+	result, err := k.Subagent(ctx, sdkCfg)
+	if result == nil {
+		return nil, &extensions.SubagentResult{Error: err}, err
+	}
+
+	extResult := &extensions.SubagentResult{
+		Response:  result.Response,
+		Error:     err,
+		SessionID: result.SessionID,
+		Elapsed:   result.Elapsed,
+	}
+	if result.Usage != nil {
+		extResult.Usage = &extensions.SubagentUsage{
+			InputTokens:  result.Usage.InputTokens,
+			OutputTokens: result.Usage.OutputTokens,
+		}
+	}
+	return nil, extResult, err
+}

internal/extensions/api.go

@@ -341,6 +341,13 @@ type Context struct {
 	// The data survives across session restarts and can be retrieved via
 	// GetEntries. Use entryType to namespace your data (e.g. "myext:state").
 	//
+	// AppendEntry is append-only and lives in the conversation tree, which
+	// makes it the right tool for audit logs and event histories. For
+	// last-write-wins snapshot state — "what's the current value of X?" —
+	// prefer SetState / GetState instead. Those primitives store data in a
+	// sidecar file outside the conversation tree, are O(1) to read/write,
+	// and do not bloat branch reads or duplicate on fork.
+	//
 	// Example:
 	//
 	//   data, _ := json.Marshal(myState)
@@ -360,6 +367,45 @@ type Context struct {
 	//   }
 	GetEntries func(entryType string) []ExtensionEntry
 
+	// SetState stores a key-value pair in session-scoped, last-write-wins
+	// extension state. Unlike AppendEntry the value is kept in a sidecar
+	// file outside the conversation tree, so:
+	//   - reads are O(1) (no branch walk)
+	//   - writes don't bloat the session JSONL
+	//   - state is not duplicated on fork (branches share the sidecar)
+	//   - state is invisible to the LLM
+	//
+	// Use SetState for snapshot state ("current value of X"); use
+	// AppendEntry for audit logs and event histories. Namespace keys with
+	// your extension name to avoid collisions (e.g. "myext:budget-cap").
+	//
+	// State persists for the lifetime of the session. For ephemeral or
+	// in-memory sessions the state lives only in memory.
+	//
+	// Example:
+	//
+	//   ctx.SetState("myext:budget-cap", "10.00")
+	SetState func(key string, value string)
+
+	// GetState returns the value previously stored via SetState. The bool
+	// is false when the key was never written. Returns ("", false) when
+	// state is unavailable.
+	//
+	// Example:
+	//
+	//   if cap, ok := ctx.GetState("myext:budget-cap"); ok {
+	//       fmt.Println("current cap:", cap)
+	//   }
+	GetState func(key string) (string, bool)
+
+	// DeleteState removes a key from session-scoped extension state.
+	// No-op when the key is missing.
+	DeleteState func(key string)
+
+	// ListState returns all keys currently stored in session-scoped
+	// extension state, in unspecified order.
+	ListState func() []string
+
 	// SetEditorText sets the text content of the input editor. This can
 	// be used to pre-fill the editor with suggested text (e.g. extracted
 	// questions, handoff prompts). The cursor is moved to the end.
@@ -918,7 +964,7 @@ type ExtensionEntry struct {
 type ContextMessage struct {
 	// Index is the position of this message in the original context array
 	// (0-based). When returning messages from a ContextPrepareResult,
-	// messages with Index >= 0 reuse the original fantasy.Message at that
+	// messages with Index >= 0 reuse the original LLM message at that
 	// position (preserving tool calls, reasoning, and other complex parts).
 	// Set Index to -1 for newly injected messages (created from Role + Content).
 	Index int
@@ -1063,6 +1109,9 @@ type PrintBlockOpts struct {
 type API struct {
 	// Event-specific registration functions (wired by the loader).
 	onToolCall                func(func(ToolCallEvent, Context) *ToolCallResult)
+	onToolCallInputStart      func(func(ToolCallInputStartEvent, Context))
+	onToolCallInputDelta      func(func(ToolCallInputDeltaEvent, Context))
+	onToolCallInputEnd        func(func(ToolCallInputEndEvent, Context))
 	onToolExecStart           func(func(ToolExecutionStartEvent, Context))
 	onToolExecEnd             func(func(ToolExecutionEndEvent, Context))
 	onToolOutput              func(func(ToolOutputEvent, Context))
@@ -1091,6 +1140,15 @@ type API struct {
 	onSubagentStart           func(func(SubagentStartEvent, Context))
 	onSubagentChunk           func(func(SubagentChunkEvent, Context))
 	onSubagentEnd             func(func(SubagentEndEvent, Context))
+	onStepStart               func(func(StepStartEvent, Context))
+	onStepFinish              func(func(StepFinishEvent, Context))
+	onReasoningStart          func(func(ReasoningStartEvent, Context))
+	onWarnings                func(func(WarningsEvent, Context))
+	onSource                  func(func(SourceEvent, Context))
+	onError                   func(func(ErrorEvent, Context))
+	onRetry                   func(func(RetryEvent, Context))
+	onPrepareStep             func(func(PrepareStepEvent, Context) *PrepareStepResult)
+	onLLMUsage                func(func(LLMUsageEvent, Context))
 }
 
 // OnToolCall registers a handler that fires before a tool executes.
@@ -1099,6 +1157,26 @@ func (a *API) OnToolCall(handler func(ToolCallEvent, Context) *ToolCallResult) {
 	a.onToolCall(handler)
 }
 
+// OnToolCallInputStart registers a handler that fires when the LLM begins
+// generating tool call arguments. The tool name is known but the full
+// argument JSON is still being streamed. Useful for showing a "running"
+// indicator immediately without waiting for the full arguments.
+func (a *API) OnToolCallInputStart(handler func(ToolCallInputStartEvent, Context)) {
+	a.onToolCallInputStart(handler)
+}
+
+// OnToolCallInputDelta registers a handler that fires for each streamed
+// fragment of tool call arguments as they arrive from the LLM.
+func (a *API) OnToolCallInputDelta(handler func(ToolCallInputDeltaEvent, Context)) {
+	a.onToolCallInputDelta(handler)
+}
+
+// OnToolCallInputEnd registers a handler that fires when tool argument
+// streaming is complete, before the tool call is parsed and execution begins.
+func (a *API) OnToolCallInputEnd(handler func(ToolCallInputEndEvent, Context)) {
+	a.onToolCallInputEnd(handler)
+}
+
 // OnToolExecutionStart registers a handler for tool execution start.
 func (a *API) OnToolExecutionStart(handler func(ToolExecutionStartEvent, Context)) {
 	a.onToolExecStart(handler)
@@ -1278,6 +1356,69 @@ func (a *API) OnBeforeCompact(handler func(BeforeCompactEvent, Context) *BeforeC
 	a.onBeforeCompact(handler)
 }
 
+// OnStepStart registers a handler that fires when a new LLM call begins
+// within a multi-step agent turn.
+func (a *API) OnStepStart(handler func(StepStartEvent, Context)) {
+	a.onStepStart(handler)
+}
+
+// OnStepFinish registers a handler that fires when a step completes,
+// providing step number, finish reason, and decomposed token usage.
+func (a *API) OnStepFinish(handler func(StepFinishEvent, Context)) {
+	a.onStepFinish(handler)
+}
+
+// OnReasoningStart registers a handler that fires when the LLM begins
+// reasoning/thinking.
+func (a *API) OnReasoningStart(handler func(ReasoningStartEvent, Context)) {
+	a.onReasoningStart(handler)
+}
+
+// OnWarnings registers a handler that fires when the LLM provider returns
+// warnings about the request.
+func (a *API) OnWarnings(handler func(WarningsEvent, Context)) {
+	a.onWarnings(handler)
+}
+
+// OnSource registers a handler that fires when the LLM references a source
+// (e.g. from web search tools).
+func (a *API) OnSource(handler func(SourceEvent, Context)) {
+	a.onSource(handler)
+}
+
+// OnError registers a handler that fires when an agent-level error occurs
+// during streaming.
+func (a *API) OnError(handler func(ErrorEvent, Context)) {
+	a.onError(handler)
+}
+
+// OnRetry registers a handler that fires when the LLM provider request is
+// retried after a transient error.
+func (a *API) OnRetry(handler func(RetryEvent, Context)) {
+	a.onRetry(handler)
+}
+
+// OnPrepareStep registers a handler that fires between steps within a
+// multi-step agent turn, after steering messages are injected and before
+// messages are sent to the LLM. Return a non-nil PrepareStepResult with
+// Messages to replace the context window for this step.
+func (a *API) OnPrepareStep(handler func(PrepareStepEvent, Context) *PrepareStepResult) {
+	a.onPrepareStep(handler)
+}
+
+// OnLLMUsage registers a handler that fires after each LLM provider call
+// with the token and cost deltas for that single call. Use this for
+// per-call usage attribution, real-time budget enforcement, and cost
+// dashboards that need to react between calls within a single agent turn.
+//
+// Handlers receive an LLMUsageEvent describing the call's input/output
+// tokens, cache tokens, computed cost, model, and provider. A single agent
+// turn typically fires multiple LLMUsageEvents (one per tool-loop
+// iteration).
+func (a *API) OnLLMUsage(handler func(LLMUsageEvent, Context)) {
+	a.onLLMUsage(handler)
+}
+
 // RegisterToolRenderer registers a custom renderer for a specific tool's
 // display in the TUI. The renderer controls the header (parameter summary)
 // and/or body (result display) of the tool's output block. If multiple
@@ -1890,6 +2031,34 @@ type ToolCallResult struct {
 
 func (ToolCallResult) isResult() {}
 
+// ToolCallInputStartEvent fires when the LLM begins generating tool call
+// arguments. The tool name is known but the full argument JSON is still
+// being streamed.
+type ToolCallInputStartEvent struct {
+	ToolCallID string
+	ToolName   string
+	ToolKind   string // Tool classification: "execute", "edit", "read", "search", "agent"
+}
+
+func (e ToolCallInputStartEvent) Type() EventType { return ToolCallInputStart }
+
+// ToolCallInputDeltaEvent fires for each streamed fragment of tool call
+// arguments as they arrive from the LLM.
+type ToolCallInputDeltaEvent struct {
+	ToolCallID string
+	Delta      string // JSON fragment of tool arguments
+}
+
+func (e ToolCallInputDeltaEvent) Type() EventType { return ToolCallInputDelta }
+
+// ToolCallInputEndEvent fires when tool argument streaming is complete,
+// before the tool call is parsed and execution begins.
+type ToolCallInputEndEvent struct {
+	ToolCallID string
+}
+
+func (e ToolCallInputEndEvent) Type() EventType { return ToolCallInputEnd }
+
 // ToolExecutionStartEvent fires when a tool begins executing.
 type ToolExecutionStartEvent struct {
 	ToolCallID string
@@ -1982,10 +2151,47 @@ type AgentStartEvent struct {
 
 func (e AgentStartEvent) Type() EventType { return AgentStart }
 
-// AgentEndEvent fires when the agent finishes responding.
+// AgentEndEvent fires when the agent finishes responding. In addition to the
+// final response and stop reason, the event carries per-turn aggregates so
+// observer-style extensions don't have to maintain parallel bookkeeping in
+// OnToolResult / OnStepFinish handlers.
 type AgentEndEvent struct {
 	Response   string
 	StopReason string // "completed", "cancelled", "error"
+
+	// ToolCallCount is the total number of tool invocations observed during
+	// this turn (sum across all steps).
+	ToolCallCount int
+
+	// ToolNames lists the tool names invoked during this turn, in call order.
+	// Duplicates are preserved (e.g. two bash calls produce ["bash", "bash"]).
+	ToolNames []string
+
+	// LLMCallCount is the number of LLM round-trips (tool-loop iterations)
+	// performed during this turn. Always >= 1 for a successful turn.
+	LLMCallCount int
+
+	// InputTokensDelta is the sum of input tokens consumed during this turn
+	// across every LLM call (including cache-hit input tokens).
+	InputTokensDelta int
+
+	// OutputTokensDelta is the sum of output tokens generated during this turn.
+	OutputTokensDelta int
+
+	// CacheReadTokensDelta is the sum of cache-read tokens during this turn.
+	CacheReadTokensDelta int
+
+	// CacheWriteTokensDelta is the sum of cache-write tokens during this turn.
+	CacheWriteTokensDelta int
+
+	// CostDelta is the total cost in USD attributable to this turn. Computed
+	// from per-step usage and current model pricing. Zero when pricing is
+	// unknown or OAuth credentials are in use.
+	CostDelta float64
+
+	// DurationMs is the elapsed wall-clock time from AgentStart to AgentEnd,
+	// in milliseconds.
+	DurationMs int64
 }
 
 func (e AgentEndEvent) Type() EventType { return AgentEnd }
@@ -2202,6 +2408,135 @@ type SubagentEndEvent struct {
 
 func (e SubagentEndEvent) Type() EventType { return SubagentEnd }
 
+// ---------------------------------------------------------------------------
+// Step lifecycle events (exposed to Yaegi — concrete structs)
+// ---------------------------------------------------------------------------
+
+// StepStartEvent fires when a new LLM call begins within a multi-step agent turn.
+type StepStartEvent struct {
+	StepNumber int
+}
+
+func (e StepStartEvent) Type() EventType { return StepStart }
+
+// StepFinishEvent fires when a step completes, providing step metadata and
+// token usage. Usage fields are plain int64 (not LLMUsage) because Yaegi
+// cannot handle fantasy types across the interpreter boundary.
+type StepFinishEvent struct {
+	StepNumber       int
+	HasToolCalls     bool
+	FinishReason     string
+	InputTokens      int64
+	OutputTokens     int64
+	CacheReadTokens  int64
+	CacheWriteTokens int64
+}
+
+func (e StepFinishEvent) Type() EventType { return StepFinish }
+
+// ReasoningStartEvent fires when the LLM begins reasoning/thinking.
+type ReasoningStartEvent struct {
+	ID string
+}
+
+func (e ReasoningStartEvent) Type() EventType { return ReasoningStart }
+
+// WarningsEvent fires when the LLM provider returns warnings about the request.
+type WarningsEvent struct {
+	Warnings []string
+}
+
+func (e WarningsEvent) Type() EventType { return Warnings }
+
+// SourceEvent fires when the LLM references a source (e.g. from web search).
+type SourceEvent struct {
+	SourceType string
+	ID         string
+	URL        string
+	Title      string
+}
+
+func (e SourceEvent) Type() EventType { return Source }
+
+// ErrorEvent fires when an agent-level error occurs during streaming.
+// Uses string instead of error because Yaegi cannot handle the error
+// interface reliably across the interpreter boundary.
+type ErrorEvent struct {
+	Error string
+}
+
+func (e ErrorEvent) Type() EventType { return Error }
+
+// RetryEvent fires when the LLM provider request is retried after a
+// transient error.
+type RetryEvent struct {
+	Attempt int
+	Error   string
+}
+
+func (e RetryEvent) Type() EventType { return Retry }
+
+// PrepareStepEvent fires between steps within a multi-step agent turn,
+// after steering messages are injected and before messages are sent to
+// the LLM. Handlers can inspect and replace the context window.
+type PrepareStepEvent struct {
+	// StepNumber is the zero-based step index within the current turn.
+	StepNumber int
+	// Messages is the current context window that will be sent to the LLM.
+	Messages []ContextMessage
+}
+
+func (e PrepareStepEvent) Type() EventType { return PrepareStep }
+
+// PrepareStepResult allows extensions to replace the context window between
+// steps. Return nil Messages to leave the context unchanged.
+type PrepareStepResult struct {
+	// Messages replaces the entire context window for this step. If nil,
+	// the original messages are used unchanged. Messages with a non-negative
+	// Index reuse the original message at that position; messages with
+	// Index < 0 are created fresh from Role + Content.
+	Messages []ContextMessage
+}
+
+func (PrepareStepResult) isResult() {}
+
+// LLMUsageEvent fires after each LLM provider call with the per-call token
+// and cost deltas. Use this for accurate budget tracking, cost dashboards,
+// and any logic that needs to react between LLM calls within a single agent
+// turn (rather than only at turn boundaries).
+//
+// A single agent turn typically produces multiple LLMUsageEvents (one per
+// tool-loop iteration). The Model and Provider fields reflect the model used
+// for that specific call, which may differ from earlier calls if the
+// extension switched models mid-turn via ctx.SetModel().
+type LLMUsageEvent struct {
+	// InputTokens is the number of input tokens for this call.
+	InputTokens int
+	// OutputTokens is the number of output tokens generated by this call.
+	OutputTokens int
+	// CacheReadTokens is the number of cache-hit input tokens (provider-specific).
+	CacheReadTokens int
+	// CacheWriteTokens is the number of cache-write tokens.
+	CacheWriteTokens int
+	// Cost is the USD cost of this call computed from the model's per-token
+	// pricing. Zero when pricing is unknown or OAuth credentials are in use.
+	Cost float64
+	// Model is the model identifier used for this call (e.g. "claude-sonnet-4-5-20250929").
+	Model string
+	// Provider is the provider identifier (e.g. "anthropic", "openai").
+	Provider string
+	// RequestID is an optional correlation id for the underlying provider
+	// call. May be empty when the provider does not surface one.
+	RequestID string
+	// StepNumber is the zero-based step index within the current agent turn.
+	StepNumber int
+	// FinishReason mirrors the provider's finish reason for this call
+	// (e.g. "stop", "tool_calls", "length"). May be empty.
+	FinishReason string
+}
+
+func (e LLMUsageEvent) Type() EventType { return LLMUsage }
+
 // ThemeColor is an adaptive color pair with light and dark hex values.
 // Either field may be empty to inherit from the default theme.
 type ThemeColor struct {

internal/extensions/events.go

@@ -13,6 +13,19 @@ const (
 	// ToolCall fires before a tool executes. Handlers can block execution.
 	ToolCall EventType = "tool_call"
 
+	// ToolCallInputStart fires when the LLM begins generating tool call
+	// arguments. The tool name is known but the full argument JSON is still
+	// being streamed.
+	ToolCallInputStart EventType = "tool_call_input_start"
+
+	// ToolCallInputDelta fires for each streamed fragment of tool call
+	// arguments as they arrive from the LLM.
+	ToolCallInputDelta EventType = "tool_call_input_delta"
+
+	// ToolCallInputEnd fires when tool argument streaming is complete,
+	// before the tool call is parsed and execution begins.
+	ToolCallInputEnd EventType = "tool_call_input_end"
+
 	// ToolExecutionStart fires when a tool begins executing.
 	ToolExecutionStart EventType = "tool_execution_start"
 
@@ -83,18 +96,55 @@ const (
 	// SubagentEnd fires when a subagent tool call completes (success
 	// or error). Carries the final response and any error message.
 	SubagentEnd EventType = "subagent_end"
+
+	// StepStart fires when a new LLM call begins within a multi-step
+	// agent turn.
+	StepStart EventType = "step_start"
+
+	// StepFinish fires when a step completes, providing step number,
+	// finish reason, and token usage.
+	StepFinish EventType = "step_finish"
+
+	// ReasoningStart fires when the LLM begins reasoning/thinking.
+	ReasoningStart EventType = "reasoning_start"
+
+	// Warnings fires when the LLM provider returns warnings.
+	Warnings EventType = "warnings"
+
+	// Source fires when the LLM references a source (e.g. web search).
+	Source EventType = "source"
+
+	// Error fires when an agent-level error occurs during streaming.
+	Error EventType = "error"
+
+	// Retry fires when the LLM provider request is retried after a
+	// transient error.
+	Retry EventType = "retry"
+
+	// PrepareStep fires between steps within a multi-step agent turn,
+	// after steering messages are injected and before messages are sent
+	// to the LLM. Handlers can replace the context window for this step.
+	PrepareStep EventType = "prepare_step"
+
+	// LLMUsage fires after each LLM provider call with the token and cost
+	// deltas for that single call. Extensions use it to attribute usage to
+	// specific calls/models and to drive budget enforcement between calls.
+	LLMUsage EventType = "llm_usage"
 )
 
 // AllEventTypes returns every supported event type.
 func AllEventTypes() []EventType {
 	return []EventType{
-		ToolCall, ToolExecutionStart, ToolExecutionEnd, ToolResult,
+		ToolCall, ToolCallInputStart, ToolCallInputDelta, ToolCallInputEnd,
+		ToolExecutionStart, ToolExecutionEnd, ToolResult,
 		Input, BeforeAgentStart, AgentStart, AgentEnd,
 		MessageStart, MessageUpdate, MessageEnd,
 		SessionStart, SessionShutdown,
 		ModelChange, ContextPrepare,
 		BeforeFork, BeforeSessionSwitch, BeforeCompact,
 		SubagentStart, SubagentChunk, SubagentEnd,
+		StepStart, StepFinish, ReasoningStart, Warnings, Source, Error, Retry,
+		PrepareStep, LLMUsage,
 	}
 }
 

internal/extensions/events_test.go

@@ -4,8 +4,8 @@ import "testing"
 
 func TestAllEventTypes_Count(t *testing.T) {
 	all := AllEventTypes()
-	if len(all) != 21 {
-		t.Fatalf("expected 21 event types, got %d", len(all))
+	if len(all) != 33 {
+		t.Fatalf("expected 33 event types, got %d", len(all))
 	}
 }
 
@@ -38,6 +38,9 @@ func TestEventType_TypeMethod(t *testing.T) {
 		want  EventType
 	}{
 		{ToolCallEvent{ToolName: "test"}, ToolCall},
+		{ToolCallInputStartEvent{ToolCallID: "x", ToolName: "test"}, ToolCallInputStart},
+		{ToolCallInputDeltaEvent{ToolCallID: "x", Delta: "{"}, ToolCallInputDelta},
+		{ToolCallInputEndEvent{ToolCallID: "x"}, ToolCallInputEnd},
 		{ToolExecutionStartEvent{ToolName: "test"}, ToolExecutionStart},
 		{ToolExecutionEndEvent{ToolName: "test"}, ToolExecutionEnd},
 		{ToolResultEvent{ToolName: "test"}, ToolResult},

internal/extensions/installer.go

@@ -450,25 +450,6 @@ func globalGitInstallRoot() string {
 	return filepath.Join(base, "kit", "git")
 }
 
-// GetInstalledPackages returns all installed packages from both scopes.
-func (i *Installer) GetInstalledPackages() ([]ManifestEntry, error) {
-	var all []ManifestEntry
-
-	global, err := i.loadManifest(ScopeGlobal)
-	if err != nil {
-		return nil, fmt.Errorf("loading global manifest: %w", err)
-	}
-	all = append(all, global.Packages...)
-
-	project, err := i.loadManifest(ScopeProject)
-	if err != nil {
-		return nil, fmt.Errorf("loading project manifest: %w", err)
-	}
-	all = append(all, project.Packages...)
-
-	return all, nil
-}
-
 // IsInstalled checks if a package is installed in either scope.
 // Returns (scope, true) if installed, ("", false) otherwise.
 func (i *Installer) IsInstalled(source *GitSource) (InstallScope, bool) {

internal/extensions/installer_test.go

@@ -245,14 +245,21 @@ func TestManifestEntryIdentity(t *testing.T) {
 	}
 }
 
+// TestLoadAndSaveManifest exercises the live *Installer.loadManifest /
+// saveManifest round-trip against a temp directory, ensuring an absent
+// manifest loads as empty and a saved manifest reads back identically.
 func TestLoadAndSaveManifest(t *testing.T) {
 	tempDir := t.TempDir()
+	installer := &Installer{
+		projectGitRoot: tempDir,
+		globalGitRoot:  tempDir,
+	}
 	manifestPath := filepath.Join(tempDir, "packages.json")
 
 	// Test loading non-existent manifest
-	manifest, err := loadManifestFromPath(manifestPath)
+	manifest, err := installer.loadManifest(ScopeGlobal)
 	if err != nil {
-		t.Fatalf("loadManifestFromPath() error = %v", err)
+		t.Fatalf("loadManifest() error = %v", err)
 	}
 	if len(manifest.Packages) != 0 {
 		t.Errorf("Expected empty packages, got %d", len(manifest.Packages))
@@ -273,15 +280,20 @@ func TestLoadAndSaveManifest(t *testing.T) {
 	}
 
 	// Save it
-	err = saveManifestToPath(manifest, manifestPath)
+	err = installer.saveManifest(manifest, ScopeGlobal)
 	if err != nil {
-		t.Fatalf("saveManifestToPath() error = %v", err)
+		t.Fatalf("saveManifest() error = %v", err)
+	}
+
+	// Verify it was written to expected path
+	if _, err := os.Stat(manifestPath); err != nil {
+		t.Fatalf("manifest file not created: %v", err)
 	}
 
 	// Load it back
-	loaded, err := loadManifestFromPath(manifestPath)
+	loaded, err := installer.loadManifest(ScopeGlobal)
 	if err != nil {
-		t.Fatalf("loadManifestFromPath() error = %v", err)
+		t.Fatalf("loadManifest() error = %v", err)
 	}
 	if len(loaded.Packages) != 1 {
 		t.Errorf("Expected 1 package, got %d", len(loaded.Packages))
@@ -291,21 +303,15 @@ func TestLoadAndSaveManifest(t *testing.T) {
 	}
 }
 
+// TestAddAndRemoveFromManifest verifies that *Installer.addToManifest
+// followed by removeFromManifest leaves the manifest in its original
+// (empty) state, using a temp-directory installer scope.
 func TestAddAndRemoveFromManifest(t *testing.T) {
 	tempDir := t.TempDir()
-
-	// Set up environment for manifest path
-	if err := os.Setenv("XDG_DATA_HOME", tempDir); err != nil {
-		t.Fatalf("Setenv() error = %v", err)
+	installer := &Installer{
+		projectGitRoot: tempDir,
+		globalGitRoot:  tempDir,
 	}
-	defer func() {
-		if err := os.Unsetenv("XDG_DATA_HOME"); err != nil {
-			t.Logf("Unsetenv() error = %v", err)
-		}
-	}()
-
-	// The manifest path when XDG_DATA_HOME is set
-	manifestPath := filepath.Join(tempDir, "kit", "git", "packages.json")
 
 	// Add an entry
 	entry := ManifestEntry{
@@ -315,58 +321,51 @@ func TestAddAndRemoveFromManifest(t *testing.T) {
 		Scope:  ScopeGlobal,
 	}
 
-	err := addEntryToManifest(entry, ScopeGlobal)
-	if err != nil {
-		t.Fatalf("addEntryToManifest() error = %v", err)
+	if err := installer.addToManifest(entry, ScopeGlobal); err != nil {
+		t.Fatalf("addToManifest() error = %v", err)
 	}
 
 	// Verify it was added
-	manifest, err := loadManifestFromPath(manifestPath)
+	manifest, err := installer.loadManifest(ScopeGlobal)
 	if err != nil {
-		t.Fatalf("loadManifestFromPath() error = %v", err)
+		t.Fatalf("loadManifest() error = %v", err)
 	}
 	if len(manifest.Packages) != 1 {
 		t.Errorf("Expected 1 package, got %d", len(manifest.Packages))
 	}
 
 	// Remove it
-	err = removeEntryFromManifest("github.com/user/repo", ScopeGlobal)
-	if err != nil {
-		t.Fatalf("removeEntryFromManifest() error = %v", err)
+	if err := installer.removeFromManifest("github.com/user/repo", ScopeGlobal); err != nil {
+		t.Fatalf("removeFromManifest() error = %v", err)
 	}
 
 	// Verify it was removed
-	manifest, err = loadManifestFromPath(manifestPath)
+	manifest, err = installer.loadManifest(ScopeGlobal)
 	if err != nil {
-		t.Fatalf("loadManifestFromPath() error = %v", err)
+		t.Fatalf("loadManifest() error = %v", err)
 	}
 	if len(manifest.Packages) != 0 {
 		t.Errorf("Expected 0 packages, got %d", len(manifest.Packages))
 	}
 }
 
+// TestFindInManifest writes a manifest file directly to the path
+// resolved by the package-level manifestPathForScope helper and then
+// confirms FindInManifest locates the entry by identity (and returns
+// nil for a non-existent identity).
 func TestFindInManifest(t *testing.T) {
 	tempDir := t.TempDir()
-	if err := os.Setenv("XDG_DATA_HOME", tempDir); err != nil {
-		t.Fatalf("Setenv() error = %v", err)
-	}
-	defer func() {
-		if err := os.Unsetenv("XDG_DATA_HOME"); err != nil {
-			t.Logf("Unsetenv() error = %v", err)
-		}
-	}()
+	t.Setenv("XDG_DATA_HOME", tempDir)
 
-	// Add an entry to global manifest
-	entry := ManifestEntry{
-		Source: "git:github.com/user/repo",
-		Host:   "github.com",
-		Path:   "user/repo",
-		Scope:  ScopeGlobal,
+	// Write a manifest entry directly via the package-level path resolver
+	// so FindInManifest (which uses manifestPathForScope) can read it back.
+	manifestPath := manifestPathForScope(ScopeGlobal)
+	if err := os.MkdirAll(filepath.Dir(manifestPath), 0755); err != nil {
+		t.Fatalf("MkdirAll() error = %v", err)
 	}
-
-	err := addEntryToManifest(entry, ScopeGlobal)
-	if err != nil {
-		t.Fatalf("addEntryToManifest() error = %v", err)
+	data := []byte(`{"packages":[{"source":"git:github.com/user/repo","repo":"","host":"github.com","path":"user/repo","pinned":false,"scope":"global","installed":"0001-01-01T00:00:00Z"}]}`)
+	if err := os.WriteFile(manifestPath, data, 0644); err != nil {
+		t.Fatalf("WriteFile() error = %v", err)
 	}
 
 	// Find it

internal/extensions/llmusage_test.go

@@ -0,0 +1,119 @@
+package extensions
+
+import "testing"
+
+func TestRunner_EmitLLMUsage(t *testing.T) {
+	var got LLMUsageEvent
+	var called bool
+	ext := makeHandlerExt("llmusage.go", map[EventType][]HandlerFunc{
+		LLMUsage: {
+			func(e Event, c Context) Result {
+				got = e.(LLMUsageEvent)
+				called = true
+				return nil
+			},
+		},
+	})
+
+	r := makeRunner(ext)
+	_, err := r.Emit(LLMUsageEvent{
+		InputTokens:  100,
+		OutputTokens: 50,
+		Cost:         0.0012,
+		Model:        "claude-sonnet-4-5-20250929",
+		Provider:     "anthropic",
+		StepNumber:   2,
+		FinishReason: "tool_calls",
+	})
+	if err != nil {
+		t.Fatalf("emit: %v", err)
+	}
+	if !called {
+		t.Fatal("expected LLMUsage handler to be called")
+	}
+	if got.InputTokens != 100 || got.OutputTokens != 50 {
+		t.Errorf("token fields not propagated: %+v", got)
+	}
+	if got.Cost != 0.0012 {
+		t.Errorf("cost not propagated, got %v", got.Cost)
+	}
+	if got.Model != "claude-sonnet-4-5-20250929" || got.Provider != "anthropic" {
+		t.Errorf("model/provider not propagated: %+v", got)
+	}
+	if got.StepNumber != 2 || got.FinishReason != "tool_calls" {
+		t.Errorf("step/finish reason not propagated: %+v", got)
+	}
+}
+
+func TestRunner_LLMUsageRegisteredViaTestAPI(t *testing.T) {
+	// Verify NewTestAPI wires up onLLMUsage so the extension can call
+	// api.OnLLMUsage during Init.
+	ext := &LoadedExtension{Handlers: make(map[EventType][]HandlerFunc)}
+	api := NewTestAPI(ext)
+
+	var calls int
+	api.OnLLMUsage(func(e LLMUsageEvent, c Context) {
+		calls++
+	})
+
+	if len(ext.Handlers[LLMUsage]) != 1 {
+		t.Fatalf("expected 1 LLMUsage handler registered, got %d", len(ext.Handlers[LLMUsage]))
+	}
+
+	r := makeRunner(*ext)
+	_, _ = r.Emit(LLMUsageEvent{InputTokens: 1})
+	if calls != 1 {
+		t.Errorf("expected handler called once, got %d", calls)
+	}
+}
+
+func TestAgentEndEvent_EnrichedFields(t *testing.T) {
+	// Verify the enriched event carries through Emit without mangling.
+	var got AgentEndEvent
+	ext := makeHandlerExt("end.go", map[EventType][]HandlerFunc{
+		AgentEnd: {
+			func(e Event, c Context) Result {
+				got = e.(AgentEndEvent)
+				return nil
+			},
+		},
+	})
+	r := makeRunner(ext)
+	_, err := r.Emit(AgentEndEvent{
+		Response:              "done",
+		StopReason:            "completed",
+		ToolCallCount:         3,
+		ToolNames:             []string{"bash", "read", "bash"},
+		LLMCallCount:          4,
+		InputTokensDelta:      1500,
+		OutputTokensDelta:     400,
+		CacheReadTokensDelta:  200,
+		CacheWriteTokensDelta: 100,
+		CostDelta:             0.0123,
+		DurationMs:            2500,
+	})
+	if err != nil {
+		t.Fatalf("emit: %v", err)
+	}
+	if got.ToolCallCount != 3 {
+		t.Errorf("ToolCallCount: got %d want 3", got.ToolCallCount)
+	}
+	if len(got.ToolNames) != 3 || got.ToolNames[0] != "bash" || got.ToolNames[2] != "bash" {
+		t.Errorf("ToolNames: %v", got.ToolNames)
+	}
+	if got.LLMCallCount != 4 {
+		t.Errorf("LLMCallCount: got %d want 4", got.LLMCallCount)
+	}
+	if got.InputTokensDelta != 1500 || got.OutputTokensDelta != 400 {
+		t.Errorf("token deltas: %+v", got)
+	}
+	if got.CacheReadTokensDelta != 200 || got.CacheWriteTokensDelta != 100 {
+		t.Errorf("cache deltas: %+v", got)
+	}
+	if got.CostDelta != 0.0123 {
+		t.Errorf("CostDelta: got %v", got.CostDelta)
+	}
+	if got.DurationMs != 2500 {
+		t.Errorf("DurationMs: got %d", got.DurationMs)
+	}
+}

internal/extensions/loader.go

@@ -34,15 +34,10 @@ func LoadExtensions(extraPaths []string) ([]LoadedExtension, error) {
 	for _, p := range paths {
 		ext, err := loadSingleExtension(p)
 		if err != nil {
-			log.Warn("skipping extension", "path", p, "err", err)
 			continue
 		}
 		loaded = append(loaded, *ext)
-		log.Debug("loaded extension", "path", p,
-			"handlers", countHandlers(ext),
-			"tools", len(ext.Tools),
-			"commands", len(ext.Commands),
-			"tool_renderers", len(ext.ToolRenderers))
+		log.Debug("loaded extension", "path", p, "handlers", countHandlers(ext), "tools", len(ext.Tools), "commands", len(ext.Commands), "tool_renderers", len(ext.ToolRenderers))
 	}
 	return loaded, nil
 }
@@ -434,6 +429,24 @@ func loadSingleExtension(path string) (*LoadedExtension, error) {
 				return *r
 			})
 		},
+		onToolCallInputStart: func(h func(ToolCallInputStartEvent, Context)) {
+			reg(ToolCallInputStart, func(e Event, c Context) Result {
+				h(e.(ToolCallInputStartEvent), c)
+				return nil
+			})
+		},
+		onToolCallInputDelta: func(h func(ToolCallInputDeltaEvent, Context)) {
+			reg(ToolCallInputDelta, func(e Event, c Context) Result {
+				h(e.(ToolCallInputDeltaEvent), c)
+				return nil
+			})
+		},
+		onToolCallInputEnd: func(h func(ToolCallInputEndEvent, Context)) {
+			reg(ToolCallInputEnd, func(e Event, c Context) Result {
+				h(e.(ToolCallInputEndEvent), c)
+				return nil
+			})
+		},
 		onToolExecStart: func(h func(ToolExecutionStartEvent, Context)) {
 			reg(ToolExecutionStart, func(e Event, c Context) Result {
 				h(e.(ToolExecutionStartEvent), c)
@@ -605,6 +618,63 @@ func loadSingleExtension(path string) (*LoadedExtension, error) {
 				return nil
 			})
 		},
+		onStepStart: func(h func(StepStartEvent, Context)) {
+			reg(StepStart, func(e Event, c Context) Result {
+				h(e.(StepStartEvent), c)
+				return nil
+			})
+		},
+		onStepFinish: func(h func(StepFinishEvent, Context)) {
+			reg(StepFinish, func(e Event, c Context) Result {
+				h(e.(StepFinishEvent), c)
+				return nil
+			})
+		},
+		onReasoningStart: func(h func(ReasoningStartEvent, Context)) {
+			reg(ReasoningStart, func(e Event, c Context) Result {
+				h(e.(ReasoningStartEvent), c)
+				return nil
+			})
+		},
+		onWarnings: func(h func(WarningsEvent, Context)) {
+			reg(Warnings, func(e Event, c Context) Result {
+				h(e.(WarningsEvent), c)
+				return nil
+			})
+		},
+		onSource: func(h func(SourceEvent, Context)) {
+			reg(Source, func(e Event, c Context) Result {
+				h(e.(SourceEvent), c)
+				return nil
+			})
+		},
+		onError: func(h func(ErrorEvent, Context)) {
+			reg(Error, func(e Event, c Context) Result {
+				h(e.(ErrorEvent), c)
+				return nil
+			})
+		},
+		onRetry: func(h func(RetryEvent, Context)) {
+			reg(Retry, func(e Event, c Context) Result {
+				h(e.(RetryEvent), c)
+				return nil
+			})
+		},
+		onPrepareStep: func(h func(PrepareStepEvent, Context) *PrepareStepResult) {
+			reg(PrepareStep, func(e Event, c Context) Result {
+				r := h(e.(PrepareStepEvent), c)
+				if r == nil {
+					return nil
+				}
+				return *r
+			})
+		},
+		onLLMUsage: func(h func(LLMUsageEvent, Context)) {
+			reg(LLMUsage, func(e Event, c Context) Result {
+				h(e.(LLMUsageEvent), c)
+				return nil
+			})
+		},
 	}
 
 	// Call Init — the extension registers its handlers, tools, commands.

internal/extensions/manifest.go

@@ -72,30 +72,6 @@ func loadManifestFromPath(path string) (*Manifest, error) {
 	return &manifest, nil
 }
 
-// saveManifestToScope saves the manifest to the given scope.
-func saveManifestToScope(manifest *Manifest, scope InstallScope) error {
-	path := manifestPathForScope(scope)
-	return saveManifestToPath(manifest, path)
-}
-
-// saveManifestToPath saves a manifest to a specific file path.
-func saveManifestToPath(manifest *Manifest, path string) error {
-	if err := os.MkdirAll(filepath.Dir(path), 0755); err != nil {
-		return fmt.Errorf("creating manifest directory: %w", err)
-	}
-
-	data, err := json.MarshalIndent(manifest, "", "  ")
-	if err != nil {
-		return fmt.Errorf("encoding manifest: %w", err)
-	}
-
-	if err := os.WriteFile(path, data, 0644); err != nil {
-		return fmt.Errorf("writing manifest: %w", err)
-	}
-
-	return nil
-}
-
 // manifestPathForScope returns the manifest file path for a scope.
 func manifestPathForScope(scope InstallScope) string {
 	if scope == ScopeProject {
@@ -113,55 +89,6 @@ func manifestPathForScope(scope InstallScope) string {
 	return filepath.Join(base, "kit", "git", "packages.json")
 }
 
-// GetGlobalManifest returns the global manifest.
-func GetGlobalManifest() (*Manifest, error) {
-	return loadManifestFromScope(ScopeGlobal)
-}
-
-// GetProjectManifest returns the project manifest.
-func GetProjectManifest() (*Manifest, error) {
-	return loadManifestFromScope(ScopeProject)
-}
-
-// addEntryToManifest adds or replaces an entry in the manifest for a scope.
-func addEntryToManifest(entry ManifestEntry, scope InstallScope) error {
-	manifest, err := loadManifestFromScope(scope)
-	if err != nil {
-		return err
-	}
-
-	// Remove any existing entry with same identity
-	identity := entry.Identity()
-	filtered := make([]ManifestEntry, 0, len(manifest.Packages))
-	for _, p := range manifest.Packages {
-		if p.Identity() != identity {
-			filtered = append(filtered, p)
-		}
-	}
-	filtered = append(filtered, entry)
-	manifest.Packages = filtered
-
-	return saveManifestToScope(manifest, scope)
-}
-
-// removeEntryFromManifest removes an entry by identity from the manifest for a scope.
-func removeEntryFromManifest(identity string, scope InstallScope) error {
-	manifest, err := loadManifestFromScope(scope)
-	if err != nil {
-		return err
-	}
-
-	filtered := make([]ManifestEntry, 0, len(manifest.Packages))
-	for _, p := range manifest.Packages {
-		if p.Identity() != identity {
-			filtered = append(filtered, p)
-		}
-	}
-	manifest.Packages = filtered
-
-	return saveManifestToScope(manifest, scope)
-}
-
 // FindInManifest finds an entry by identity in either global or project manifest.
 // Returns the entry and its scope, or nil if not found.
 func FindInManifest(identity string) (*ManifestEntry, InstallScope, error) {

internal/extensions/runner.go

@@ -1,21 +1,96 @@
 package extensions
 
 import (
+	"bytes"
+	"encoding/json"
 	"fmt"
+	"log"
+	"maps"
 	"os"
+	"path/filepath"
+	"runtime"
 	"sort"
+	"strconv"
 	"strings"
 	"sync"
 
-	"github.com/charmbracelet/log"
 	"github.com/spf13/viper"
 )
 
+// ---------------------------------------------------------------------------
+// reentrantMu — a per-extension mutex that allows the same goroutine to
+// re-enter (e.g. handler → ctx.EmitCustomEvent → handler in same extension).
+// Different goroutines are serialized, preventing concurrent state mutation.
+// ---------------------------------------------------------------------------
+
+type reentrantMu struct {
+	mu    sync.Mutex
+	cond  *sync.Cond
+	owner int64 // goroutine ID that holds the lock, or 0
+	depth int   // re-entrancy depth
+}
+
+// initReentrantMu initializes the reentrant mutex in-place. Must be called
+// after the struct is at its final memory location (not before copying).
+func (r *reentrantMu) init() {
+	r.cond = sync.NewCond(&r.mu)
+}
+
+// lock acquires the mutex. If the calling goroutine already holds it, the
+// call succeeds immediately (re-entrant). Every call to lock must be paired
+// with a call to unlock.
+func (r *reentrantMu) lock() {
+	gid := goroutineID()
+	r.mu.Lock()
+	if r.owner == gid {
+		// Re-entrant: same goroutine already holds the lock.
+		r.depth++
+		r.mu.Unlock()
+		return
+	}
+	// Wait for the current owner to release.
+	for r.owner != 0 {
+		r.cond.Wait() // releases mu, blocks, re-acquires mu on wake
+	}
+	r.owner = gid
+	r.depth = 1
+	r.mu.Unlock()
+}
+
+// unlock releases the mutex (or decrements re-entrancy depth).
+func (r *reentrantMu) unlock() {
+	r.mu.Lock()
+	r.depth--
+	if r.depth == 0 {
+		r.owner = 0
+		r.cond.Signal()
+	}
+	r.mu.Unlock()
+}
+
+// goroutineID extracts the current goroutine's ID from runtime.Stack output.
+// This is a well-known technique used by Go testing infrastructure.
+func goroutineID() int64 {
+	var buf [64]byte
+	n := runtime.Stack(buf[:], false)
+	// Stack output starts with "goroutine NNN ["
+	s := buf[:n]
+	s = s[len("goroutine "):]
+	s = s[:bytes.IndexByte(s, ' ')]
+	id, _ := strconv.ParseInt(string(s), 10, 64)
+	return id
+}
+
 // Runner manages loaded extensions and dispatches events to their handlers
 // sequentially. Handlers execute in extension
 // load order; for cancellable events the first blocking result wins.
+//
+// Each extension has a dedicated reentrant mutex so that handlers for the
+// same extension are serialized (preventing data races on shared package-level
+// state), while handlers for different extensions may execute concurrently.
 type Runner struct {
 	extensions      []LoadedExtension
+	extMu           []reentrantMu // per-extension reentrant mutex, indexed by extension position
 	ctx             Context
 	widgets         map[string]WidgetConfig   // keyed by widget ID
 	statusEntries   map[string]StatusBarEntry // keyed by status key
@@ -26,9 +101,24 @@ type Runner struct {
 	disabledTools   map[string]bool           // nil = all tools enabled
 	customEventSubs map[string][]func(string) // inter-extension event bus
 	optionOverrides map[string]string         // runtime option overrides
+	configStore     *viper.Viper              // per-instance config store (nil = global)
+	state           map[string]string         // session-scoped extension state (last-write-wins)
+	stateMu         sync.RWMutex              // guards state independently of mu
+	saverMu         sync.Mutex                // serializes stateSaver invocations so atomic-rename writes don't interleave
+	stateSaver      func()                    // optional persistence hook invoked after each state mutation
 	mu              sync.RWMutex
 }
 
+// SetConfigStore sets the per-instance configuration store used by GetOption
+// to resolve "options.<name>" config values. When unset (nil), GetOption falls
+// back to the process-global viper store. Threading a per-Kit store keeps
+// extension option resolution isolated between Kit instances.
+func (r *Runner) SetConfigStore(v *viper.Viper) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	r.configStore = v
+}
+
 // ShortcutEntry pairs a shortcut definition with its handler.
 type ShortcutEntry struct {
 	Def     ShortcutDef
@@ -52,7 +142,11 @@ type LoadedExtension struct {
 
 // NewRunner creates a Runner from a set of loaded extensions.
 func NewRunner(exts []LoadedExtension) *Runner {
-	return &Runner{extensions: exts}
+	mus := make([]reentrantMu, len(exts))
+	for i := range mus {
+		mus[i].init()
+	}
+	return &Runner{extensions: exts, extMu: mus}
 }
 
 // SetContext updates the runtime context (session ID, model, etc.) that is
@@ -177,6 +271,18 @@ func normalizeContext(ctx Context) Context {
 	if ctx.GetEntries == nil {
 		ctx.GetEntries = func(string) []ExtensionEntry { return nil }
 	}
+	if ctx.SetState == nil {
+		ctx.SetState = func(string, string) {}
+	}
+	if ctx.GetState == nil {
+		ctx.GetState = func(string) (string, bool) { return "", false }
+	}
+	if ctx.DeleteState == nil {
+		ctx.DeleteState = func(string) {}
+	}
+	if ctx.ListState == nil {
+		ctx.ListState = func() []string { return nil }
+	}
 	if ctx.GetOption == nil {
 		ctx.GetOption = func(string) string { return "" }
 	}
@@ -367,13 +473,15 @@ func (r *Runner) Emit(event Event) (Result, error) {
 	for i := range r.extensions {
 		ext := &r.extensions[i]
 		handlers := ext.Handlers[event.Type()]
+		if len(handlers) == 0 {
+			continue
+		}
+
+		r.extMu[i].lock()
 		for _, handler := range handlers {
 			result, err := safeCall(handler, event, ctx)
 			if err != nil {
-				log.Warn("extension handler error",
-					"path", ext.Path,
-					"event", event.Type(),
-					"err", err)
+				log.Printf("WARN extension handler error: path=%s event=%s err=%v", ext.Path, event.Type(), err)
 				continue
 			}
 			if result == nil {
@@ -382,6 +490,7 @@ func (r *Runner) Emit(event Event) (Result, error) {
 
 			// Check for blocking/short-circuit results.
 			if isBlocking(result) {
+				r.extMu[i].unlock()
 				return result, nil
 			}
 
@@ -389,6 +498,7 @@ func (r *Runner) Emit(event Event) (Result, error) {
 			// the caller is responsible for applying the modifications.
 			accumulated = result
 		}
+		r.extMu[i].unlock()
 	}
 	return accumulated, nil
 }
@@ -654,6 +764,168 @@ func (r *Runner) GetMessageRenderer(name string) *MessageRendererConfig {
 	return nil
 }
 
+// ---------------------------------------------------------------------------
+// Extension state store (session-scoped, last-write-wins)
+// ---------------------------------------------------------------------------
+
+// SetState records a key-value pair in the runner's session-scoped extension
+// state store. The store is in-memory; callers wire SetStateSaver to persist
+// changes to a sidecar file. Thread-safe.
+//
+// When a saver is installed, concurrent SetState/DeleteState invocations are
+// serialized through saverMu so that overlapping snapshot-and-rename writes
+// cannot interleave (which would otherwise race on the shared tmp file and
+// risk persisting an older snapshot after a newer one).
+func (r *Runner) SetState(key, value string) {
+	r.stateMu.Lock()
+	if r.state == nil {
+		r.state = make(map[string]string)
+	}
+	r.state[key] = value
+	saver := r.stateSaver
+	r.stateMu.Unlock()
+	r.runSaver(saver)
+}
+
+// GetState returns the value previously stored via SetState, plus a bool
+// indicating whether the key was present. Thread-safe.
+func (r *Runner) GetState(key string) (string, bool) {
+	r.stateMu.RLock()
+	defer r.stateMu.RUnlock()
+	v, ok := r.state[key]
+	return v, ok
+}
+
+// DeleteState removes a key from the state store. No-op if the key is
+// missing. Thread-safe. Saver invocations are serialized via saverMu — see
+// SetState for the rationale.
+func (r *Runner) DeleteState(key string) {
+	r.stateMu.Lock()
+	_, existed := r.state[key]
+	if existed {
+		delete(r.state, key)
+	}
+	saver := r.stateSaver
+	r.stateMu.Unlock()
+	if !existed {
+		return
+	}
+	r.runSaver(saver)
+}
+
+// runSaver invokes the optional persistence callback under saverMu so
+// concurrent SetState/DeleteState writers cannot race on the shared tmp
+// file used by SaveStateToFile's atomic rename. The deferred Unlock
+// guarantees saverMu is released even if the saver panics.
+func (r *Runner) runSaver(saver func()) {
+	if saver == nil {
+		return
+	}
+	r.saverMu.Lock()
+	defer r.saverMu.Unlock()
+	saver()
+}
+
+// ListState returns all keys currently in the state store, in unspecified
+// order. Thread-safe.
+func (r *Runner) ListState() []string {
+	r.stateMu.RLock()
+	defer r.stateMu.RUnlock()
+	if len(r.state) == 0 {
+		return nil
+	}
+	keys := make([]string, 0, len(r.state))
+	for k := range r.state {
+		keys = append(keys, k)
+	}
+	return keys
+}
+
+// SetStateSaver installs an optional persistence hook invoked after each
+// mutation to the state store (SetState / DeleteState / LoadStateFromFile).
+// Pass nil to disable persistence. Thread-safe.
+func (r *Runner) SetStateSaver(saver func()) {
+	r.stateMu.Lock()
+	defer r.stateMu.Unlock()
+	r.stateSaver = saver
+}
+
+// SnapshotState returns a copy of the current state store as a
+// fresh map. Useful for persisting to disk without holding the lock.
+// Thread-safe.
+func (r *Runner) SnapshotState() map[string]string {
+	r.stateMu.RLock()
+	defer r.stateMu.RUnlock()
+	if len(r.state) == 0 {
+		return nil
+	}
+	copyMap := make(map[string]string, len(r.state))
+	maps.Copy(copyMap, r.state)
+	return copyMap
+}
+
+// LoadStateFromFile reads a JSON map from path and replaces the in-memory
+// state store with its contents. Missing or empty files are treated as
+// "no prior state": the in-memory store is replaced with an empty map so
+// callers can safely switch sessions without leaking keys from a prior
+// session into a new one. Malformed JSON returns the parse error without
+// touching the existing store. Thread-safe.
+func (r *Runner) LoadStateFromFile(path string) error {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			r.stateMu.Lock()
+			r.state = map[string]string{}
+			r.stateMu.Unlock()
+			return nil
+		}
+		return fmt.Errorf("reading extension state: %w", err)
+	}
+	if len(data) == 0 {
+		r.stateMu.Lock()
+		r.state = map[string]string{}
+		r.stateMu.Unlock()
+		return nil
+	}
+	var loaded map[string]string
+	if err := json.Unmarshal(data, &loaded); err != nil {
+		return fmt.Errorf("parsing extension state: %w", err)
+	}
+	r.stateMu.Lock()
+	r.state = loaded
+	r.stateMu.Unlock()
+	return nil
+}
+
+// SaveStateToFile writes the current state store to path as JSON, creating
+// parent directories as needed. An empty store writes an empty object so
+// that consumers can distinguish "loaded but empty" from "never saved".
+// Writes are atomic via a tmp-file-and-rename sequence. Thread-safe.
+func (r *Runner) SaveStateToFile(path string) error {
+	snap := r.SnapshotState()
+	if snap == nil {
+		snap = map[string]string{}
+	}
+	data, err := json.MarshalIndent(snap, "", "  ")
+	if err != nil {
+		return fmt.Errorf("marshalling extension state: %w", err)
+	}
+	if dir := filepath.Dir(path); dir != "." && dir != "" {
+		if err := os.MkdirAll(dir, 0o755); err != nil {
+			return fmt.Errorf("creating state directory: %w", err)
+		}
+	}
+	tmp := path + ".tmp"
+	if err := os.WriteFile(tmp, data, 0o644); err != nil {
+		return fmt.Errorf("writing extension state: %w", err)
+	}
+	if err := os.Rename(tmp, path); err != nil {
+		_ = os.Remove(tmp)
+		return fmt.Errorf("renaming extension state: %w", err)
+	}
+	return nil
+}
+
 // ---------------------------------------------------------------------------
 // Hot-reload
 // ---------------------------------------------------------------------------
@@ -677,7 +949,9 @@ func (r *Runner) Reload(exts []LoadedExtension) {
 	r.uiVisibility = nil
 	r.disabledTools = nil
 	r.customEventSubs = nil
-	// optionOverrides are intentionally preserved.
+	// optionOverrides and state are intentionally preserved across reloads:
+	// they represent user/session intent (not extension code) and would be
+	// surprising to lose on a hot-reload.
 }
 
 // ---------------------------------------------------------------------------
@@ -707,9 +981,7 @@ func (r *Runner) EmitCustomEvent(name, data string) {
 	safeInvoke := func(h func(string)) {
 		defer func() {
 			if rec := recover(); rec != nil {
-				log.Warn("custom event handler panicked",
-					"event", name,
-					"err", fmt.Sprintf("%v", rec))
+				log.Printf("WARN custom event handler panicked: event=%s err=%v", name, rec)
 			}
 		}()
 		h(data)
@@ -717,11 +989,17 @@ func (r *Runner) EmitCustomEvent(name, data string) {
 
 	// Extension-registered handlers first (in load order).
 	for i := range r.extensions {
-		for _, h := range r.extensions[i].CustomEventHandlers[name] {
+		extHandlers := r.extensions[i].CustomEventHandlers[name]
+		if len(extHandlers) == 0 {
+			continue
+		}
+		r.extMu[i].lock()
+		for _, h := range extHandlers {
 			safeInvoke(h)
 		}
+		r.extMu[i].unlock()
 	}
-	// Then dynamic subscriptions.
+	// Then dynamic subscriptions (not extension-scoped, no per-ext lock).
 	for _, h := range dynamicHandlers {
 		safeInvoke(h)
 	}
@@ -788,7 +1066,13 @@ func (r *Runner) GetOption(name string) string {
 
 	// 3. Viper config: options.<name>
 	configKey := "options." + name
-	if v := viper.GetString(configKey); v != "" {
+	r.mu.RLock()
+	store := r.configStore
+	r.mu.RUnlock()
+	if store == nil {
+		store = viper.GetViper()
+	}
+	if v := store.GetString(configKey); v != "" {
 		return v
 	}
 

internal/extensions/runner_test.go

@@ -1,6 +1,7 @@
 package extensions
 
 import (
+	"sync"
 	"testing"
 )
 
@@ -571,3 +572,142 @@ func TestRunner_ContextPrintNilSafe(t *testing.T) {
 		t.Fatalf("unexpected error: %v", err)
 	}
 }
+
+func TestRunner_ConcurrentEmitSameExtension(t *testing.T) {
+	// Verify that concurrent Emit calls for the same extension are serialized
+	// and don't cause data races on shared handler state.
+	var counter int
+	ext := makeHandlerExt("shared-state.go", map[EventType][]HandlerFunc{
+		SubagentStart: {
+			func(e Event, c Context) Result {
+				// Read-modify-write: racy without serialization.
+				v := counter
+				counter = v + 1
+				return nil
+			},
+		},
+		SubagentChunk: {
+			func(e Event, c Context) Result {
+				v := counter
+				counter = v + 1
+				return nil
+			},
+		},
+	})
+
+	r := makeRunner(ext)
+	var wg sync.WaitGroup
+	const goroutines = 20
+	const iterations = 50
+	wg.Add(goroutines)
+	for range goroutines {
+		go func() {
+			defer wg.Done()
+			for range iterations {
+				_, _ = r.Emit(SubagentStartEvent{ToolCallID: "x"})
+				_, _ = r.Emit(SubagentChunkEvent{ToolCallID: "x"})
+			}
+		}()
+	}
+	wg.Wait()
+	if counter != goroutines*iterations*2 {
+		t.Errorf("expected counter=%d, got %d (race detected)", goroutines*iterations*2, counter)
+	}
+}
+
+func TestRunner_ConcurrentEmitDifferentExtensions(t *testing.T) {
+	// Two extensions with independent state should not block each other
+	// and should both run correctly under concurrent Emit calls.
+	var counter1, counter2 int
+	ext1 := makeHandlerExt("ext1.go", map[EventType][]HandlerFunc{
+		SubagentStart: {
+			func(e Event, c Context) Result {
+				v := counter1
+				counter1 = v + 1
+				return nil
+			},
+		},
+	})
+	ext2 := makeHandlerExt("ext2.go", map[EventType][]HandlerFunc{
+		SubagentStart: {
+			func(e Event, c Context) Result {
+				v := counter2
+				counter2 = v + 1
+				return nil
+			},
+		},
+	})
+
+	r := makeRunner(ext1, ext2)
+	var wg sync.WaitGroup
+	const goroutines = 20
+	const iterations = 50
+	wg.Add(goroutines)
+	for range goroutines {
+		go func() {
+			defer wg.Done()
+			for range iterations {
+				_, _ = r.Emit(SubagentStartEvent{ToolCallID: "x"})
+			}
+		}()
+	}
+	wg.Wait()
+	expected := goroutines * iterations
+	if counter1 != expected {
+		t.Errorf("ext1 counter: expected %d, got %d", expected, counter1)
+	}
+	if counter2 != expected {
+		t.Errorf("ext2 counter: expected %d, got %d", expected, counter2)
+	}
+}
+
+func TestRunner_ReentrantEmitCustomEvent(t *testing.T) {
+	// Verify that a handler can call EmitCustomEvent (which dispatches to
+	// the same extension's custom event handlers) without deadlocking.
+	var order []string
+	ext := LoadedExtension{
+		Path: "reentrant.go",
+		Handlers: map[EventType][]HandlerFunc{
+			SessionStart: {
+				func(e Event, c Context) Result {
+					order = append(order, "session_start")
+					// This triggers EmitCustomEvent for the same extension
+					// via a direct runner call (simulating ctx.EmitCustomEvent).
+					return nil
+				},
+			},
+		},
+		CustomEventHandlers: map[string][]func(string){
+			"test-event": {
+				func(data string) {
+					order = append(order, "custom:"+data)
+				},
+			},
+		},
+	}
+
+	r := makeRunner(ext)
+
+	// Wire up the handler to call EmitCustomEvent re-entrantly.
+	ext.Handlers[SessionStart] = []HandlerFunc{
+		func(e Event, c Context) Result {
+			order = append(order, "session_start")
+			r.EmitCustomEvent("test-event", "hello")
+			return nil
+		},
+	}
+	r.extensions[0] = ext
+	// Rebuild mutexes after modifying extensions slice.
+	r.extMu = make([]reentrantMu, len(r.extensions))
+	for i := range r.extMu {
+		r.extMu[i].init()
+	}
+
+	_, err := r.Emit(SessionStartEvent{})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if len(order) != 2 || order[0] != "session_start" || order[1] != "custom:hello" {
+		t.Errorf("expected [session_start, custom:hello], got %v", order)
+	}
+}

internal/extensions/state_test.go

@@ -0,0 +1,262 @@
+package extensions
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"sync"
+	"testing"
+	"time"
+)
+
+func TestRunner_State_BasicSetGetDelete(t *testing.T) {
+	r := NewRunner(nil)
+
+	if _, ok := r.GetState("missing"); ok {
+		t.Fatal("expected GetState to return ok=false for missing key")
+	}
+
+	r.SetState("a", "1")
+	r.SetState("b", "2")
+	r.SetState("a", "3") // last-write-wins
+
+	if v, ok := r.GetState("a"); !ok || v != "3" {
+		t.Errorf("expected GetState(a)=(3,true), got (%q,%v)", v, ok)
+	}
+	if v, ok := r.GetState("b"); !ok || v != "2" {
+		t.Errorf("expected GetState(b)=(2,true), got (%q,%v)", v, ok)
+	}
+
+	keys := r.ListState()
+	if len(keys) != 2 {
+		t.Errorf("expected 2 keys, got %d (%v)", len(keys), keys)
+	}
+
+	r.DeleteState("a")
+	if _, ok := r.GetState("a"); ok {
+		t.Error("expected key a to be gone after DeleteState")
+	}
+	if len(r.ListState()) != 1 {
+		t.Errorf("expected 1 key after delete, got %v", r.ListState())
+	}
+
+	// Deleting missing key is a no-op.
+	r.DeleteState("never-there")
+}
+
+func TestRunner_State_SaverFires(t *testing.T) {
+	r := NewRunner(nil)
+	var calls int
+	var mu sync.Mutex
+	r.SetStateSaver(func() {
+		mu.Lock()
+		calls++
+		mu.Unlock()
+	})
+
+	r.SetState("a", "1")
+	r.SetState("a", "2")
+	r.DeleteState("a")
+	r.DeleteState("a") // missing → no save
+
+	mu.Lock()
+	defer mu.Unlock()
+	if calls != 3 {
+		t.Errorf("expected saver to fire 3 times (2 sets + 1 delete), got %d", calls)
+	}
+}
+
+func TestRunner_State_SaveAndLoadRoundTrip(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "ext-state.json")
+
+	r1 := NewRunner(nil)
+	r1.SetState("k1", "v1")
+	r1.SetState("k2", `{"json":"value"}`)
+	if err := r1.SaveStateToFile(path); err != nil {
+		t.Fatalf("SaveStateToFile: %v", err)
+	}
+
+	// Verify file contains JSON map.
+	data, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("reading saved file: %v", err)
+	}
+	var parsed map[string]string
+	if err := json.Unmarshal(data, &parsed); err != nil {
+		t.Fatalf("unmarshalling: %v", err)
+	}
+	if parsed["k1"] != "v1" || parsed["k2"] != `{"json":"value"}` {
+		t.Errorf("unexpected file contents: %v", parsed)
+	}
+
+	r2 := NewRunner(nil)
+	if err := r2.LoadStateFromFile(path); err != nil {
+		t.Fatalf("LoadStateFromFile: %v", err)
+	}
+	if v, ok := r2.GetState("k1"); !ok || v != "v1" {
+		t.Errorf("expected k1=v1 after load, got (%q,%v)", v, ok)
+	}
+	if v, ok := r2.GetState("k2"); !ok || v != `{"json":"value"}` {
+		t.Errorf("expected k2 to round-trip, got %q", v)
+	}
+}
+
+func TestRunner_State_LoadMissingFileClearsState(t *testing.T) {
+	// LoadStateFromFile is documented to "replace the in-memory state store
+	// with its contents"; for a missing file that means clearing the store.
+	// This is what makes session-switching safe: a new session that has not
+	// yet written a sidecar must not inherit keys from a prior session.
+	r := NewRunner(nil)
+	r.SetState("a", "1")
+	if err := r.LoadStateFromFile(filepath.Join(t.TempDir(), "does-not-exist.json")); err != nil {
+		t.Errorf("expected nil error for missing file, got %v", err)
+	}
+	if _, ok := r.GetState("a"); ok {
+		t.Error("expected pre-existing state to be cleared when target file is missing")
+	}
+	if keys := r.ListState(); keys != nil {
+		t.Errorf("expected ListState() to be nil after clearing, got %v", keys)
+	}
+}
+
+func TestRunner_State_LoadEmptyFileClearsState(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "empty.json")
+	if err := os.WriteFile(path, nil, 0o644); err != nil {
+		t.Fatal(err)
+	}
+	r := NewRunner(nil)
+	r.SetState("a", "1")
+	if err := r.LoadStateFromFile(path); err != nil {
+		t.Errorf("expected nil error for empty file, got %v", err)
+	}
+	if _, ok := r.GetState("a"); ok {
+		t.Error("expected pre-existing state to be cleared when target file is empty")
+	}
+}
+
+func TestRunner_State_LoadMalformedFileError(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "bad.json")
+	if err := os.WriteFile(path, []byte("{not json"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	r := NewRunner(nil)
+	if err := r.LoadStateFromFile(path); err == nil {
+		t.Error("expected error loading malformed JSON, got nil")
+	}
+}
+
+func TestRunner_State_PersistenceViaSaver(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "ext-state.json")
+
+	r := NewRunner(nil)
+	r.SetStateSaver(func() {
+		_ = r.SaveStateToFile(path)
+	})
+	r.SetState("hello", "world")
+
+	// File should exist with the value already.
+	data, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("reading saved file: %v", err)
+	}
+	var parsed map[string]string
+	if err := json.Unmarshal(data, &parsed); err != nil {
+		t.Fatalf("unmarshalling: %v", err)
+	}
+	if parsed["hello"] != "world" {
+		t.Errorf("expected file to contain hello=world, got %v", parsed)
+	}
+}
+
+func TestRunner_State_ConcurrentSet(t *testing.T) {
+	r := NewRunner(nil)
+	var wg sync.WaitGroup
+	const goroutines = 16
+	const iterations = 100
+	wg.Add(goroutines)
+	for range goroutines {
+		go func() {
+			defer wg.Done()
+			for range iterations {
+				r.SetState("k", "v")
+				_, _ = r.GetState("k")
+			}
+		}()
+	}
+	wg.Wait()
+	if v, ok := r.GetState("k"); !ok || v != "v" {
+		t.Errorf("expected k=v after concurrent writes, got (%q,%v)", v, ok)
+	}
+}
+
+func TestRunner_State_ContextNoOpsWhenUnset(t *testing.T) {
+	// Verify normalizeContext installs safe no-ops for SetState/GetState/etc.
+	// when not provided by the caller.
+	ext := makeHandlerExt("state.go", map[EventType][]HandlerFunc{
+		SessionStart: {
+			func(e Event, c Context) Result {
+				// All four state functions should be non-nil and safe to call.
+				c.SetState("a", "b")
+				if v, ok := c.GetState("a"); ok || v != "" {
+					t.Errorf("no-op GetState should return (\"\", false); got (%q,%v)", v, ok)
+				}
+				c.DeleteState("a")
+				if keys := c.ListState(); keys != nil {
+					t.Errorf("no-op ListState should return nil; got %v", keys)
+				}
+				return nil
+			},
+		},
+	})
+	r := makeRunner(ext)
+	// SetContext with empty Context to exercise normalizeContext defaults.
+	r.SetContext(Context{})
+	_, err := r.Emit(SessionStartEvent{})
+	if err != nil {
+		t.Fatalf("emit: %v", err)
+	}
+}
+
+func TestRunner_State_SaverPanicReleasesSaverMu(t *testing.T) {
+	// If the saver callback panics (e.g. disk full mid-write), runSaver
+	// must still release saverMu so subsequent SetState/DeleteState calls
+	// can make progress. Without `defer Unlock()` the lock would be
+	// permanently held and the next write would deadlock.
+	r := NewRunner(nil)
+	var calls int
+	r.SetStateSaver(func() {
+		calls++
+		if calls == 1 {
+			panic("simulated disk-write failure")
+		}
+	})
+
+	// First call panics. Recover, then verify a follow-up call still works
+	// without blocking (proving saverMu was released).
+	func() {
+		defer func() {
+			if rec := recover(); rec == nil {
+				t.Fatal("expected panic from first saver invocation")
+			}
+		}()
+		r.SetState("a", "1")
+	}()
+
+	done := make(chan struct{})
+	go func() {
+		r.SetState("b", "2") // would deadlock if saverMu were still held
+		close(done)
+	}()
+	select {
+	case <-done:
+	case <-time.After(2 * time.Second):
+		t.Fatal("SetState after saver panic blocked — saverMu was not released")
+	}
+	if calls != 2 {
+		t.Errorf("expected saver to fire twice (panic + recovery write), got %d", calls)
+	}
+}

internal/extensions/subagent.go

@@ -2,22 +2,15 @@
 package extensions
 
 import (
-	"bufio"
-	"context"
-	"encoding/json"
 	"fmt"
 	"os"
-	"os/exec"
-	"strings"
 	"sync"
-	"sync/atomic"
 	"time"
 )
 
 // ---------------------------------------------------------------------------
 // Subagent types
 // ---------------------------------------------------------------------------
-
 // SubagentConfig configures a subagent spawn.
 type SubagentConfig struct {
 	// Prompt is the task/instruction for the subagent (required).
@@ -157,221 +150,3 @@ func (h *SubagentHandle) Wait() SubagentResult {
 func (h *SubagentHandle) Done() <-chan struct{} {
 	return h.done
 }
-
-// ---------------------------------------------------------------------------
-// Internal helpers
-// ---------------------------------------------------------------------------
-
-// subagentJSONOutput matches the JSON envelope produced by `kit --json`.
-type subagentJSONOutput struct {
-	Response   string `json:"response"`
-	StopReason string `json:"stop_reason,omitempty"`
-	SessionID  string `json:"session_id,omitempty"`
-	Usage      *struct {
-		InputTokens  int64 `json:"input_tokens"`
-		OutputTokens int64 `json:"output_tokens"`
-	} `json:"usage,omitempty"`
-}
-
-var subagentCounter atomic.Uint64
-
-func generateSubagentID() string {
-	n := subagentCounter.Add(1)
-	return fmt.Sprintf("sub-%d-%d", time.Now().UnixNano(), n)
-}
-
-func findKitBinary() string {
-	// Try the current process executable first.
-	if exe, err := os.Executable(); err == nil {
-		if _, err := os.Stat(exe); err == nil {
-			return exe
-		}
-	}
-	// Fall back to PATH lookup.
-	if p, err := exec.LookPath("kit"); err == nil {
-		return p
-	}
-	return "kit"
-}
-
-// ---------------------------------------------------------------------------
-// SpawnSubagent implementation
-// ---------------------------------------------------------------------------
-
-// SpawnSubagent spawns a child Kit instance to perform a task.
-//
-// When config.Blocking is true, blocks until completion and returns the result
-// directly (handle is nil). When false, returns immediately with a handle for
-// monitoring/cancellation.
-//
-// The subagent runs with --json --no-session --no-extensions flags by default,
-// ensuring isolation from the parent's extensions and session state.
-func SpawnSubagent(cfg SubagentConfig) (*SubagentHandle, *SubagentResult, error) {
-	if cfg.Prompt == "" {
-		return nil, nil, fmt.Errorf("prompt is required")
-	}
-
-	timeout := cfg.Timeout
-	if timeout == 0 {
-		timeout = 5 * time.Minute
-	}
-
-	kitBinary := findKitBinary()
-
-	// Build subprocess arguments.
-	args := []string{
-		"--json",
-		"--no-extensions",
-	}
-	if cfg.NoSession {
-		args = append(args, "--no-session")
-	}
-	if cfg.Model != "" {
-		args = append(args, "--model", cfg.Model)
-	}
-
-	// Handle system prompt - write to temp file if provided.
-	var tmpFile *os.File
-	if cfg.SystemPrompt != "" {
-		var err error
-		tmpFile, err = os.CreateTemp("", "kit-subagent-*.txt")
-		if err != nil {
-			return nil, nil, fmt.Errorf("create temp file: %w", err)
-		}
-		if _, err := tmpFile.WriteString(cfg.SystemPrompt); err != nil {
-			_ = tmpFile.Close()
-			_ = os.Remove(tmpFile.Name())
-			return nil, nil, fmt.Errorf("write system prompt: %w", err)
-		}
-		_ = tmpFile.Close()
-		args = append(args, "--system-prompt", tmpFile.Name())
-	}
-
-	// Add the prompt as a positional argument.
-	args = append(args, cfg.Prompt)
-
-	// Create command with timeout context.
-	ctx, cancel := context.WithTimeout(context.Background(), timeout)
-
-	cmd := exec.CommandContext(ctx, kitBinary, args...)
-	cmd.Env = os.Environ()
-
-	stdout, err := cmd.StdoutPipe()
-	if err != nil {
-		cancel()
-		if tmpFile != nil {
-			_ = os.Remove(tmpFile.Name())
-		}
-		return nil, nil, fmt.Errorf("stdout pipe: %w", err)
-	}
-	stderr, err := cmd.StderrPipe()
-	if err != nil {
-		cancel()
-		if tmpFile != nil {
-			_ = os.Remove(tmpFile.Name())
-		}
-		return nil, nil, fmt.Errorf("stderr pipe: %w", err)
-	}
-
-	handle := &SubagentHandle{
-		ID:   generateSubagentID(),
-		done: make(chan struct{}),
-	}
-
-	// Start the subprocess.
-	start := time.Now()
-	if err := cmd.Start(); err != nil {
-		cancel()
-		if tmpFile != nil {
-			_ = os.Remove(tmpFile.Name())
-		}
-		return nil, nil, fmt.Errorf("start subprocess: %w", err)
-	}
-
-	handle.mu.Lock()
-	handle.proc = cmd.Process
-	handle.mu.Unlock()
-
-	// Run the subprocess monitoring in a goroutine.
-	go func() {
-		defer close(handle.done)
-		defer cancel()
-		if tmpFile != nil {
-			defer func() { _ = os.Remove(tmpFile.Name()) }()
-		}
-
-		var wg sync.WaitGroup
-		var stdoutBuf strings.Builder
-
-		// Read stderr (live output).
-		wg.Go(func() {
-			scanner := bufio.NewScanner(stderr)
-			scanner.Buffer(make([]byte, 256*1024), 256*1024)
-			for scanner.Scan() {
-				line := scanner.Text()
-				if cfg.OnOutput != nil && strings.TrimSpace(line) != "" {
-					cfg.OnOutput(line + "\n")
-				}
-			}
-		})
-
-		// Read stdout (JSON output).
-		scanner := bufio.NewScanner(stdout)
-		scanner.Buffer(make([]byte, 256*1024), 256*1024)
-		for scanner.Scan() {
-			stdoutBuf.WriteString(scanner.Text() + "\n")
-		}
-
-		wg.Wait()
-		waitErr := cmd.Wait()
-		elapsed := time.Since(start)
-
-		// Build result.
-		result := SubagentResult{Elapsed: elapsed}
-		if waitErr != nil {
-			result.Error = waitErr
-			if exitErr, ok := waitErr.(*exec.ExitError); ok {
-				result.ExitCode = exitErr.ExitCode()
-			} else {
-				result.ExitCode = 1
-			}
-		}
-
-		// Parse JSON output.
-		raw := strings.TrimSpace(stdoutBuf.String())
-		var parsed subagentJSONOutput
-		if raw != "" && json.Unmarshal([]byte(raw), &parsed) == nil {
-			result.Response = parsed.Response
-			result.SessionID = parsed.SessionID
-			if parsed.Usage != nil {
-				result.Usage = &SubagentUsage{
-					InputTokens:  parsed.Usage.InputTokens,
-					OutputTokens: parsed.Usage.OutputTokens,
-				}
-			}
-		} else {
-			// Fallback: use raw stdout.
-			result.Response = raw
-		}
-
-		handle.mu.Lock()
-		handle.result = &result
-		handle.proc = nil
-		handle.mu.Unlock()
-
-		if cfg.OnComplete != nil {
-			cfg.OnComplete(result)
-		}
-	}()
-
-	if cfg.Blocking {
-		// Wait for completion and return result directly.
-		<-handle.done
-		handle.mu.Lock()
-		r := handle.result
-		handle.mu.Unlock()
-		return nil, r, nil
-	}
-
-	return handle, nil, nil
-}

internal/extensions/symbols.go

@@ -152,6 +152,9 @@ func Symbols() interp.Exports {
 			// Event structs
 			"ToolCallEvent":           reflect.ValueOf((*ToolCallEvent)(nil)),
 			"ToolCallResult":          reflect.ValueOf((*ToolCallResult)(nil)),
+			"ToolCallInputStartEvent": reflect.ValueOf((*ToolCallInputStartEvent)(nil)),
+			"ToolCallInputDeltaEvent": reflect.ValueOf((*ToolCallInputDeltaEvent)(nil)),
+			"ToolCallInputEndEvent":   reflect.ValueOf((*ToolCallInputEndEvent)(nil)),
 			"ToolExecutionStartEvent": reflect.ValueOf((*ToolExecutionStartEvent)(nil)),
 			"ToolExecutionEndEvent":   reflect.ValueOf((*ToolExecutionEndEvent)(nil)),
 			"ToolOutputEvent":         reflect.ValueOf((*ToolOutputEvent)(nil)),
@@ -169,6 +172,18 @@ func Symbols() interp.Exports {
 			"SessionStartEvent":       reflect.ValueOf((*SessionStartEvent)(nil)),
 			"SessionShutdownEvent":    reflect.ValueOf((*SessionShutdownEvent)(nil)),
 			"ModelChangeEvent":        reflect.ValueOf((*ModelChangeEvent)(nil)),
+
+			// Step lifecycle events
+			"StepStartEvent":      reflect.ValueOf((*StepStartEvent)(nil)),
+			"StepFinishEvent":     reflect.ValueOf((*StepFinishEvent)(nil)),
+			"ReasoningStartEvent": reflect.ValueOf((*ReasoningStartEvent)(nil)),
+			"WarningsEvent":       reflect.ValueOf((*WarningsEvent)(nil)),
+			"SourceEvent":         reflect.ValueOf((*SourceEvent)(nil)),
+			"ErrorEvent":          reflect.ValueOf((*ErrorEvent)(nil)),
+			"RetryEvent":          reflect.ValueOf((*RetryEvent)(nil)),
+			"PrepareStepEvent":    reflect.ValueOf((*PrepareStepEvent)(nil)),
+			"PrepareStepResult":   reflect.ValueOf((*PrepareStepResult)(nil)),
+			"LLMUsageEvent":       reflect.ValueOf((*LLMUsageEvent)(nil)),
 		},
 	}
 }

internal/extensions/test_api.go

@@ -189,5 +189,11 @@ func NewTestAPI(ext *LoadedExtension) API {
 				return nil
 			})
 		},
+		onLLMUsage: func(h func(LLMUsageEvent, Context)) {
+			reg(LLMUsage, func(e Event, c Context) Result {
+				h(e.(LLMUsageEvent), c)
+				return nil
+			})
+		},
 	}
 }

internal/extensions/toolkinds.go

@@ -0,0 +1,38 @@
+package extensions
+
+// ToolKind constants classify what a tool does, enabling UIs to render
+// appropriate visualizations (e.g. diff view for edit tools, command+output
+// for execute tools) and file trackers to identify which results contain
+// modifications.
+//
+// This is the single source of truth for tool-kind classification; the
+// pkg/kit SDK re-exports these constants.
+const (
+	ToolKindExecute  = "execute" // Shell execution (bash)
+	ToolKindEdit     = "edit"    // File modification (edit, write)
+	ToolKindRead     = "read"    // File reading (read, ls)
+	ToolKindSearch   = "search"  // Content/file search (grep, find)
+	ToolKindSubagent = "agent"   // Subagent spawning (subagent)
+)
+
+// coreToolKinds maps built-in tool names to their kind classification.
+// MCP and extension tools without an entry default to ToolKindExecute.
+var coreToolKinds = map[string]string{
+	"bash":     ToolKindExecute,
+	"edit":     ToolKindEdit,
+	"write":    ToolKindEdit,
+	"read":     ToolKindRead,
+	"ls":       ToolKindRead,
+	"grep":     ToolKindSearch,
+	"find":     ToolKindSearch,
+	"subagent": ToolKindSubagent,
+}
+
+// ToolKindFor returns the ToolKind for a given tool name, defaulting to
+// ToolKindExecute for unknown tools (including MCP tools).
+func ToolKindFor(toolName string) string {
+	if kind, ok := coreToolKinds[toolName]; ok {
+		return kind
+	}
+	return ToolKindExecute
+}

internal/extensions/watcher.go

@@ -1,143 +1,32 @@
 package extensions
 
 import (
-	"context"
-	"fmt"
 	"os"
 	"path/filepath"
-	"strings"
-	"sync"
-	"time"
 
-	"github.com/charmbracelet/log"
-	"github.com/fsnotify/fsnotify"
+	"github.com/mark3labs/kit/internal/watcher"
 )
 
-// Watcher monitors extension directories for file changes and triggers
-// a reload callback when .go files are created, modified, or removed.
-// It uses fsnotify for kernel-level file notifications (inotify on Linux,
-// kqueue on macOS) with debouncing to coalesce rapid editor writes.
-type Watcher struct {
-	watcher  *fsnotify.Watcher
-	onReload func()
-	debounce time.Duration
-	cancel   context.CancelFunc
-	done     chan struct{}
-	mu       sync.Mutex
-}
+// Watcher monitors extension directories for .go file changes and triggers
+// a reload callback when changes are detected. It is implemented in terms
+// of the general-purpose internal/watcher.ContentWatcher.
+//
+// Type-aliasing here lets existing call sites (cmd/root.go and the
+// watcher_test.go suite) keep using `extensions.NewWatcher` / `*Watcher`
+// without knowing about the underlying implementation.
+type Watcher = watcher.ContentWatcher
 
 // NewWatcher creates a file watcher that monitors the given directories
 // for .go file changes. When a change is detected (after debouncing),
 // onReload is called. The watcher must be started with Start() and
 // stopped with Close().
 func NewWatcher(dirs []string, onReload func()) (*Watcher, error) {
-	fsw, err := fsnotify.NewWatcher()
-	if err != nil {
-		return nil, fmt.Errorf("creating file watcher: %w", err)
-	}
-
-	for _, dir := range dirs {
-		// Watch the directory itself.
-		if err := fsw.Add(dir); err != nil {
-			log.Debug("watcher: skipping directory", "dir", dir, "err", err)
-			continue
-		}
-
-		// Also watch immediate subdirectories (for */main.go pattern).
-		entries, err := os.ReadDir(dir)
-		if err != nil {
-			continue
-		}
-		for _, entry := range entries {
-			if entry.IsDir() {
-				subdir := filepath.Join(dir, entry.Name())
-				if err := fsw.Add(subdir); err != nil {
-					log.Debug("watcher: skipping subdirectory", "dir", subdir, "err", err)
-				}
-			}
-		}
-	}
-
-	return &Watcher{
-		watcher:  fsw,
-		onReload: onReload,
-		debounce: 300 * time.Millisecond,
-		done:     make(chan struct{}),
-	}, nil
-}
-
-// Start begins watching for file changes. It blocks until the context
-// is cancelled or Close() is called. Typically called in a goroutine.
-func (w *Watcher) Start(ctx context.Context) {
-	w.mu.Lock()
-	ctx, w.cancel = context.WithCancel(ctx)
-	w.mu.Unlock()
-
-	defer close(w.done)
-
-	var timer *time.Timer
-	var timerC <-chan time.Time
-
-	for {
-		select {
-		case <-ctx.Done():
-			if timer != nil {
-				timer.Stop()
-			}
-			return
-
-		case event, ok := <-w.watcher.Events:
-			if !ok {
-				return
-			}
-
-			// Only care about .go files.
-			if !strings.HasSuffix(event.Name, ".go") {
-				continue
-			}
-
-			// React to write, create, remove, rename events.
-			if event.Op&(fsnotify.Write|fsnotify.Create|fsnotify.Remove|fsnotify.Rename) == 0 {
-				continue
-			}
-
-			log.Debug("watcher: file changed", "file", event.Name, "op", event.Op)
-
-			// Debounce: reset timer on each event.
-			if timer != nil {
-				timer.Stop()
-			}
-			timer = time.NewTimer(w.debounce)
-			timerC = timer.C
-
-		case <-timerC:
-			timerC = nil
-			timer = nil
-			log.Debug("watcher: reloading extensions")
-			w.onReload()
-
-		case err, ok := <-w.watcher.Errors:
-			if !ok {
-				return
-			}
-			log.Warn("watcher: error", "err", err)
-		}
-	}
-}
-
-// Close stops the watcher and releases resources.
-func (w *Watcher) Close() error {
-	w.mu.Lock()
-	cancel := w.cancel
-	w.mu.Unlock()
-
-	if cancel != nil {
-		cancel()
-	}
-
-	// Wait for the event loop to finish.
-	<-w.done
-	return w.watcher.Close()
+	return watcher.New(watcher.Options{
+		Dirs:       dirs,
+		Extensions: []string{".go"},
+		OnReload:   onReload,
+		Label:      "extensions",
+	})
 }
 
 // WatchedDirs returns the directories to watch for extension changes.
@@ -146,47 +35,25 @@ func (w *Watcher) Close() error {
 // point to directories are also included; explicit file paths cause
 // their parent directory to be watched instead.
 func WatchedDirs(extraPaths []string) []string {
-	var dirs []string
-	seen := make(map[string]bool)
-
-	add := func(dir string) {
-		abs, err := filepath.Abs(dir)
-		if err != nil {
-			return
-		}
-		if seen[abs] {
-			return
-		}
-
-		// Verify the directory exists.
-		info, err := os.Stat(abs)
-		if err != nil || !info.IsDir() {
-			return
-		}
-
-		seen[abs] = true
-		dirs = append(dirs, abs)
+	standard := []string{
+		globalExtensionsDir(),
+		filepath.Join(".kit", "extensions"),
 	}
 
-	// Global extensions dir.
-	add(globalExtensionsDir())
-
-	// Project-local extensions dir.
-	add(filepath.Join(".kit", "extensions"))
-
-	// Explicit paths that are directories.
+	// Filter explicit paths into directories (passed through) and files
+	// (parent dir watched) for CollectDirs to dedupe.
+	var extras []string
 	for _, p := range extraPaths {
 		info, err := os.Stat(p)
 		if err != nil {
 			continue
 		}
 		if info.IsDir() {
-			add(p)
+			extras = append(extras, p)
 		} else {
-			// For explicit files, watch the parent directory.
-			add(filepath.Dir(p))
+			extras = append(extras, filepath.Dir(p))
 		}
 	}
 
-	return dirs
+	return watcher.CollectDirs(standard, extras)
 }

internal/extensions/wrapper.go

@@ -28,11 +28,11 @@ func WrapToolsWithExtensions(tools []fantasy.AgentTool, runner *Runner) []fantas
 	return wrapped
 }
 
-// ExtensionToolsAsFantasy converts ToolDef values registered by extensions
-// into fantasy.AgentTool implementations so the LLM can invoke them.
+// ExtensionToolsAsLLMTools converts ToolDef values registered by extensions
+// into LLM agent tool implementations so the LLM can invoke them.
 // The runner is optional; if provided, ToolContext.OnProgress routes
 // progress messages through the runner's Print function.
-func ExtensionToolsAsFantasy(defs []ToolDef, runner *Runner) []fantasy.AgentTool {
+func ExtensionToolsAsLLMTools(defs []ToolDef, runner *Runner) []fantasy.AgentTool {
 	tools := make([]fantasy.AgentTool, 0, len(defs))
 	for _, def := range defs {
 		tools = append(tools, &extensionTool{def: def, runner: runner})
@@ -40,27 +40,6 @@ func ExtensionToolsAsFantasy(defs []ToolDef, runner *Runner) []fantasy.AgentTool
 	return tools
 }
 
-// coreToolKinds maps built-in tool names to their kind classification.
-var coreToolKinds = map[string]string{
-	"bash":     "execute",
-	"edit":     "edit",
-	"write":    "edit",
-	"read":     "read",
-	"ls":       "read",
-	"grep":     "search",
-	"find":     "search",
-	"subagent": "agent",
-}
-
-// toolKindFor returns the ToolKind for a given tool name, defaulting to
-// "execute" for unknown tools (including MCP tools).
-func toolKindFor(toolName string) string {
-	if kind, ok := coreToolKinds[toolName]; ok {
-		return kind
-	}
-	return "execute"
-}
-
 // parseToolArgsJSON attempts to parse JSON-encoded tool args into a map.
 // Returns nil on failure (non-fatal convenience parsing).
 func parseToolArgsJSON(input string) map[string]any {
@@ -90,11 +69,10 @@ func (w *wrappedTool) Run(ctx context.Context, call fantasy.ToolCall) (fantasy.T
 	// 0. Check if tool is disabled via SetActiveTools.
 	if w.runner.IsToolDisabled(toolName) {
 		return fantasy.NewTextErrorResponse(
-				fmt.Sprintf("Error: tool %q is currently disabled", toolName)),
-			fmt.Errorf("tool %q disabled by extension", toolName)
+			fmt.Sprintf("Error: tool %q is currently disabled", toolName)), nil
 	}
 
-	kind := toolKindFor(toolName)
+	kind := ToolKindFor(toolName)
 
 	// 1. Emit ToolCall — extensions can block execution.
 	if w.runner.HasHandlers(ToolCall) {
@@ -111,8 +89,7 @@ func (w *wrappedTool) Run(ctx context.Context, call fantasy.ToolCall) (fantasy.T
 			if reason == "" {
 				reason = "blocked by extension"
 			}
-			return fantasy.NewTextErrorResponse(fmt.Sprintf("Error: %s", reason)),
-				fmt.Errorf("tool blocked by extension: %s", reason)
+			return fantasy.NewTextErrorResponse(fmt.Sprintf("Error: %s", reason)), nil
 		}
 	}
 
@@ -154,7 +131,7 @@ func (w *wrappedTool) Run(ctx context.Context, call fantasy.ToolCall) (fantasy.T
 }
 
 // ---------------------------------------------------------------------------
-// extensionTool — wraps a ToolDef into a fantasy.AgentTool
+// extensionTool — wraps a ToolDef into an LLM agent tool
 // ---------------------------------------------------------------------------
 
 type extensionTool struct {
@@ -182,7 +159,7 @@ func (t *extensionTool) Info() fantasy.ToolInfo {
 				info.Parameters = props
 			} else {
 				// Schema doesn't have "properties" — use as-is (may be
-				// a flat property map already matching fantasy's format).
+				// a flat property map already matching the expected format).
 				info.Parameters = schema
 			}
 			// Extract required fields if present.
@@ -238,7 +215,7 @@ func (t *extensionTool) Run(ctx context.Context, call fantasy.ToolCall) (fantasy
 	}
 
 	if err != nil {
-		return fantasy.NewTextErrorResponse(err.Error()), err
+		return fantasy.NewTextErrorResponse(err.Error()), nil
 	}
 	return fantasy.NewTextResponse(result), nil
 }

internal/extensions/wrapper_test.go

@@ -142,8 +142,8 @@ func TestWrappedTool_BlockExecution(t *testing.T) {
 	if toolRan {
 		t.Error("tool should not have run after block")
 	}
-	if err == nil {
-		t.Error("expected error from blocked tool")
+	if err != nil {
+		t.Error("expected nil error for blocked tool (error is conveyed via IsError response)")
 	}
 	if resp.IsError != true {
 		t.Error("expected IsError=true from blocked response")
@@ -192,7 +192,7 @@ func TestWrappedTool_ExecutionStartEnd(t *testing.T) {
 	}
 }
 
-func TestExtensionToolsAsFantasy(t *testing.T) {
+func TestExtensionToolsAsLLMTools(t *testing.T) {
 	defs := []ToolDef{
 		{
 			Name:        "greet",
@@ -202,7 +202,7 @@ func TestExtensionToolsAsFantasy(t *testing.T) {
 		},
 	}
 
-	tools := ExtensionToolsAsFantasy(defs, nil)
+	tools := ExtensionToolsAsLLMTools(defs, nil)
 	if len(tools) != 1 {
 		t.Fatalf("expected 1 tool, got %d", len(tools))
 	}
@@ -232,10 +232,10 @@ func TestExtensionTool_Error(t *testing.T) {
 		},
 	}
 
-	tools := ExtensionToolsAsFantasy(defs, nil)
+	tools := ExtensionToolsAsLLMTools(defs, nil)
 	resp, err := tools[0].Run(context.Background(), fantasy.ToolCall{Input: "x"})
-	if err == nil {
-		t.Error("expected error")
+	if err != nil {
+		t.Error("expected nil error (error is conveyed via IsError response)")
 	}
 	if !resp.IsError {
 		t.Error("expected IsError=true")
@@ -259,7 +259,7 @@ func TestExtensionTool_ExecuteWithContext(t *testing.T) {
 	}
 
 	// Without runner, OnProgress is a no-op.
-	tools := ExtensionToolsAsFantasy(defs, nil)
+	tools := ExtensionToolsAsLLMTools(defs, nil)
 	resp, err := tools[0].Run(context.Background(), fantasy.ToolCall{Input: "test"})
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
@@ -285,7 +285,7 @@ func TestExtensionTool_ExecuteWithContext(t *testing.T) {
 			},
 		},
 	}
-	tools2 := ExtensionToolsAsFantasy(defs2, runner)
+	tools2 := ExtensionToolsAsLLMTools(defs2, runner)
 	_, err = tools2[0].Run(context.Background(), fantasy.ToolCall{Input: ""})
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
@@ -306,7 +306,7 @@ func TestExtensionTool_ExecuteWithContextPriority(t *testing.T) {
 			},
 		},
 	}
-	tools := ExtensionToolsAsFantasy(defs, nil)
+	tools := ExtensionToolsAsLLMTools(defs, nil)
 	resp, err := tools[0].Run(context.Background(), fantasy.ToolCall{Input: ""})
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
@@ -330,7 +330,7 @@ func TestExtensionTool_CancelledContext(t *testing.T) {
 			},
 		},
 	}
-	tools := ExtensionToolsAsFantasy(defs, nil)
+	tools := ExtensionToolsAsLLMTools(defs, nil)
 	_, _ = tools[0].Run(ctx, fantasy.ToolCall{Input: ""})
 	if !sawCancelled {
 		t.Error("expected IsCancelled=true for cancelled context")
@@ -339,7 +339,7 @@ func TestExtensionTool_CancelledContext(t *testing.T) {
 
 func TestExtensionTool_ProviderOptions(t *testing.T) {
 	defs := []ToolDef{{Name: "test", Execute: func(string) (string, error) { return "", nil }}}
-	tools := ExtensionToolsAsFantasy(defs, nil)
+	tools := ExtensionToolsAsLLMTools(defs, nil)
 
 	// Initially nil.
 	opts := tools[0].ProviderOptions()

internal/fences/fences.go

@@ -0,0 +1,248 @@
+// Package fences provides utilities for detecting markdown code regions
+// (fenced code blocks and inline code spans) and applying transformations
+// only to text outside those regions.
+//
+// This prevents special tokens like $1, $@, or @file from being interpreted
+// when they appear inside ``` fences, ~~~ fences, or `inline` code spans.
+package fences
+
+import "strings"
+
+// Ranges returns byte ranges [start, end) of fenced code blocks in content.
+// Recognises both backtick (```) and tilde (~~~) fences, with optional
+// leading indentation (up to 3 spaces) and optional info strings.
+// An unclosed fence extends to the end of content.
+func Ranges(content string) [][2]int {
+	var result [][2]int
+	var inFence bool
+	var fenceChar byte
+	var fenceCount int
+	var fenceStart int
+
+	pos := 0
+	for pos < len(content) {
+		// Find the end of the current line.
+		lineEnd := strings.IndexByte(content[pos:], '\n')
+		var line string
+		var nextPos int
+		if lineEnd < 0 {
+			line = content[pos:]
+			nextPos = len(content)
+		} else {
+			line = content[pos : pos+lineEnd]
+			nextPos = pos + lineEnd + 1
+		}
+
+		trimmed := strings.TrimLeft(line, " ")
+		indent := len(line) - len(trimmed)
+
+		if !inFence {
+			if indent <= 3 {
+				if ch, n := parseFenceOpen(trimmed); n > 0 {
+					inFence = true
+					fenceChar = ch
+					fenceCount = n
+					fenceStart = pos
+				}
+			}
+		} else {
+			if indent <= 3 && isFenceClose(trimmed, fenceChar, fenceCount) {
+				result = append(result, [2]int{fenceStart, nextPos})
+				inFence = false
+			}
+		}
+
+		pos = nextPos
+	}
+
+	// Unclosed fence extends to end of content.
+	if inFence {
+		result = append(result, [2]int{fenceStart, len(content)})
+	}
+
+	return result
+}
+
+// ReplaceOutside applies fn to each text segment that is outside fenced code
+// blocks and inline code spans, leaving code content unchanged. This is the
+// primary entry point for callers that need to do regex replacement only on
+// non-code text.
+func ReplaceOutside(content string, fn func(string) string) string {
+	ranges := Ranges(content)
+	if len(ranges) == 0 {
+		return replaceOutsideInline(content, fn)
+	}
+
+	var b strings.Builder
+	b.Grow(len(content))
+	pos := 0
+	for _, r := range ranges {
+		if pos < r[0] {
+			// Within non-fenced segments, also skip inline code spans.
+			b.WriteString(replaceOutsideInline(content[pos:r[0]], fn))
+		}
+		// Preserve fenced content verbatim.
+		b.WriteString(content[r[0]:r[1]])
+		pos = r[1]
+	}
+	if pos < len(content) {
+		b.WriteString(replaceOutsideInline(content[pos:], fn))
+	}
+	return b.String()
+}
+
+// StripCode returns content with fenced code blocks and inline code spans
+// removed. Useful for detection/matching where only non-code text matters.
+func StripCode(content string) string {
+	// First strip fenced blocks.
+	stripped := StripFenced(content)
+	// Then strip inline code spans from what remains.
+	return stripInlineCode(stripped)
+}
+
+// StripFenced returns content with fenced code block regions removed.
+// Useful for detection/matching where only non-fenced text matters.
+// NOTE: this does NOT strip inline code spans; use StripCode for both.
+func StripFenced(content string) string {
+	ranges := Ranges(content)
+	if len(ranges) == 0 {
+		return content
+	}
+
+	var b strings.Builder
+	b.Grow(len(content))
+	pos := 0
+	for _, r := range ranges {
+		b.WriteString(content[pos:r[0]])
+		pos = r[1]
+	}
+	b.WriteString(content[pos:])
+	return b.String()
+}
+
+// parseFenceOpen checks whether trimmed (leading spaces already removed)
+// starts a fenced code block. Returns the fence character and count, or
+// (0, 0) if it is not a fence opener.
+func parseFenceOpen(trimmed string) (byte, int) {
+	if len(trimmed) == 0 {
+		return 0, 0
+	}
+	ch := trimmed[0]
+	if ch != '`' && ch != '~' {
+		return 0, 0
+	}
+	count := 0
+	for count < len(trimmed) && trimmed[count] == ch {
+		count++
+	}
+	if count < 3 {
+		return 0, 0
+	}
+	// Per CommonMark: backtick fences cannot have backticks in the info string.
+	if ch == '`' && strings.ContainsRune(trimmed[count:], '`') {
+		return 0, 0
+	}
+	return ch, count
+}
+
+// isFenceClose checks whether trimmed is a closing fence matching fenceChar
+// with at least minCount characters. A closing fence line contains only the
+// fence characters and optional trailing spaces.
+func isFenceClose(trimmed string, fenceChar byte, minCount int) bool {
+	if len(trimmed) == 0 || trimmed[0] != fenceChar {
+		return false
+	}
+	count := 0
+	for count < len(trimmed) && trimmed[count] == fenceChar {
+		count++
+	}
+	if count < minCount {
+		return false
+	}
+	// Closing fence must contain only fence chars (and optional trailing spaces).
+	return strings.TrimRight(trimmed[count:], " ") == ""
+}
+
+// --------------------------------------------------------------------------
+// Inline code span handling
+// --------------------------------------------------------------------------
+
+// inlineCodeRanges returns byte ranges [start, end) of inline code spans
+// in segment. Per CommonMark, a code span opens with N backticks and closes
+// with exactly N backticks.
+func inlineCodeRanges(s string) [][2]int {
+	var result [][2]int
+	i := 0
+	for i < len(s) {
+		if s[i] != '`' {
+			i++
+			continue
+		}
+		// Count opening backticks.
+		start := i
+		n := 0
+		for i < len(s) && s[i] == '`' {
+			n++
+			i++
+		}
+		// Scan for a closing run of exactly n backticks.
+		for j := i; j < len(s); {
+			if s[j] != '`' {
+				j++
+				continue
+			}
+			m := 0
+			for j < len(s) && s[j] == '`' {
+				m++
+				j++
+			}
+			if m == n {
+				result = append(result, [2]int{start, j})
+				i = j
+				break
+			}
+		}
+		// If no closing run was found, i is already past the opening
+		// backticks so the outer loop advances naturally.
+	}
+	return result
+}
+
+// replaceOutsideInline applies fn only to text outside inline code spans.
+func replaceOutsideInline(segment string, fn func(string) string) string {
+	ranges := inlineCodeRanges(segment)
+	if len(ranges) == 0 {
+		return fn(segment)
+	}
+	var b strings.Builder
+	b.Grow(len(segment))
+	pos := 0
+	for _, r := range ranges {
+		if pos < r[0] {
+			b.WriteString(fn(segment[pos:r[0]]))
+		}
+		b.WriteString(segment[r[0]:r[1]])
+		pos = r[1]
+	}
+	if pos < len(segment) {
+		b.WriteString(fn(segment[pos:]))
+	}
+	return b.String()
+}
+
+// stripInlineCode removes inline code spans from s.
+func stripInlineCode(s string) string {
+	ranges := inlineCodeRanges(s)
+	if len(ranges) == 0 {
+		return s
+	}
+	var b strings.Builder
+	b.Grow(len(s))
+	pos := 0
+	for _, r := range ranges {
+		b.WriteString(s[pos:r[0]])
+		pos = r[1]
+	}
+	b.WriteString(s[pos:])
+	return b.String()
+}

internal/fences/fences_test.go

@@ -0,0 +1,313 @@
+package fences
+
+import (
+	"testing"
+)
+
+func TestRanges(t *testing.T) {
+	tests := []struct {
+		name    string
+		content string
+		want    [][2]int
+	}{
+		{
+			name:    "no fences",
+			content: "hello world\nno code here",
+			want:    nil,
+		},
+		{
+			name:    "single backtick fence",
+			content: "before\n```\ncode\n```\nafter",
+			want:    [][2]int{{7, 20}},
+		},
+		{
+			name:    "single tilde fence",
+			content: "before\n~~~\ncode\n~~~\nafter",
+			want:    [][2]int{{7, 20}},
+		},
+		{
+			name:    "fence with info string",
+			content: "before\n```go\ncode\n```\nafter",
+			want:    [][2]int{{7, 22}},
+		},
+		{
+			name:    "multiple fences",
+			content: "a\n```\nx\n```\nb\n~~~\ny\n~~~\nc",
+			want:    [][2]int{{2, 12}, {14, 24}},
+		},
+		{
+			name:    "unclosed fence",
+			content: "before\n```\ncode\nmore code",
+			want:    [][2]int{{7, 25}},
+		},
+		{
+			name:    "longer closing fence",
+			content: "before\n```\ncode\n`````\nafter",
+			want:    [][2]int{{7, 22}},
+		},
+		{
+			name:    "shorter closing fence ignored",
+			content: "before\n`````\ncode\n```\nmore\n`````\nafter",
+			want:    [][2]int{{7, 33}},
+		},
+		{
+			name:    "indented fence up to 3 spaces",
+			content: "before\n   ```\ncode\n   ```\nafter",
+			want:    [][2]int{{7, 26}},
+		},
+		{
+			name:    "4 space indent is not a fence",
+			content: "before\n    ```\ncode\n    ```\nafter",
+			want:    nil,
+		},
+		{
+			name: "backtick in info string rejects open",
+			// The ```foo`bar line is not a valid opener (backtick in info).
+			// The standalone ``` becomes an opener with no close.
+			content: "before\n```foo`bar\ncode\n```\nafter",
+			want:    [][2]int{{23, 32}},
+		},
+		{
+			name:    "empty content",
+			content: "",
+			want:    nil,
+		},
+		{
+			name:    "fence only",
+			content: "```\ncode\n```",
+			want:    [][2]int{{0, 12}},
+		},
+		{
+			name:    "fence at end without trailing newline",
+			content: "```\ncode\n```",
+			want:    [][2]int{{0, 12}},
+		},
+		{
+			name:    "tilde fence does not close with backticks",
+			content: "~~~\ncode\n```\nmore\n~~~\nafter",
+			want:    [][2]int{{0, 22}},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := Ranges(tt.content)
+			if len(got) != len(tt.want) {
+				t.Fatalf("Ranges() = %v, want %v", got, tt.want)
+			}
+			for i := range got {
+				if got[i] != tt.want[i] {
+					t.Errorf("Ranges()[%d] = %v, want %v", i, got[i], tt.want[i])
+				}
+			}
+		})
+	}
+}
+
+func TestReplaceOutside(t *testing.T) {
+	upper := func(s string) string {
+		b := []byte(s)
+		for i, c := range b {
+			if c >= 'a' && c <= 'z' {
+				b[i] = c - 32
+			}
+		}
+		return string(b)
+	}
+
+	tests := []struct {
+		name    string
+		content string
+		want    string
+	}{
+		{
+			name:    "no fences",
+			content: "hello world",
+			want:    "HELLO WORLD",
+		},
+		{
+			name:    "text around fence",
+			content: "before\n```\ncode\n```\nafter",
+			want:    "BEFORE\n```\ncode\n```\nAFTER",
+		},
+		{
+			name:    "multiple fences",
+			content: "aaa\n```\nxxx\n```\nbbb\n~~~\nyyy\n~~~\nccc",
+			want:    "AAA\n```\nxxx\n```\nBBB\n~~~\nyyy\n~~~\nCCC",
+		},
+		{
+			name:    "unclosed fence preserves code",
+			content: "before\n```\ncode",
+			want:    "BEFORE\n```\ncode",
+		},
+		{
+			name:    "only fenced content",
+			content: "```\ncode\n```",
+			want:    "```\ncode\n```",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := ReplaceOutside(tt.content, upper)
+			if got != tt.want {
+				t.Errorf("ReplaceOutside() =\n%s\nwant:\n%s", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestStripFenced(t *testing.T) {
+	tests := []struct {
+		name    string
+		content string
+		want    string
+	}{
+		{
+			name:    "no fences",
+			content: "hello $1 world",
+			want:    "hello $1 world",
+		},
+		{
+			name:    "strips fenced code",
+			content: "before $1\n```\n$2 inside\n```\nafter $3",
+			want:    "before $1\nafter $3",
+		},
+		{
+			name:    "multiple fences",
+			content: "a\n```\nx\n```\nb\n~~~\ny\n~~~\nc",
+			want:    "a\nb\nc",
+		},
+		{
+			name:    "unclosed fence",
+			content: "before\n```\n$1 inside",
+			want:    "before\n",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := StripFenced(tt.content)
+			if got != tt.want {
+				t.Errorf("StripFenced() = %q, want %q", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestInlineCodeRanges(t *testing.T) {
+	tests := []struct {
+		name string
+		s    string
+		want [][2]int
+	}{
+		{"no backticks", "hello world", nil},
+		{"single backtick span", "use `$1` here", [][2]int{{4, 8}}},
+		{"double backtick span", "use ``$1`` here", [][2]int{{4, 10}}},
+		{"multiple spans", "`$1` and `$2`", [][2]int{{0, 4}, {9, 13}}},
+		{"unmatched backtick", "use `$1 here", nil},
+		{"mismatched backtick counts", "use ``$1` here", nil},
+		{"empty inline content", "use `` `` here", [][2]int{{4, 9}}},
+		{"backticks inside double", "use ``foo`bar`` here", [][2]int{{4, 15}}},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := inlineCodeRanges(tt.s)
+			if len(got) != len(tt.want) {
+				t.Fatalf("inlineCodeRanges() = %v, want %v", got, tt.want)
+			}
+			for i := range got {
+				if got[i] != tt.want[i] {
+					t.Errorf("inlineCodeRanges()[%d] = %v, want %v", i, got[i], tt.want[i])
+				}
+			}
+		})
+	}
+}
+
+func TestReplaceOutside_InlineCode(t *testing.T) {
+	upper := func(s string) string {
+		b := []byte(s)
+		for i, c := range b {
+			if c >= 'a' && c <= 'z' {
+				b[i] = c - 32
+			}
+		}
+		return string(b)
+	}
+
+	tests := []struct {
+		name    string
+		content string
+		want    string
+	}{
+		{
+			name:    "inline code preserved",
+			content: "use `code` here",
+			want:    "USE `code` HERE",
+		},
+		{
+			name:    "double backtick inline code",
+			content: "use ``co`de`` here",
+			want:    "USE ``co`de`` HERE",
+		},
+		{
+			name:    "mixed fenced and inline",
+			content: "before `x` mid\n```\nfenced\n```\nafter `y` end",
+			want:    "BEFORE `x` MID\n```\nfenced\n```\nAFTER `y` END",
+		},
+		{
+			name:    "only inline code",
+			content: "`code`",
+			want:    "`code`",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := ReplaceOutside(tt.content, upper)
+			if got != tt.want {
+				t.Errorf("ReplaceOutside() =\n%s\nwant:\n%s", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestStripCode(t *testing.T) {
+	tests := []struct {
+		name    string
+		content string
+		want    string
+	}{
+		{
+			name:    "no code",
+			content: "hello $1 world",
+			want:    "hello $1 world",
+		},
+		{
+			name:    "strips inline code",
+			content: "use `$1` and `$2` for positional args",
+			want:    "use  and  for positional args",
+		},
+		{
+			name:    "strips fenced and inline",
+			content: "before `$1`\n```\n$2 inside\n```\nafter",
+			want:    "before \nafter",
+		},
+		{
+			name:    "real world prompt template",
+			content: "Use $@ for all args.\n`$1`, `$2` for positional.\n```bash\necho $1\n```\n",
+			want:    "Use $@ for all args.\n,  for positional.\n",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := StripCode(tt.content)
+			if got != tt.want {
+				t.Errorf("StripCode() = %q, want %q", got, tt.want)
+			}
+		})
+	}
+}

internal/kitsetup/setup.go

@@ -33,6 +33,10 @@ type AgentSetupOptions struct {
 	// CoreTools overrides the default core tool set. If empty, core.AllTools()
 	// is used. Allows SDK users to pass custom tools (e.g. with WithWorkDir).
 	CoreTools []fantasy.AgentTool
+	// DisableCoreTools, when true, prevents loading any core tools.
+	// If both DisableCoreTools is true and CoreTools is empty, the agent
+	// will have no tools (useful for simple chat completions).
+	DisableCoreTools bool
 	// ExtraTools are additional tools added alongside core, MCP, and extension
 	// tools. They do not replace the defaults — they extend them.
 	ExtraTools []fantasy.AgentTool
@@ -42,9 +46,9 @@ type AgentSetupOptions struct {
 	ToolWrapper func([]fantasy.AgentTool) []fantasy.AgentTool
 
 	// ProviderConfig, when non-nil, is used directly instead of calling
-	// BuildProviderConfig(). Callers that already hold viperInitMu can
-	// pre-build this and release the lock before calling SetupAgent, so the
-	// slow agent/MCP initialisation runs concurrently with other New() calls.
+	// BuildProviderConfig(). Callers (e.g. Kit.New) pre-build this from their
+	// per-instance config store and pass it here, so the slow agent/MCP
+	// initialisation can run without further config reads.
 	ProviderConfig *models.ProviderConfig
 	// Debug enables debug logging. When zero-value, viper is consulted.
 	// Only meaningful when ProviderConfig is also set.
@@ -61,6 +65,21 @@ type AgentSetupOptions struct {
 	// AuthHandler handles OAuth authorization for remote MCP servers.
 	// When set, remote transports are configured with OAuth support.
 	AuthHandler tools.MCPAuthHandler
+	// TokenStoreFactory, if non-nil, creates a custom token store for each
+	// remote MCP server's OAuth tokens. When nil, the default file-based
+	// token store is used.
+	TokenStoreFactory tools.TokenStoreFactory
+	// OnMCPServerLoaded, if non-nil, is called when each MCP server finishes
+	// loading (successfully or with error). Called from the background goroutine.
+	OnMCPServerLoaded func(serverName string, toolCount int, err error)
+	// MCPTaskConfig configures task-augmented tools/call execution. The
+	// zero value preserves historical synchronous-only behaviour.
+	MCPTaskConfig tools.MCPTaskConfig
+	// Viper is the per-instance configuration store. When set, it is used for
+	// any fallback config reads (debug, no-extensions, max-steps, stream,
+	// extension paths) and is attached to the extension runner. When nil, the
+	// process-global viper store is used.
+	Viper *viper.Viper
 }
 
 // AgentSetupResult bundles the created agent and any debug logger so the caller
@@ -73,34 +92,62 @@ type AgentSetupResult struct {
 	ExtRunner *extensions.Runner
 }
 
-// BuildProviderConfig creates a *models.ProviderConfig from the current viper
-// state. All entry points (root, script, SDK) converge through this function.
-func BuildProviderConfig() (*models.ProviderConfig, string, error) {
-	systemPrompt, err := config.LoadSystemPrompt(viper.GetString("system-prompt"))
+// BuildProviderConfig creates a *models.ProviderConfig from the supplied viper
+// store (or the process-global store when v is nil). All entry points (root,
+// script, SDK) converge through this function.
+//
+// Generation parameter pointers (Temperature, TopP, etc.) are only set when
+// the user has explicitly configured them via CLI flag, environment variable,
+// or global config file. This allows per-model defaults from modelSettings
+// and customModels to fill in unset parameters downstream.
+func BuildProviderConfig(v *viper.Viper) (*models.ProviderConfig, string, error) {
+	if v == nil {
+		v = viper.GetViper()
+	}
+	systemPrompt, err := config.LoadSystemPrompt(v.GetString("system-prompt"))
 	if err != nil {
 		return nil, "", fmt.Errorf("failed to load system prompt: %w", err)
 	}
 
-	temperature := float32(viper.GetFloat64("temperature"))
-	topP := float32(viper.GetFloat64("top-p"))
-	topK := int32(viper.GetInt("top-k"))
-	numGPU := int32(viper.GetInt("num-gpu-layers"))
-	mainGPU := int32(viper.GetInt("main-gpu"))
+	numGPU := int32(v.GetInt("num-gpu-layers"))
+	mainGPU := int32(v.GetInt("main-gpu"))
 
 	cfg := &models.ProviderConfig{
-		ModelString:    viper.GetString("model"),
+		ModelString:    v.GetString("model"),
 		SystemPrompt:   systemPrompt,
-		ProviderAPIKey: viper.GetString("provider-api-key"),
-		ProviderURL:    viper.GetString("provider-url"),
-		MaxTokens:      viper.GetInt("max-tokens"),
-		Temperature:    &temperature,
-		TopP:           &topP,
-		TopK:           &topK,
-		StopSequences:  viper.GetStringSlice("stop-sequences"),
+		ProviderAPIKey: v.GetString("provider-api-key"),
+		ProviderURL:    v.GetString("provider-url"),
+		MaxTokens:      v.GetInt("max-tokens"),
+		StopSequences:  v.GetStringSlice("stop-sequences"),
 		NumGPU:         &numGPU,
 		MainGPU:        &mainGPU,
-		TLSSkipVerify:  viper.GetBool("tls-skip-verify"),
-		ThinkingLevel:  models.ParseThinkingLevel(viper.GetString("thinking-level")),
+		TLSSkipVerify:  v.GetBool("tls-skip-verify"),
+		ThinkingLevel:  models.ParseThinkingLevel(v.GetString("thinking-level")),
+		ConfigStore:    v,
+	}
+
+	// Only set generation parameter pointers when the user has explicitly
+	// provided a value. This leaves nil pointers for unset params, allowing
+	// per-model defaults (modelSettings / customModels params) to apply.
+	if v.IsSet("temperature") {
+		val := float32(v.GetFloat64("temperature"))
+		cfg.Temperature = &val
+	}
+	if v.IsSet("top-p") {
+		val := float32(v.GetFloat64("top-p"))
+		cfg.TopP = &val
+	}
+	if v.IsSet("top-k") {
+		val := int32(v.GetInt("top-k"))
+		cfg.TopK = &val
+	}
+	if v.IsSet("frequency-penalty") {
+		val := float32(v.GetFloat64("frequency-penalty"))
+		cfg.FrequencyPenalty = &val
+	}
+	if v.IsSet("presence-penalty") {
+		val := float32(v.GetFloat64("presence-penalty"))
+		cfg.PresencePenalty = &val
 	}
 
 	return cfg, systemPrompt, nil
@@ -112,14 +159,21 @@ func SetupAgent(ctx context.Context, opts AgentSetupOptions) (*AgentSetupResult,
 	var modelConfig *models.ProviderConfig
 	var systemPrompt string
 
+	// Resolve the config store: prefer the per-instance store, falling back to
+	// the process-global store.
+	v := opts.Viper
+	if v == nil {
+		v = viper.GetViper()
+	}
+
 	if opts.ProviderConfig != nil {
-		// Pre-built config supplied by caller (e.g. Kit.New after releasing
-		// viperInitMu). Use it directly — no viper reads needed here.
+		// Pre-built config supplied by caller (e.g. Kit.New after building the
+		// per-instance store). Use it directly — no viper reads needed here.
 		modelConfig = opts.ProviderConfig
 		systemPrompt = modelConfig.SystemPrompt
 	} else {
 		var err error
-		modelConfig, systemPrompt, err = BuildProviderConfig()
+		modelConfig, systemPrompt, err = BuildProviderConfig(v)
 		if err != nil {
 			return nil, err
 		}
@@ -127,13 +181,13 @@ func SetupAgent(ctx context.Context, opts AgentSetupOptions) (*AgentSetupResult,
 
 	// Resolve debug / no-extensions / max-steps / streaming: prefer explicit
 	// fields (set when ProviderConfig was pre-built) over viper fallback.
-	debugEnabled := opts.Debug || viper.GetBool("debug")
-	noExtensions := opts.NoExtensions || viper.GetBool("no-extensions")
+	debugEnabled := opts.Debug || v.GetBool("debug")
+	noExtensions := opts.NoExtensions || v.GetBool("no-extensions")
 	maxSteps := opts.MaxSteps
 	if maxSteps == 0 {
-		maxSteps = viper.GetInt("max-steps")
+		maxSteps = v.GetInt("max-steps")
 	}
-	streamingEnabled := opts.StreamingEnabled || viper.GetBool("stream")
+	streamingEnabled := opts.StreamingEnabled || v.GetBool("stream")
 
 	// Create the appropriate debug logger.
 	var debugLogger tools.DebugLogger
@@ -152,7 +206,7 @@ func SetupAgent(ctx context.Context, opts AgentSetupOptions) (*AgentSetupResult,
 	var extCreationOpts extensionCreationOpts
 	if !noExtensions {
 		var extErr error
-		extRunner, extCreationOpts, extErr = loadExtensions()
+		extRunner, extCreationOpts, extErr = loadExtensions(v)
 		if extErr != nil {
 			fmt.Printf("Warning: Failed to load extensions: %v\n", extErr)
 		}
@@ -179,19 +233,23 @@ func SetupAgent(ctx context.Context, opts AgentSetupOptions) (*AgentSetupResult,
 	}
 
 	a, err := agent.CreateAgent(ctx, &agent.AgentCreationOptions{
-		ModelConfig:      modelConfig,
-		MCPConfig:        opts.MCPConfig,
-		SystemPrompt:     systemPrompt,
-		MaxSteps:         maxSteps,
-		StreamingEnabled: streamingEnabled,
-		ShowSpinner:      opts.ShowSpinner,
-		Quiet:            opts.Quiet,
-		SpinnerFunc:      opts.SpinnerFunc,
-		DebugLogger:      debugLogger,
-		AuthHandler:      opts.AuthHandler,
-		CoreTools:        opts.CoreTools,
-		ToolWrapper:      toolWrapper,
-		ExtraTools:       extraTools,
+		ModelConfig:       modelConfig,
+		MCPConfig:         opts.MCPConfig,
+		SystemPrompt:      systemPrompt,
+		MaxSteps:          maxSteps,
+		StreamingEnabled:  streamingEnabled,
+		ShowSpinner:       opts.ShowSpinner,
+		Quiet:             opts.Quiet,
+		SpinnerFunc:       opts.SpinnerFunc,
+		DebugLogger:       debugLogger,
+		AuthHandler:       opts.AuthHandler,
+		TokenStoreFactory: opts.TokenStoreFactory,
+		CoreTools:         opts.CoreTools,
+		DisableCoreTools:  opts.DisableCoreTools,
+		ToolWrapper:       toolWrapper,
+		ExtraTools:        extraTools,
+		OnMCPServerLoaded: opts.OnMCPServerLoaded,
+		MCPTaskConfig:     opts.MCPTaskConfig,
 	})
 	if err != nil {
 		return nil, fmt.Errorf("failed to create agent: %w", err)
@@ -212,9 +270,14 @@ type extensionCreationOpts struct {
 }
 
 // loadExtensions discovers and loads Yaegi extensions, builds the runner,
-// and returns the tool wrapper/extra tools.
-func loadExtensions() (*extensions.Runner, extensionCreationOpts, error) {
-	extraPaths := viper.GetStringSlice("extension")
+// and returns the tool wrapper/extra tools. The supplied store is used to
+// resolve the "extension" config key and is attached to the runner so
+// extension option lookups stay isolated to this Kit instance.
+func loadExtensions(v *viper.Viper) (*extensions.Runner, extensionCreationOpts, error) {
+	if v == nil {
+		v = viper.GetViper()
+	}
+	extraPaths := v.GetStringSlice("extension")
 	loaded, err := extensions.LoadExtensions(extraPaths)
 	if err != nil {
 		return nil, extensionCreationOpts{}, err
@@ -225,12 +288,13 @@ func loadExtensions() (*extensions.Runner, extensionCreationOpts, error) {
 	}
 
 	runner := extensions.NewRunner(loaded)
+	runner.SetConfigStore(v)
 
 	wrapper := func(tools []fantasy.AgentTool) []fantasy.AgentTool {
 		return extensions.WrapToolsWithExtensions(tools, runner)
 	}
 
-	extTools := extensions.ExtensionToolsAsFantasy(runner.RegisteredTools(), runner)
+	extTools := extensions.ExtensionToolsAsLLMTools(runner.RegisteredTools(), runner)
 
 	return runner, extensionCreationOpts{
 		toolWrapper: wrapper,

internal/message/content.go

@@ -325,12 +325,6 @@ func UnmarshalParts(data []byte) ([]ContentPart, error) {
 // mixed TextPart and ToolCallPart content. Tool-role messages produce
 // ToolResultPart entries.
 func (m *Message) ToLLMMessages() []fantasy.Message {
-	return m.ToFantasyMessages()
-}
-
-// Deprecated: Use ToLLMMessages instead.
-// ToFantasyMessages converts a Message to one or more LLM message values.
-func (m *Message) ToFantasyMessages() []fantasy.Message {
 	switch m.Role {
 	case RoleAssistant:
 		var parts []fantasy.MessagePart
@@ -431,13 +425,6 @@ func (m *Message) ToFantasyMessages() []fantasy.Message {
 // FromLLMMessage converts an LLM message into our Message type,
 // extracting all content parts into the appropriate block types.
 func FromLLMMessage(msg fantasy.Message) Message {
-	return FromFantasyMessage(msg)
-}
-
-// Deprecated: Use FromLLMMessage instead.
-// FromFantasyMessage converts an LLM message into our Message type,
-// extracting all content parts into the appropriate block types.
-func FromFantasyMessage(msg fantasy.Message) Message {
 	m := Message{
 		Role:      MessageRole(msg.Role),
 		Parts:     make([]ContentPart, 0),

internal/models/autoroute_test.go

@@ -0,0 +1,282 @@
+package models
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+// TestNpmToWireProtocol documents the wire protocols that the auto-router
+// understands. Provider-specific bundles that need bespoke auth or URL
+// templating (azure, bedrock, openrouter, google-vertex*, @ai-sdk/gateway)
+// are intentionally absent — they have native top-level cases in
+// CreateProvider and never reach the auto-router.
+func TestNpmToWireProtocol(t *testing.T) {
+	want := map[string]wireProtocol{
+		"@ai-sdk/openai":            wireOpenAI,
+		"@ai-sdk/openai-compatible": wireOpenAI,
+		"@ai-sdk/anthropic":         wireAnthropic,
+		"@ai-sdk/google":            wireGoogle,
+
+		// Thin OpenAI-compatible wrappers — routed via openaicompat using
+		// the SDK's hard-coded default base URL (sdkDefaultBaseURL).
+		"@ai-sdk/groq":                  wireOpenAI,
+		"@ai-sdk/cerebras":              wireOpenAI,
+		"@ai-sdk/perplexity":            wireOpenAI,
+		"@ai-sdk/togetherai":            wireOpenAI,
+		"@ai-sdk/xai":                   wireOpenAI,
+		"@ai-sdk/deepinfra":             wireOpenAI,
+		"@ai-sdk/mistral":               wireOpenAI,
+		"@ai-sdk/cohere":                wireOpenAI,
+		"@ai-sdk/vercel":                wireOpenAI,
+		"@aihubmix/ai-sdk-provider":     wireOpenAI,
+		"venice-ai-sdk-provider":        wireOpenAI,
+		"merge-gateway-ai-sdk-provider": wireOpenAI,
+	}
+	for npm, wire := range want {
+		if got := npmToWireProtocol[npm]; got != wire {
+			t.Errorf("npmToWireProtocol[%q] = %d, want %d", npm, got, wire)
+		}
+	}
+
+	// Bundle packages must NOT be in the table — they need bespoke auth or
+	// URL templating that the auto-router cannot satisfy.
+	for _, npm := range []string{
+		"@ai-sdk/google-vertex",
+		"@ai-sdk/google-vertex/anthropic",
+		"@ai-sdk/amazon-bedrock",
+		"@ai-sdk/azure",
+		"@openrouter/ai-sdk-provider",
+		"@ai-sdk/gateway",
+	} {
+		if _, ok := npmToWireProtocol[npm]; ok {
+			t.Errorf("npmToWireProtocol unexpectedly contains bundle package %q", npm)
+		}
+	}
+}
+
+// newTestRegistry builds a registry containing a single proxy-style provider
+// ("testproxy") with the given default npm, plus one model that carries the
+// given per-model npm override.
+func newTestRegistry(api, defaultNPM, modelID, modelNPMOverride string) *ModelsRegistry {
+	return &ModelsRegistry{
+		providers: map[string]ProviderInfo{
+			"testproxy": {
+				ID:   "testproxy",
+				Name: "Test Proxy",
+				Env:  []string{"TESTPROXY_API_KEY"},
+				NPM:  defaultNPM,
+				API:  api,
+				Models: map[string]ModelInfo{
+					modelID: {
+						ID:          modelID,
+						Name:        modelID,
+						ProviderNPM: modelNPMOverride,
+					},
+				},
+			},
+		},
+	}
+}
+
+// TestAutoRouteProvider_WireRouting verifies that autoRouteProvider routes each
+// npm package to the correct fantasy provider implementation. This is the core
+// regression test for issue #41: previously any npm that resolved to a
+// non-openai/anthropic/openaicompat LLM provider (notably @ai-sdk/google) hit a
+// dead `default` branch and failed with "has no LLM provider mapping".
+func TestAutoRouteProvider_WireRouting(t *testing.T) {
+	tests := []struct {
+		name        string
+		modelID     string
+		defaultNPM  string
+		overrideNPM string
+		// wantType is the concrete fantasy LanguageModel type the model should
+		// be routed to, identified by reflect type string.
+		wantType string
+	}{
+		{
+			name:       "openai-compatible default",
+			modelID:    "test-model",
+			defaultNPM: "@ai-sdk/openai-compatible",
+			wantType:   "openai.languageModel",
+		},
+		{
+			name:        "anthropic override",
+			modelID:     "test-model",
+			defaultNPM:  "@ai-sdk/openai-compatible",
+			overrideNPM: "@ai-sdk/anthropic",
+			wantType:    "anthropic.languageModel",
+		},
+		{
+			name:        "openai (responses) override",
+			modelID:     "gpt-4o",
+			defaultNPM:  "@ai-sdk/openai-compatible",
+			overrideNPM: "@ai-sdk/openai",
+			wantType:    "openai.responsesLanguageModel",
+		},
+		{
+			// The bug: opencode's gemini-* models override the default
+			// openai-compatible npm with @ai-sdk/google.
+			name:        "google override (issue #41)",
+			modelID:     "gemini-3.5-flash",
+			defaultNPM:  "@ai-sdk/openai-compatible",
+			overrideNPM: "@ai-sdk/google",
+			wantType:    "*google.languageModel",
+		},
+		{
+			// Unknown npm but provider has an API URL → openai-compatible fallback.
+			name:       "unknown npm with API URL falls back to openai-compat",
+			modelID:    "test-model",
+			defaultNPM: "@ai-sdk/some-future-thing",
+			wantType:   "openai.languageModel",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			reg := newTestRegistry("https://proxy.example/v1", tt.defaultNPM, tt.modelID, tt.overrideNPM)
+			config := &ProviderConfig{ProviderAPIKey: "test-key"}
+
+			result, err := autoRouteProvider(context.Background(), config, "testproxy", tt.modelID, reg)
+			if err != nil {
+				t.Fatalf("autoRouteProvider returned error: %v", err)
+			}
+			if result == nil || result.Model == nil {
+				t.Fatalf("autoRouteProvider returned nil model")
+			}
+
+			gotType := reflect.TypeOf(result.Model).String()
+			if gotType != tt.wantType {
+				t.Errorf("routed to %s, want %s", gotType, tt.wantType)
+			}
+		})
+	}
+}
+
+// TestAutoRouteProvider_UnknownNpmNoAPI verifies the improved error message for
+// a provider whose npm has no known wire protocol and that has no API URL to
+// fall back on.
+func TestAutoRouteProvider_UnknownNpmNoAPI(t *testing.T) {
+	reg := newTestRegistry("", "@ai-sdk/unmapped", "test-model", "")
+	config := &ProviderConfig{ProviderAPIKey: "test-key"}
+
+	_, err := autoRouteProvider(context.Background(), config, "testproxy", "test-model", reg)
+	if err == nil {
+		t.Fatal("expected error for unknown npm with no API URL, got nil")
+	}
+	if !strings.Contains(err.Error(), "cannot auto-route provider testproxy") {
+		t.Errorf("unexpected error message: %v", err)
+	}
+	if !strings.Contains(err.Error(), "--provider-url") {
+		t.Errorf("error should suggest --provider-url, got: %v", err)
+	}
+}
+
+// TestAutoRouteProvider_UnknownProvider verifies the not-in-database error.
+func TestAutoRouteProvider_UnknownProvider(t *testing.T) {
+	reg := newTestRegistry("https://proxy.example/v1", "@ai-sdk/openai-compatible", "test-model", "")
+	config := &ProviderConfig{ProviderAPIKey: "test-key"}
+
+	_, err := autoRouteProvider(context.Background(), config, "does-not-exist", "test-model", reg)
+	if err == nil {
+		t.Fatal("expected error for unknown provider, got nil")
+	}
+	if !strings.Contains(err.Error(), "not found in model database") {
+		t.Errorf("unexpected error message: %v", err)
+	}
+}
+
+// TestIsProviderLLMSupported_Google verifies that a provider whose npm is
+// @ai-sdk/google is reported as supported (it now maps to a wire protocol).
+func TestIsProviderLLMSupported_Google(t *testing.T) {
+	info := &ProviderInfo{ID: "testproxy", NPM: "@ai-sdk/google"}
+	if !isProviderLLMSupported("testproxy", info) {
+		t.Error("expected @ai-sdk/google provider to be LLM-supported")
+	}
+}
+
+// TestVersionedBasePath verifies detection of proxy base URLs that already
+// carry an API version segment (which collides with the genai SDK's injected
+// version).
+func TestVersionedBasePath(t *testing.T) {
+	tests := []struct {
+		rawURL string
+		want   string
+	}{
+		{"https://opencode.ai/zen/v1", "/zen/v1"},
+		{"https://opencode.ai/zen/v1/", "/zen/v1"},
+		{"https://example.com/api/v1beta", "/api/v1beta"},
+		{"https://example.com/api/v2alpha", "/api/v2alpha"},
+		{"https://generativelanguage.googleapis.com", ""},
+		{"https://proxy.example/openai", ""},
+		{"", ""},
+	}
+	for _, tt := range tests {
+		if got := versionedBasePath(tt.rawURL); got != tt.want {
+			t.Errorf("versionedBasePath(%q) = %q, want %q", tt.rawURL, got, tt.want)
+		}
+	}
+}
+
+// recordingRoundTripper captures the path of the request it receives.
+type recordingRoundTripper struct{ gotPath string }
+
+func (r *recordingRoundTripper) RoundTrip(req *http.Request) (*http.Response, error) {
+	r.gotPath = req.URL.Path
+	return &http.Response{
+		StatusCode: 200,
+		Body:       io.NopCloser(strings.NewReader("{}")),
+		Header:     make(http.Header),
+	}, nil
+}
+
+// TestGeminiProxyTransport_StripsInjectedVersion verifies that the transport
+// collapses the genai-injected "/v1beta" segment that follows a proxy base
+// URL which already carries its own version segment. This is the second-order
+// fix that makes opencode/gemini-* actually reach the proxy (issue #41).
+func TestGeminiProxyTransport_StripsInjectedVersion(t *testing.T) {
+	tests := []struct {
+		name     string
+		basePath string
+		reqPath  string
+		wantPath string
+	}{
+		{
+			name:     "strips doubled v1beta after /zen/v1",
+			basePath: "/zen/v1",
+			reqPath:  "/zen/v1/v1beta/models/gemini-3.5-flash:generateContent",
+			wantPath: "/zen/v1/models/gemini-3.5-flash:generateContent",
+		},
+		{
+			name:     "strips doubled v1beta1 after /zen/v1",
+			basePath: "/zen/v1",
+			reqPath:  "/zen/v1/v1beta1/models/gemini-3.5-flash:generateContent",
+			wantPath: "/zen/v1/models/gemini-3.5-flash:generateContent",
+		},
+		{
+			name:     "leaves non-matching path untouched",
+			basePath: "/zen/v1",
+			reqPath:  "/other/v1beta/models/x:generateContent",
+			wantPath: "/other/v1beta/models/x:generateContent",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			rec := &recordingRoundTripper{}
+			tr := &geminiProxyTransport{base: rec, basePath: tt.basePath}
+			req, err := http.NewRequest(http.MethodPost, "https://host"+tt.reqPath, nil)
+			if err != nil {
+				t.Fatalf("NewRequest: %v", err)
+			}
+			if _, err := tr.RoundTrip(req); err != nil {
+				t.Fatalf("RoundTrip: %v", err)
+			}
+			if rec.gotPath != tt.wantPath {
+				t.Errorf("forwarded path = %q, want %q", rec.gotPath, tt.wantPath)
+			}
+		})
+	}
+}

internal/models/cache_options.go

@@ -3,7 +3,6 @@ package models
 import (
 	"crypto/sha256"
 	"encoding/hex"
-	"maps"
 	"os"
 
 	"charm.land/fantasy"
@@ -69,19 +68,3 @@ func generateCacheKey(systemPrompt, modelID string) string {
 	// Prefix with "kit-" to identify KIT-generated cache keys
 	return "kit-" + hex.EncodeToString(h.Sum(nil))[:24]
 }
-
-// mergeProviderOptions merges multiple ProviderOptions maps.
-// Later maps take precedence over earlier ones.
-func mergeProviderOptions(opts ...fantasy.ProviderOptions) fantasy.ProviderOptions {
-	result := make(fantasy.ProviderOptions)
-
-	for _, opt := range opts {
-		maps.Copy(result, opt)
-	}
-
-	if len(result) == 0 {
-		return nil
-	}
-
-	return result
-}

internal/models/cache_options_test.go

@@ -3,8 +3,6 @@ package models
 import (
 	"os"
 	"testing"
-
-	"charm.land/fantasy"
 )
 
 func TestModelInfo_SupportsCaching(t *testing.T) {
@@ -192,57 +190,3 @@ func TestCachingPriorityOverThinking(t *testing.T) {
 		t.Errorf("OpenAI caching should work when thinking is OFF")
 	}
 }
-
-func TestMergeProviderOptions(t *testing.T) {
-	opts1 := fantasy.ProviderOptions{
-		"provider1": &testProviderData{value: "value1"},
-	}
-	opts2 := fantasy.ProviderOptions{
-		"provider2": &testProviderData{value: "value2"},
-	}
-
-	merged := mergeProviderOptions(opts1, opts2)
-
-	if len(merged) != 2 {
-		t.Errorf("mergeProviderOptions should combine options from multiple maps, got %d items", len(merged))
-	}
-
-	if _, ok := merged["provider1"]; !ok {
-		t.Errorf("merged options should contain 'provider1' key")
-	}
-
-	if _, ok := merged["provider2"]; !ok {
-		t.Errorf("merged options should contain 'provider2' key")
-	}
-
-	// Later options should override earlier ones
-	opts3 := fantasy.ProviderOptions{
-		"provider1": &testProviderData{value: "overridden"},
-	}
-	merged2 := mergeProviderOptions(opts1, opts3)
-
-	if data, ok := merged2["provider1"].(*testProviderData); ok {
-		if data.value != "overridden" {
-			t.Errorf("later options should override earlier ones, got %q", data.value)
-		}
-	}
-
-	if mergeProviderOptions() != nil {
-		t.Errorf("mergeProviderOptions with no args should return nil")
-	}
-}
-
-// testProviderData is a simple implementation of ProviderOptionsData for testing
-type testProviderData struct {
-	value string
-}
-
-func (t *testProviderData) Options() {}
-
-func (t *testProviderData) MarshalJSON() ([]byte, error) {
-	return []byte(`"` + t.value + `"`), nil
-}
-
-func (t *testProviderData) UnmarshalJSON(data []byte) error {
-	return nil
-}

internal/models/copilot_test.go

@@ -0,0 +1,84 @@
+package models
+
+import (
+	"net/http"
+	"testing"
+	"time"
+)
+
+func TestCopilotProviderAliasUsesCatalog(t *testing.T) {
+	registry := NewModelsRegistry()
+
+	models, err := registry.GetModelsForProvider("copilot")
+	if err != nil {
+		t.Fatalf("GetModelsForProvider(copilot) failed: %v", err)
+	}
+	if len(models) == 0 {
+		t.Fatal("expected copilot alias to return github-copilot catalog models")
+	}
+	if registry.LookupModel("copilot", "gpt-5.5") == nil {
+		t.Fatal("expected copilot/gpt-5.5 to resolve through github-copilot catalog")
+	}
+	if registry.GetProviderInfo("copilot") == nil {
+		t.Fatal("expected copilot alias to return github-copilot provider info")
+	}
+}
+
+func TestCopilotRejectsNonGPTModels(t *testing.T) {
+	_, err := CreateProvider(t.Context(), &ProviderConfig{ModelString: "copilot/claude-sonnet-4.6"})
+	if err == nil {
+		t.Fatal("expected non-GPT Copilot model to be rejected")
+	}
+}
+
+func TestCopilotHTTPClientCachesToken(t *testing.T) {
+	client := createCopilotHTTPClient("cached-token", time.Now().Add(time.Hour).Unix(), false)
+	transport, ok := client.Transport.(*copilotTransport)
+	if !ok {
+		t.Fatal("expected *copilotTransport")
+	}
+
+	token := transport.cachedToken(t.Context())
+	if token != "cached-token" {
+		t.Fatalf("expected cached token, got %q", token)
+	}
+}
+
+func TestCopilotTransportHeaders(t *testing.T) {
+	req, err := http.NewRequest(http.MethodGet, "https://example.com", nil)
+	if err != nil {
+		t.Fatalf("NewRequest failed: %v", err)
+	}
+
+	transport := &copilotTransport{
+		base: roundTripFunc(func(req *http.Request) (*http.Response, error) {
+			if req.Header.Get("Authorization") != "Bearer cached-token" {
+				t.Fatalf("unexpected Authorization header: %q", req.Header.Get("Authorization"))
+			}
+			if req.Header.Get("Copilot-Integration-Id") != copilotIntegrationID {
+				t.Fatalf("unexpected Copilot-Integration-Id header: %q", req.Header.Get("Copilot-Integration-Id"))
+			}
+			if req.Header.Get("Editor-Version") != copilotEditorVersion {
+				t.Fatalf("unexpected Editor-Version header: %q", req.Header.Get("Editor-Version"))
+			}
+			if req.Header.Get("User-Agent") != copilotUserAgent {
+				t.Fatalf("unexpected User-Agent header: %q", req.Header.Get("User-Agent"))
+			}
+			return &http.Response{StatusCode: http.StatusOK, Body: http.NoBody}, nil
+		}),
+		token:     "cached-token",
+		expiresAt: time.Now().Add(time.Hour).Unix(),
+	}
+
+	resp, err := transport.RoundTrip(req)
+	if err != nil {
+		t.Fatalf("RoundTrip failed: %v", err)
+	}
+	_ = resp.Body.Close()
+}
+
+type roundTripFunc func(*http.Request) (*http.Response, error)
+
+func (f roundTripFunc) RoundTrip(req *http.Request) (*http.Response, error) {
+	return f(req)
+}

internal/models/custom.go

@@ -2,20 +2,32 @@ package models
 
 import (
 	"log"
+	"os"
+	"strings"
 
 	"github.com/spf13/viper"
 )
 
 // loadCustomModelsFromConfig loads custom model definitions from the config file
 // and returns them as a map of model ID -> ModelInfo. Returns nil if no custom
-// models are configured.
+// models are configured. Reads from the process-global viper store (the model
+// registry is a process-global singleton).
 func loadCustomModelsFromConfig() map[string]ModelInfo {
-	if !viper.IsSet("customModels") {
+	return loadCustomModelsFrom(viper.GetViper())
+}
+
+// loadCustomModelsFrom loads custom model definitions from the supplied store.
+// When v is nil the process-global store is used.
+func loadCustomModelsFrom(v *viper.Viper) map[string]ModelInfo {
+	if v == nil {
+		v = viper.GetViper()
+	}
+	if !v.IsSet("customModels") {
 		return nil
 	}
 
 	var customModels map[string]CustomModelConfig
-	if err := viper.UnmarshalKey("customModels", &customModels); err != nil {
+	if err := v.UnmarshalKey("customModels", &customModels); err != nil {
 		log.Printf("Warning: Failed to parse customModels: %v", err)
 		return nil
 	}
@@ -31,7 +43,7 @@ func loadCustomModelsFromConfig() map[string]ModelInfo {
 
 // modelConfigToModelInfo converts a CustomModelConfig to a ModelInfo.
 func modelConfigToModelInfo(modelID string, cfg CustomModelConfig) ModelInfo {
-	return ModelInfo{
+	info := ModelInfo{
 		ID:          modelID,
 		Name:        cfg.Name,
 		Attachment:  cfg.Attachment,
@@ -48,21 +60,258 @@ func modelConfigToModelInfo(modelID string, cfg CustomModelConfig) ModelInfo {
 			Output:  cfg.Limit.Output,
 		},
 	}
+
+	// Convert custom model generation params if any are set.
+	if p := convertGenerationParams(cfg.Params); p != nil {
+		info.Params = p
+	}
+
+	return info
+}
+
+// LoadModelSettingsFrom loads per-model generation parameter overrides from the
+// supplied per-instance store. When v is nil the process-global store is used.
+// Keys are "provider/model" strings. Returns nil if no model settings are
+// configured.
+func LoadModelSettingsFrom(v *viper.Viper) map[string]*GenerationParams {
+	if v == nil {
+		v = viper.GetViper()
+	}
+	if !v.IsSet("modelSettings") {
+		return nil
+	}
+
+	var settings map[string]GenerationParamsConfig
+	if err := v.UnmarshalKey("modelSettings", &settings); err != nil {
+		log.Printf("Warning: Failed to parse modelSettings: %v", err)
+		return nil
+	}
+
+	result := make(map[string]*GenerationParams, len(settings))
+	for modelKey, cfg := range settings {
+		if p := convertGenerationParams(cfg); p != nil {
+			result[modelKey] = p
+		}
+	}
+
+	return result
+}
+
+// convertGenerationParams converts a GenerationParamsConfig to a GenerationParams.
+// Returns nil if no parameters are set.
+func convertGenerationParams(cfg GenerationParamsConfig) *GenerationParams {
+	p := &GenerationParams{}
+	any := false
+
+	if cfg.MaxTokens != nil {
+		p.MaxTokens = cfg.MaxTokens
+		any = true
+	}
+	if cfg.Temperature != nil {
+		p.Temperature = cfg.Temperature
+		any = true
+	}
+	if cfg.TopP != nil {
+		p.TopP = cfg.TopP
+		any = true
+	}
+	if cfg.TopK != nil {
+		p.TopK = cfg.TopK
+		any = true
+	}
+	if cfg.FrequencyPenalty != nil {
+		p.FrequencyPenalty = cfg.FrequencyPenalty
+		any = true
+	}
+	if cfg.PresencePenalty != nil {
+		p.PresencePenalty = cfg.PresencePenalty
+		any = true
+	}
+	if len(cfg.StopSequences) > 0 {
+		p.StopSequences = cfg.StopSequences
+		any = true
+	}
+	if cfg.ThinkingLevel != "" {
+		p.ThinkingLevel = ParseThinkingLevel(cfg.ThinkingLevel)
+		any = true
+	}
+	if cfg.SystemPrompt != "" {
+		p.SystemPrompt = cfg.SystemPrompt
+		any = true
+	}
+
+	if !any {
+		return nil
+	}
+	return p
+}
+
+// ApplyModelSettings merges per-model generation parameter defaults from the
+// registry into a ProviderConfig. Model-level params are only applied for
+// fields where the user has not explicitly set a value (i.e., the
+// corresponding viper key is not set via CLI flag or global config).
+//
+// The lookup order is:
+//  1. modelSettings["provider/model"] from config (highest model-level priority)
+//  2. ModelInfo.Params from custom model definitions
+//
+// Both are overridden by explicit CLI flags / global config values.
+func ApplyModelSettings(config *ProviderConfig, modelInfo *ModelInfo) {
+	provider, modelName, err := ParseModelString(config.ModelString)
+	if err != nil {
+		return
+	}
+
+	// Resolve the config store: prefer the per-instance store carried on the
+	// ProviderConfig (set by BuildProviderConfig / Kit.New), falling back to
+	// the process-global store for callers that don't thread one through.
+	store := config.ConfigStore
+
+	// Collect model-level params: modelSettings override > custom model params.
+	// modelSettings takes priority because it's the more specific/intentional config.
+	var params *GenerationParams
+
+	// First check modelSettings from config.
+	if settings := LoadModelSettingsFrom(store); settings != nil {
+		modelKey := provider + "/" + modelName
+		if p, ok := settings[modelKey]; ok {
+			params = p
+		}
+	}
+
+	// Fall back to ModelInfo.Params (from custom model definitions).
+	if params == nil && modelInfo != nil && modelInfo.Params != nil {
+		params = modelInfo.Params
+	}
+
+	if params == nil {
+		return
+	}
+
+	// Apply each parameter only when the user hasn't explicitly set it.
+	// We check viper.IsSet() which returns true only when the key was
+	// set via CLI flag, environment variable, or config file global section.
+
+	if params.MaxTokens != nil && !isExplicitlySet(store, "max-tokens") {
+		config.MaxTokens = *params.MaxTokens
+	}
+	if params.Temperature != nil && !isExplicitlySet(store, "temperature") {
+		config.Temperature = params.Temperature
+	}
+	if params.TopP != nil && !isExplicitlySet(store, "top-p") {
+		config.TopP = params.TopP
+	}
+	if params.TopK != nil && !isExplicitlySet(store, "top-k") {
+		config.TopK = params.TopK
+	}
+	if params.FrequencyPenalty != nil && !isExplicitlySet(store, "frequency-penalty") {
+		config.FrequencyPenalty = params.FrequencyPenalty
+	}
+	if params.PresencePenalty != nil && !isExplicitlySet(store, "presence-penalty") {
+		config.PresencePenalty = params.PresencePenalty
+	}
+	if len(params.StopSequences) > 0 && !isExplicitlySet(store, "stop-sequences") {
+		config.StopSequences = params.StopSequences
+	}
+	if params.ThinkingLevel != "" && !isExplicitlySet(store, "thinking-level") {
+		config.ThinkingLevel = params.ThinkingLevel
+	}
+	if params.SystemPrompt != "" && config.SystemPrompt == "" {
+		// Resolve file paths: if the value points to an existing file, read it.
+		// We check config.SystemPrompt == "" rather than isExplicitlySet because
+		// viper.BindPFlag causes IsSet to return true even for unset flags.
+		config.SystemPrompt = LoadSystemPromptValue(params.SystemPrompt)
+	}
+}
+
+// LoadSystemPromptValue resolves a system prompt value that may be either
+// inline text or a file path. If the value is a path to an existing file,
+// its contents are read and returned. Otherwise the string is returned as-is.
+// This mirrors config.LoadSystemPrompt but lives in the models package to
+// avoid circular dependencies.
+func LoadSystemPromptValue(input string) string {
+	if input == "" {
+		return ""
+	}
+	if info, err := os.Stat(input); err == nil && !info.IsDir() {
+		content, err := os.ReadFile(input)
+		if err != nil {
+			log.Printf("Warning: failed to read system prompt file %q: %v", input, err)
+			return input
+		}
+		return strings.TrimSpace(string(content))
+	}
+	return input
+}
+
+// isExplicitlySet returns true when the user has explicitly set a config key
+// via CLI flag, environment variable, or the global section of the config file.
+// Model-level defaults should not override explicitly set values.
+//
+// The check runs against the supplied per-instance store when non-nil,
+// otherwise the process-global store. This keeps the "explicit vs unset"
+// precedence contract per-Kit-instance once a store is threaded through.
+func isExplicitlySet(v *viper.Viper, key string) bool {
+	if v == nil {
+		v = viper.GetViper()
+	}
+	// viper.IsSet returns true if the key has been set in any of the
+	// data stores (flag, env, config file, default). We need to check
+	// whether the value was set at the global config level (not just
+	// as a default). For generation params, the global config keys use
+	// hyphenated names (e.g. "max-tokens", "top-p").
+	//
+	// Since viper merges all sources, IsSet returns true even for config
+	// file values. This means global config file values (e.g.
+	// temperature: 0.7 at the top level) will correctly take precedence
+	// over model-level defaults, which is the desired behavior.
+	return v.IsSet(key)
+}
+
+// GenerationParams holds per-model generation parameter defaults.
+// These are stored on ModelInfo and applied during provider creation.
+// Nil pointer fields mean "no model-level default" — the global config
+// or CLI flag value (if any) will be used instead.
+type GenerationParams struct {
+	MaxTokens        *int
+	Temperature      *float32
+	TopP             *float32
+	TopK             *int32
+	FrequencyPenalty *float32
+	PresencePenalty  *float32
+	StopSequences    []string
+	ThinkingLevel    ThinkingLevel
+	SystemPrompt     string // Per-model system prompt (inline text or file path)
 }
 
 // CustomModelConfig defines a custom model configuration loaded from the config file.
 // This is a duplicate here to avoid circular dependencies with internal/config.
 type CustomModelConfig struct {
-	Name        string      `json:"name" yaml:"name"`
-	BaseURL     string      `json:"baseUrl,omitempty" yaml:"baseUrl,omitempty"`
-	APIKey      string      `json:"apiKey,omitempty" yaml:"apiKey,omitempty"`
-	Family      string      `json:"family,omitempty" yaml:"family,omitempty"`
-	Attachment  bool        `json:"attachment,omitempty" yaml:"attachment,omitempty"`
-	Reasoning   bool        `json:"reasoning,omitempty" yaml:"reasoning,omitempty"`
-	Temperature bool        `json:"temperature,omitempty" yaml:"temperature,omitempty"`
-	Knowledge   string      `json:"knowledge,omitempty" yaml:"knowledge,omitempty"`
-	Cost        CostConfig  `json:"cost" yaml:"cost"`
-	Limit       LimitConfig `json:"limit" yaml:"limit"`
+	Name        string                 `json:"name" yaml:"name"`
+	BaseURL     string                 `json:"baseUrl,omitempty" yaml:"baseUrl,omitempty"`
+	APIKey      string                 `json:"apiKey,omitempty" yaml:"apiKey,omitempty"`
+	Family      string                 `json:"family,omitempty" yaml:"family,omitempty"`
+	Attachment  bool                   `json:"attachment,omitempty" yaml:"attachment,omitempty"`
+	Reasoning   bool                   `json:"reasoning,omitempty" yaml:"reasoning,omitempty"`
+	Temperature bool                   `json:"temperature,omitempty" yaml:"temperature,omitempty"`
+	Knowledge   string                 `json:"knowledge,omitempty" yaml:"knowledge,omitempty"`
+	Cost        CostConfig             `json:"cost" yaml:"cost"`
+	Limit       LimitConfig            `json:"limit" yaml:"limit"`
+	Params      GenerationParamsConfig `json:"params,omitzero" yaml:"params,omitempty"`
+}
+
+// GenerationParamsConfig is the JSON/YAML-serializable form of generation
+// parameter defaults. Used in both customModels[].params and modelSettings[].
+type GenerationParamsConfig struct {
+	MaxTokens        *int     `json:"maxTokens,omitempty" yaml:"maxTokens,omitempty"`
+	Temperature      *float32 `json:"temperature,omitempty" yaml:"temperature,omitempty"`
+	TopP             *float32 `json:"topP,omitempty" yaml:"topP,omitempty"`
+	TopK             *int32   `json:"topK,omitempty" yaml:"topK,omitempty"`
+	FrequencyPenalty *float32 `json:"frequencyPenalty,omitempty" yaml:"frequencyPenalty,omitempty"`
+	PresencePenalty  *float32 `json:"presencePenalty,omitempty" yaml:"presencePenalty,omitempty"`
+	StopSequences    []string `json:"stopSequences,omitempty" yaml:"stopSequences,omitempty"`
+	ThinkingLevel    string   `json:"thinkingLevel,omitempty" yaml:"thinkingLevel,omitempty"`
+	SystemPrompt     string   `json:"systemPrompt,omitempty" yaml:"systemPrompt,omitempty"`
 }
 
 // CostConfig defines the pricing for a custom model.

internal/models/custom_test.go

@@ -0,0 +1,422 @@
+package models
+
+import (
+	"os"
+	"testing"
+
+	"github.com/spf13/viper"
+)
+
+func TestConvertGenerationParams(t *testing.T) {
+	t.Run("empty config returns nil", func(t *testing.T) {
+		cfg := GenerationParamsConfig{}
+		p := convertGenerationParams(cfg)
+		if p != nil {
+			t.Errorf("expected nil, got %+v", p)
+		}
+	})
+
+	t.Run("temperature only", func(t *testing.T) {
+		temp := float32(0.7)
+		cfg := GenerationParamsConfig{Temperature: &temp}
+		p := convertGenerationParams(cfg)
+		if p == nil {
+			t.Fatal("expected non-nil")
+		}
+		if p.Temperature == nil || *p.Temperature != 0.7 {
+			t.Errorf("expected temperature 0.7, got %v", p.Temperature)
+		}
+		if p.TopP != nil {
+			t.Errorf("expected nil TopP, got %v", p.TopP)
+		}
+	})
+
+	t.Run("all params set", func(t *testing.T) {
+		maxTokens := 8192
+		temp := float32(0.5)
+		topP := float32(0.9)
+		topK := int32(50)
+		freqPenalty := float32(0.1)
+		presPenalty := float32(0.2)
+		cfg := GenerationParamsConfig{
+			MaxTokens:        &maxTokens,
+			Temperature:      &temp,
+			TopP:             &topP,
+			TopK:             &topK,
+			FrequencyPenalty: &freqPenalty,
+			PresencePenalty:  &presPenalty,
+			StopSequences:    []string{"STOP"},
+			ThinkingLevel:    "high",
+		}
+		p := convertGenerationParams(cfg)
+		if p == nil {
+			t.Fatal("expected non-nil")
+		}
+		if p.MaxTokens == nil || *p.MaxTokens != 8192 {
+			t.Errorf("expected maxTokens 8192, got %v", p.MaxTokens)
+		}
+		if p.Temperature == nil || *p.Temperature != 0.5 {
+			t.Errorf("expected temperature 0.5, got %v", p.Temperature)
+		}
+		if p.TopP == nil || *p.TopP != 0.9 {
+			t.Errorf("expected topP 0.9, got %v", p.TopP)
+		}
+		if p.TopK == nil || *p.TopK != 50 {
+			t.Errorf("expected topK 50, got %v", p.TopK)
+		}
+		if p.FrequencyPenalty == nil || *p.FrequencyPenalty != 0.1 {
+			t.Errorf("expected frequencyPenalty 0.1, got %v", p.FrequencyPenalty)
+		}
+		if p.PresencePenalty == nil || *p.PresencePenalty != 0.2 {
+			t.Errorf("expected presencePenalty 0.2, got %v", p.PresencePenalty)
+		}
+		if len(p.StopSequences) != 1 || p.StopSequences[0] != "STOP" {
+			t.Errorf("expected stop sequences [STOP], got %v", p.StopSequences)
+		}
+		if p.ThinkingLevel != ThinkingHigh {
+			t.Errorf("expected thinking level high, got %v", p.ThinkingLevel)
+		}
+	})
+
+	t.Run("thinking level parsing", func(t *testing.T) {
+		cfg := GenerationParamsConfig{ThinkingLevel: "medium"}
+		p := convertGenerationParams(cfg)
+		if p == nil {
+			t.Fatal("expected non-nil")
+		}
+		if p.ThinkingLevel != ThinkingMedium {
+			t.Errorf("expected thinking level medium, got %v", p.ThinkingLevel)
+		}
+	})
+	t.Run("system prompt only", func(t *testing.T) {
+		cfg := GenerationParamsConfig{SystemPrompt: "You are helpful."}
+		p := convertGenerationParams(cfg)
+		if p == nil {
+			t.Fatal("expected non-nil")
+		}
+		if p.SystemPrompt != "You are helpful." {
+			t.Errorf("expected system prompt, got %q", p.SystemPrompt)
+		}
+	})
+}
+
+func TestModelConfigToModelInfoWithParams(t *testing.T) {
+	temp := float32(0.8)
+	topP := float32(0.95)
+	cfg := CustomModelConfig{
+		Name:        "Test Model",
+		BaseURL:     "http://localhost:8080/v1",
+		Temperature: true,
+		Params: GenerationParamsConfig{
+			Temperature: &temp,
+			TopP:        &topP,
+		},
+	}
+
+	info := modelConfigToModelInfo("test-model", cfg)
+
+	if info.Params == nil {
+		t.Fatal("expected non-nil Params")
+	}
+	if info.Params.Temperature == nil || *info.Params.Temperature != 0.8 {
+		t.Errorf("expected temperature 0.8, got %v", info.Params.Temperature)
+	}
+	if info.Params.TopP == nil || *info.Params.TopP != 0.95 {
+		t.Errorf("expected topP 0.95, got %v", info.Params.TopP)
+	}
+}
+
+func TestModelConfigToModelInfoWithoutParams(t *testing.T) {
+	cfg := CustomModelConfig{
+		Name:    "Test Model",
+		BaseURL: "http://localhost:8080/v1",
+	}
+
+	info := modelConfigToModelInfo("test-model", cfg)
+
+	if info.Params != nil {
+		t.Errorf("expected nil Params, got %+v", info.Params)
+	}
+}
+
+func TestApplyModelSettings(t *testing.T) {
+	// Save and restore viper state.
+	originalViper := viper.AllSettings()
+	defer func() {
+		viper.Reset()
+		for k, v := range originalViper {
+			viper.Set(k, v)
+		}
+	}()
+
+	t.Run("applies model params when not explicitly set", func(t *testing.T) {
+		viper.Reset()
+
+		temp := float32(0.8)
+		topK := int32(50)
+		maxTokens := 4096
+		modelInfo := &ModelInfo{
+			ID: "test-model",
+			Params: &GenerationParams{
+				Temperature: &temp,
+				TopK:        &topK,
+				MaxTokens:   &maxTokens,
+			},
+		}
+
+		config := &ProviderConfig{
+			ModelString: "custom/test-model",
+		}
+
+		ApplyModelSettings(config, modelInfo)
+
+		if config.Temperature == nil || *config.Temperature != 0.8 {
+			t.Errorf("expected temperature 0.8, got %v", config.Temperature)
+		}
+		if config.TopK == nil || *config.TopK != 50 {
+			t.Errorf("expected topK 50, got %v", config.TopK)
+		}
+		if config.MaxTokens != 4096 {
+			t.Errorf("expected maxTokens 4096, got %d", config.MaxTokens)
+		}
+	})
+
+	t.Run("explicit viper values take precedence", func(t *testing.T) {
+		viper.Reset()
+		viper.Set("temperature", 0.3)
+
+		temp := float32(0.8)
+		modelInfo := &ModelInfo{
+			ID: "test-model",
+			Params: &GenerationParams{
+				Temperature: &temp,
+			},
+		}
+
+		explicitTemp := float32(0.3)
+		config := &ProviderConfig{
+			ModelString: "custom/test-model",
+			Temperature: &explicitTemp,
+		}
+
+		ApplyModelSettings(config, modelInfo)
+
+		// Temperature should NOT be overridden because it's explicitly set in viper
+		if config.Temperature == nil || *config.Temperature != 0.3 {
+			t.Errorf("expected temperature 0.3 (explicit), got %v", config.Temperature)
+		}
+	})
+
+	t.Run("nil model info is safe", func(t *testing.T) {
+		viper.Reset()
+
+		config := &ProviderConfig{
+			ModelString: "custom/test-model",
+		}
+
+		// Should not panic
+		ApplyModelSettings(config, nil)
+
+		if config.Temperature != nil {
+			t.Errorf("expected nil temperature, got %v", config.Temperature)
+		}
+	})
+
+	t.Run("model info without params is safe", func(t *testing.T) {
+		viper.Reset()
+
+		modelInfo := &ModelInfo{ID: "test-model"}
+		config := &ProviderConfig{
+			ModelString: "custom/test-model",
+		}
+
+		ApplyModelSettings(config, modelInfo)
+
+		if config.Temperature != nil {
+			t.Errorf("expected nil temperature, got %v", config.Temperature)
+		}
+	})
+
+	t.Run("modelSettings from viper takes priority over ModelInfo.Params", func(t *testing.T) {
+		viper.Reset()
+
+		// Set up modelSettings in viper (simulating config file)
+		viper.Set("modelSettings", map[string]any{
+			"custom/test-model": map[string]any{
+				"temperature": 0.5,
+				"topK":        30,
+			},
+		})
+
+		// ModelInfo has different params
+		temp := float32(0.8)
+		topK := int32(50)
+		modelInfo := &ModelInfo{
+			ID: "test-model",
+			Params: &GenerationParams{
+				Temperature: &temp,
+				TopK:        &topK,
+			},
+		}
+
+		config := &ProviderConfig{
+			ModelString: "custom/test-model",
+		}
+
+		ApplyModelSettings(config, modelInfo)
+
+		// modelSettings should win over ModelInfo.Params
+		if config.Temperature == nil || *config.Temperature != 0.5 {
+			t.Errorf("expected temperature 0.5 (from modelSettings), got %v", config.Temperature)
+		}
+		if config.TopK == nil || *config.TopK != 30 {
+			t.Errorf("expected topK 30 (from modelSettings), got %v", config.TopK)
+		}
+	})
+
+	t.Run("stop sequences applied from model params", func(t *testing.T) {
+		viper.Reset()
+
+		modelInfo := &ModelInfo{
+			ID: "test-model",
+			Params: &GenerationParams{
+				StopSequences: []string{"STOP", "END"},
+			},
+		}
+
+		config := &ProviderConfig{
+			ModelString: "custom/test-model",
+		}
+
+		ApplyModelSettings(config, modelInfo)
+
+		if len(config.StopSequences) != 2 || config.StopSequences[0] != "STOP" {
+			t.Errorf("expected stop sequences [STOP END], got %v", config.StopSequences)
+		}
+	})
+
+	t.Run("thinking level applied from model params", func(t *testing.T) {
+		viper.Reset()
+
+		modelInfo := &ModelInfo{
+			ID: "test-model",
+			Params: &GenerationParams{
+				ThinkingLevel: ThinkingHigh,
+			},
+		}
+
+		config := &ProviderConfig{
+			ModelString: "custom/test-model",
+		}
+
+		ApplyModelSettings(config, modelInfo)
+
+		if config.ThinkingLevel != ThinkingHigh {
+			t.Errorf("expected thinking level high, got %v", config.ThinkingLevel)
+		}
+	})
+
+	t.Run("system prompt applied from model params", func(t *testing.T) {
+		viper.Reset()
+
+		modelInfo := &ModelInfo{
+			ID: "test-model",
+			Params: &GenerationParams{
+				SystemPrompt: "You are a coding assistant.",
+			},
+		}
+
+		config := &ProviderConfig{
+			ModelString: "custom/test-model",
+		}
+
+		ApplyModelSettings(config, modelInfo)
+
+		if config.SystemPrompt != "You are a coding assistant." {
+			t.Errorf("expected system prompt to be set, got %q", config.SystemPrompt)
+		}
+	})
+
+	t.Run("explicit system prompt takes precedence", func(t *testing.T) {
+		viper.Reset()
+
+		modelInfo := &ModelInfo{
+			ID: "test-model",
+			Params: &GenerationParams{
+				SystemPrompt: "Model-specific prompt",
+			},
+		}
+
+		config := &ProviderConfig{
+			ModelString:  "custom/test-model",
+			SystemPrompt: "Global prompt",
+		}
+
+		ApplyModelSettings(config, modelInfo)
+
+		// Global system prompt should NOT be overridden because config
+		// already has a non-empty SystemPrompt.
+		if config.SystemPrompt != "Global prompt" {
+			t.Errorf("expected global prompt preserved, got %q", config.SystemPrompt)
+		}
+	})
+
+	t.Run("system prompt from file path", func(t *testing.T) {
+		viper.Reset()
+
+		// Create a temp file with a system prompt
+		tmpFile, err := os.CreateTemp("", "kit-test-prompt-*.txt")
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer func() { _ = os.Remove(tmpFile.Name()) }()
+		if _, err := tmpFile.WriteString("  Prompt from file  "); err != nil {
+			t.Fatal(err)
+		}
+		_ = tmpFile.Close()
+
+		modelInfo := &ModelInfo{
+			ID: "test-model",
+			Params: &GenerationParams{
+				SystemPrompt: tmpFile.Name(),
+			},
+		}
+
+		config := &ProviderConfig{
+			ModelString: "custom/test-model",
+		}
+
+		ApplyModelSettings(config, modelInfo)
+
+		if config.SystemPrompt != "Prompt from file" {
+			t.Errorf("expected trimmed file content, got %q", config.SystemPrompt)
+		}
+	})
+
+	t.Run("modelSettings system prompt overrides custom model params", func(t *testing.T) {
+		viper.Reset()
+
+		viper.Set("modelSettings", map[string]any{
+			"custom/test-model": map[string]any{
+				"systemPrompt": "From modelSettings",
+			},
+		})
+
+		modelInfo := &ModelInfo{
+			ID: "test-model",
+			Params: &GenerationParams{
+				SystemPrompt: "From custom model",
+			},
+		}
+
+		config := &ProviderConfig{
+			ModelString: "custom/test-model",
+		}
+
+		ApplyModelSettings(config, modelInfo)
+
+		if config.SystemPrompt != "From modelSettings" {
+			t.Errorf("expected modelSettings prompt, got %q", config.SystemPrompt)
+		}
+	})
+}

internal/models/modelsdb.go

@@ -48,18 +48,87 @@ type modelsDBLimit struct {
 	Output  int `json:"output"`
 }
 
-// npmToLLMProvider maps npm package names from models.dev to LLM
-// provider identifiers. Providers not in this map but with an api URL
-// can be auto-routed through openaicompat.
-var npmToLLMProvider = map[string]string{
-	"@ai-sdk/anthropic":               "anthropic",
-	"@ai-sdk/openai":                  "openai",
-	"@ai-sdk/google":                  "google",
-	"@ai-sdk/google-vertex":           "google-vertex",
-	"@ai-sdk/google-vertex/anthropic": "google-vertex-anthropic",
-	"@ai-sdk/amazon-bedrock":          "bedrock",
-	"@ai-sdk/azure":                   "azure",
-	"@openrouter/ai-sdk-provider":     "openrouter",
-	"@ai-sdk/vercel":                  "vercel",
-	"@ai-sdk/openai-compatible":       "openaicompat",
+// wireProtocol identifies which LLM API protocol an npm package speaks.
+// Fantasy implements three native protocols (openai, anthropic, google);
+// everything else in its providers/ tree is a thin wrapper around one of
+// them with a pre-baked default URL or auth scheme.
+type wireProtocol int
+
+const (
+	wireUnknown wireProtocol = iota
+	wireOpenAI
+	wireAnthropic
+	wireGoogle
+)
+
+// npmToWireProtocol maps npm package names from models.dev to the wire
+// protocol they speak. Provider-specific bundles that need bespoke auth or
+// URL templating (azure, bedrock, openrouter, google-vertex, google-vertex-
+// anthropic, and @ai-sdk/gateway which is the Vercel AI Gateway) are
+// intentionally absent — they have native top-level cases in CreateProvider
+// and never reach the auto-router. Providers not in this map but with an
+// api URL are auto-routed through the OpenAI-compatible wire.
+//
+// The thin OpenAI-compatible npm wrappers (groq, cerebras, mistral, …) are
+// listed explicitly so that auto-routing can recover their hard-coded base
+// URL from sdkDefaultBaseURL when the registry entry has no api field.
+var npmToWireProtocol = map[string]wireProtocol{
+	// Native wires.
+	"@ai-sdk/openai":            wireOpenAI,
+	"@ai-sdk/openai-compatible": wireOpenAI,
+	"@ai-sdk/anthropic":         wireAnthropic,
+	"@ai-sdk/google":            wireGoogle,
+
+	// Thin OpenAI-compatible wrappers. Each ships with a hard-coded base URL
+	// in its JS SDK (see sdkDefaultBaseURL) but speaks the plain OpenAI chat
+	// completions wire — so we can route them all through fantasy's
+	// openaicompat provider once we supply the URL.
+	"@ai-sdk/groq":                  wireOpenAI,
+	"@ai-sdk/cerebras":              wireOpenAI,
+	"@ai-sdk/perplexity":            wireOpenAI,
+	"@ai-sdk/togetherai":            wireOpenAI,
+	"@ai-sdk/xai":                   wireOpenAI,
+	"@ai-sdk/deepinfra":             wireOpenAI,
+	"@ai-sdk/mistral":               wireOpenAI,
+	"@ai-sdk/cohere":                wireOpenAI,
+	"@ai-sdk/vercel":                wireOpenAI, // v0 API (api.v0.dev), distinct from @ai-sdk/gateway
+	"@aihubmix/ai-sdk-provider":     wireOpenAI,
+	"venice-ai-sdk-provider":        wireOpenAI,
+	"merge-gateway-ai-sdk-provider": wireOpenAI,
+}
+
+// sdkDefaultBaseURL maps an npm package name to the base URL its JavaScript
+// SDK uses by default. This lets us recover a working endpoint for providers
+// whose models.dev entry omits the `api` field because the JS SDK hard-codes
+// the URL (e.g. groq, cerebras, mistral, x.ai…).
+//
+// Only OpenAI-compatible and native-wire SDKs are listed; providers needing
+// bespoke auth or URL templating (bedrock SigV4, azure resource URLs,
+// google-vertex project/location, cloudflare gateway account IDs, gitlab,
+// sap-ai-core) are handled by native CreateProvider cases or surface a
+// targeted error that asks the user to supply --provider-url.
+var sdkDefaultBaseURL = map[string]string{
+	// Native wires.
+	"@ai-sdk/openai":    "https://api.openai.com/v1",
+	"@ai-sdk/anthropic": "https://api.anthropic.com/v1",
+	"@ai-sdk/google":    "https://generativelanguage.googleapis.com/v1beta",
+
+	// Thin OpenAI-compatible wrappers.
+	"@ai-sdk/groq":                  "https://api.groq.com/openai/v1",
+	"@ai-sdk/cerebras":              "https://api.cerebras.ai/v1",
+	"@ai-sdk/perplexity":            "https://api.perplexity.ai",
+	"@ai-sdk/togetherai":            "https://api.together.xyz/v1",
+	"@ai-sdk/xai":                   "https://api.x.ai/v1",
+	"@ai-sdk/deepinfra":             "https://api.deepinfra.com/v1/openai",
+	"@ai-sdk/mistral":               "https://api.mistral.ai/v1",
+	"@ai-sdk/cohere":                "https://api.cohere.com/compatibility/v1",
+	"@ai-sdk/vercel":                "https://api.v0.dev/v1",
+	"@aihubmix/ai-sdk-provider":     "https://aihubmix.com/v1",
+	"venice-ai-sdk-provider":        "https://api.venice.ai/api/v1",
+	"merge-gateway-ai-sdk-provider": "https://api-gateway.merge.dev/v1/ai-sdk",
+
+	// Native handlers — included for ResolveProviderBaseURL introspection
+	// even though CreateProvider routes these via dedicated cases.
+	"@ai-sdk/gateway":             "https://ai-gateway.vercel.sh/v1",
+	"@openrouter/ai-sdk-provider": "https://openrouter.ai/api/v1",
 }

internal/models/pool.go

@@ -1,168 +0,0 @@
-package models
-
-import (
-	"context"
-	"sync"
-	"time"
-
-	"charm.land/fantasy"
-)
-
-// ProviderPool manages reusable LLM provider instances to reduce overhead
-// when spawning multiple subagents or making repeated completion calls.
-type ProviderPool struct {
-	mu        sync.RWMutex
-	providers map[string]*pooledProvider
-	ttl       time.Duration
-	closed    bool
-	closeCh   chan struct{}
-}
-
-type pooledProvider struct {
-	model        fantasy.LanguageModel
-	closer       func() error
-	providerOpts fantasy.ProviderOptions
-	created      time.Time
-	lastUsed     time.Time
-	refs         int32
-}
-
-// DefaultPoolTTL is the default time-to-live for idle pooled providers.
-const DefaultPoolTTL = 5 * time.Minute
-
-// globalPool is the singleton provider pool instance.
-var globalPool *ProviderPool
-var poolOnce sync.Once
-
-// GetGlobalPool returns the singleton provider pool instance.
-func GetGlobalPool() *ProviderPool {
-	poolOnce.Do(func() {
-		globalPool = NewProviderPool(DefaultPoolTTL)
-	})
-	return globalPool
-}
-
-// NewProviderPool creates a provider pool with the given TTL for idle providers.
-func NewProviderPool(ttl time.Duration) *ProviderPool {
-	p := &ProviderPool{
-		providers: make(map[string]*pooledProvider),
-		ttl:       ttl,
-		closeCh:   make(chan struct{}),
-	}
-	go p.cleanupLoop()
-	return p
-}
-
-// Get returns a provider for the model string, creating one if needed.
-// The returned release function must be called when the provider is no longer
-// needed. The provider may be reused by subsequent Get calls.
-func (p *ProviderPool) Get(ctx context.Context, modelString string) (fantasy.LanguageModel, fantasy.ProviderOptions, func(), error) {
-	p.mu.Lock()
-
-	// Check if we have an existing provider.
-	if pp, ok := p.providers[modelString]; ok {
-		pp.refs++
-		pp.lastUsed = time.Now()
-		p.mu.Unlock()
-		return pp.model, pp.providerOpts, func() { p.release(modelString) }, nil
-	}
-
-	p.mu.Unlock()
-
-	// Create a new provider outside the lock.
-	config := &ProviderConfig{ModelString: modelString}
-	result, err := CreateProvider(ctx, config)
-	if err != nil {
-		return nil, nil, nil, err
-	}
-
-	p.mu.Lock()
-	defer p.mu.Unlock()
-
-	// Double-check: another goroutine may have created one while we were unlocked.
-	if pp, ok := p.providers[modelString]; ok {
-		// Close the one we just created and use the existing one.
-		if result.Closer != nil {
-			_ = result.Closer.Close()
-		}
-		pp.refs++
-		pp.lastUsed = time.Now()
-		return pp.model, pp.providerOpts, func() { p.release(modelString) }, nil
-	}
-
-	var closerFn func() error
-	if result.Closer != nil {
-		closerFn = result.Closer.Close
-	}
-
-	pp := &pooledProvider{
-		model:        result.Model,
-		closer:       closerFn,
-		providerOpts: result.ProviderOptions,
-		created:      time.Now(),
-		lastUsed:     time.Now(),
-		refs:         1,
-	}
-	p.providers[modelString] = pp
-
-	return pp.model, pp.providerOpts, func() { p.release(modelString) }, nil
-}
-
-func (p *ProviderPool) release(modelString string) {
-	p.mu.Lock()
-	defer p.mu.Unlock()
-
-	if pp, ok := p.providers[modelString]; ok {
-		pp.refs--
-		pp.lastUsed = time.Now()
-	}
-}
-
-func (p *ProviderPool) cleanupLoop() {
-	ticker := time.NewTicker(p.ttl / 2)
-	defer ticker.Stop()
-
-	for {
-		select {
-		case <-p.closeCh:
-			return
-		case <-ticker.C:
-			p.cleanup()
-		}
-	}
-}
-
-func (p *ProviderPool) cleanup() {
-	p.mu.Lock()
-	defer p.mu.Unlock()
-
-	now := time.Now()
-	for key, pp := range p.providers {
-		// Only clean up providers with no active references and past TTL.
-		if pp.refs <= 0 && now.Sub(pp.lastUsed) > p.ttl {
-			if pp.closer != nil {
-				_ = pp.closer()
-			}
-			delete(p.providers, key)
-		}
-	}
-}
-
-// Close shuts down the pool and releases all providers.
-func (p *ProviderPool) Close() {
-	p.mu.Lock()
-	if p.closed {
-		p.mu.Unlock()
-		return
-	}
-	p.closed = true
-	close(p.closeCh)
-
-	for key, pp := range p.providers {
-		if pp.closer != nil {
-			_ = pp.closer()
-		}
-		delete(p.providers, key)
-	}
-	p.mu.Unlock()
-}

internal/models/registry.go

@@ -4,6 +4,7 @@ import (
 	_ "embed"
 	"encoding/json"
 	"fmt"
+	"maps"
 	"os"
 	"strings"
 
@@ -26,6 +27,11 @@ type ModelInfo struct {
 	ProviderNPM string // Model-specific provider npm override (e.g. "@ai-sdk/anthropic")
 	BaseURL     string // Per-model base URL override (custom models only)
 	APIKey      string // Per-model API key override (custom models only)
+
+	// Params holds per-model generation parameter defaults. These are applied
+	// when the user hasn't explicitly set the corresponding CLI flag or global
+	// config value. Nil pointer fields mean "no model-level default".
+	Params *GenerationParams
 }
 
 // SupportsCaching returns true if this model family supports prompt caching.
@@ -106,13 +112,30 @@ func NewModelsRegistry() *ModelsRegistry {
 }
 
 // buildFromModelsDB converts models.dev provider data into our internal format.
-// It tries the on-disk cache first and falls back to the embedded database.
+// It starts from the compile-time embedded database and merges on-disk cached
+// data from `kit update-models` on top. Cached provider metadata replaces
+// embedded metadata, and model entries are merged with cached models taking
+// precedence. This means newly synced models are available while embedded
+// models that haven't been synced yet are still reachable.
 func buildFromModelsDB() map[string]ProviderInfo {
-	// Try cached data first (from `kit update-models`)
-	dbProviders, _ := LoadCachedProviders()
-	if len(dbProviders) == 0 {
-		// Fall back to compile-time embedded data
-		dbProviders = loadEmbeddedProviders()
+	// Start with compile-time embedded data as the base.
+	dbProviders := loadEmbeddedProviders()
+	if dbProviders == nil {
+		dbProviders = make(ModelsDBProviders)
+	}
+
+	// Merge on-disk cached data on top (cached takes precedence).
+	if cached, _ := LoadCachedProviders(); len(cached) > 0 {
+		for providerID, cp := range cached {
+			if existing, ok := dbProviders[providerID]; ok {
+				// Merge models: embedded base + cached overrides.
+				mergedModels := make(map[string]modelsDBModel, len(existing.Models)+len(cp.Models))
+				maps.Copy(mergedModels, existing.Models)
+				maps.Copy(mergedModels, cp.Models)
+				cp.Models = mergedModels
+			}
+			dbProviders[providerID] = cp
+		}
 	}
 
 	providers := make(map[string]ProviderInfo, len(dbProviders))
@@ -223,6 +246,7 @@ func loadEmbeddedProviders() map[string]modelsDBProvider {
 // doesn't track yet. Callers should treat a nil return as "unknown model"
 // and continue with sensible defaults.
 func (r *ModelsRegistry) LookupModel(provider, modelID string) *ModelInfo {
+	provider = catalogProviderID(provider)
 	providerInfo, exists := r.providers[provider]
 	if !exists {
 		return nil
@@ -236,8 +260,21 @@ func (r *ModelsRegistry) LookupModel(provider, modelID string) *ModelInfo {
 	return &modelInfo
 }
 
+// LookupModelForSettings is a convenience function that parses a
+// "provider/model" string and looks up the ModelInfo in the global registry.
+// Returns nil when the model string is invalid or the model is unknown.
+// Used by Kit.SetModel to pre-apply per-model settings before CreateProvider.
+func LookupModelForSettings(modelString string) *ModelInfo {
+	provider, modelName, err := ParseModelString(modelString)
+	if err != nil {
+		return nil
+	}
+	return GetGlobalRegistry().LookupModel(provider, modelName)
+}
+
 // getRequiredEnvVars returns the required environment variables for a provider.
 func (r *ModelsRegistry) getRequiredEnvVars(provider string) ([]string, error) {
+	provider = catalogProviderID(provider)
 	providerInfo, exists := r.providers[provider]
 	if !exists {
 		return nil, fmt.Errorf("unsupported provider: %s", provider)
@@ -252,6 +289,7 @@ func (r *ModelsRegistry) getRequiredEnvVars(provider string) ([]string, error) {
 // variables. Returns nil for providers not in the registry (unknown
 // providers are assumed to handle auth themselves or via --provider-api-key).
 func (r *ModelsRegistry) ValidateEnvironment(provider string, apiKey string) error {
+	provider = catalogProviderID(provider)
 	if apiKey != "" {
 		return nil
 	}
@@ -276,6 +314,15 @@ func (r *ModelsRegistry) ValidateEnvironment(provider string, apiKey string) err
 		}
 	}
 
+	// For GitHub Copilot, check stored GitHub OAuth credentials.
+	if provider == copilotProviderID {
+		if cm, err := auth.NewCredentialManager(); err == nil {
+			if has, _ := cm.HasCopilotCredentials(); has {
+				return nil
+			}
+		}
+	}
+
 	envVars, err := r.getRequiredEnvVars(provider)
 	if err != nil {
 		// Unknown provider — nothing to validate
@@ -315,6 +362,7 @@ func (r *ModelsRegistry) ValidateEnvironment(provider string, apiKey string) err
 
 // SuggestModels returns similar model names when an invalid model is provided.
 func (r *ModelsRegistry) SuggestModels(provider, invalidModel string) []string {
+	provider = catalogProviderID(provider)
 	providerInfo, exists := r.providers[provider]
 	if !exists {
 		return nil
@@ -362,11 +410,6 @@ func (r *ModelsRegistry) GetLLMProviders() []string {
 	return providers
 }
 
-// Deprecated: Use GetLLMProviders instead.
-func (r *ModelsRegistry) GetFantasyProviders() []string {
-	return r.GetLLMProviders()
-}
-
 // isProviderLLMSupported checks if a provider can be used with the LLM layer.
 func isProviderLLMSupported(providerID string, info *ProviderInfo) bool {
 	// Ollama and custom are always supported (model names are user-defined).
@@ -374,8 +417,8 @@ func isProviderLLMSupported(providerID string, info *ProviderInfo) bool {
 		return true
 	}
 
-	// Check if npm maps to an LLM provider
-	if _, ok := npmToLLMProvider[info.NPM]; ok {
+	// Check if npm maps to a known wire protocol
+	if _, ok := npmToWireProtocol[info.NPM]; ok {
 		return true
 	}
 
@@ -385,6 +428,7 @@ func isProviderLLMSupported(providerID string, info *ProviderInfo) bool {
 
 // GetModelsForProvider returns all models for a specific provider.
 func (r *ModelsRegistry) GetModelsForProvider(provider string) (map[string]ModelInfo, error) {
+	provider = catalogProviderID(provider)
 	providerInfo, exists := r.providers[provider]
 	if !exists {
 		return nil, fmt.Errorf("unsupported provider: %s", provider)
@@ -395,6 +439,7 @@ func (r *ModelsRegistry) GetModelsForProvider(provider string) (map[string]Model
 
 // GetProviderInfo returns the full provider info, or nil if not found.
 func (r *ModelsRegistry) GetProviderInfo(provider string) *ProviderInfo {
+	provider = catalogProviderID(provider)
 	info, exists := r.providers[provider]
 	if !exists {
 		return nil

internal/models/rightsize_test.go

@@ -0,0 +1,148 @@
+package models
+
+import (
+	"testing"
+
+	"github.com/spf13/pflag"
+	"github.com/spf13/viper"
+)
+
+// bindMaxTokensFlag wires a fresh pflag-backed "max-tokens" key into viper so
+// isExplicitlySet behaves the same way it does in production. Returns a
+// cleanup function that removes the binding so sibling tests see a clean
+// state.
+func bindMaxTokensFlag(t *testing.T, args []string) func() {
+	t.Helper()
+	fs := pflag.NewFlagSet("test", pflag.ContinueOnError)
+	fs.Int("max-tokens", 8192, "")
+	if err := viper.BindPFlag("max-tokens", fs.Lookup("max-tokens")); err != nil {
+		t.Fatalf("BindPFlag: %v", err)
+	}
+	if err := fs.Parse(args); err != nil {
+		t.Fatalf("fs.Parse: %v", err)
+	}
+	return func() {
+		viper.Reset()
+	}
+}
+
+func TestRightSizeMaxTokens_RaisesWhenBelowCeiling(t *testing.T) {
+	cleanup := bindMaxTokensFlag(t, nil) // no args → flag.Changed = false
+	defer cleanup()
+
+	config := &ProviderConfig{MaxTokens: 8192}
+	modelInfo := &ModelInfo{
+		ID:    "claude-sonnet-4-5",
+		Limit: Limit{Context: 200000, Output: 64000},
+	}
+
+	rightSizeMaxTokens(config, modelInfo)
+
+	if config.MaxTokens != 32768 {
+		t.Errorf("expected MaxTokens raised to defaultRightSizeCap (32768), got %d", config.MaxTokens)
+	}
+}
+
+func TestRightSizeMaxTokens_CapsAtDefaultRightSizeCap(t *testing.T) {
+	cleanup := bindMaxTokensFlag(t, nil)
+	defer cleanup()
+
+	config := &ProviderConfig{MaxTokens: 8192}
+	// Mistral Devstral has 262144 output — we should still cap at 32768.
+	modelInfo := &ModelInfo{
+		ID:    "devstral-medium-latest",
+		Limit: Limit{Context: 262144, Output: 262144},
+	}
+
+	rightSizeMaxTokens(config, modelInfo)
+
+	if config.MaxTokens != defaultRightSizeCap {
+		t.Errorf("expected MaxTokens capped at %d, got %d", defaultRightSizeCap, config.MaxTokens)
+	}
+}
+
+func TestRightSizeMaxTokens_UsesExactOutputWhenBelowCap(t *testing.T) {
+	cleanup := bindMaxTokensFlag(t, nil)
+	defer cleanup()
+
+	config := &ProviderConfig{MaxTokens: 4096}
+	// Model with output limit smaller than the cap.
+	modelInfo := &ModelInfo{
+		ID:    "gpt-4",
+		Limit: Limit{Context: 8192, Output: 8192},
+	}
+
+	rightSizeMaxTokens(config, modelInfo)
+
+	if config.MaxTokens != 8192 {
+		t.Errorf("expected MaxTokens raised to model output ceiling (8192), got %d", config.MaxTokens)
+	}
+}
+
+func TestRightSizeMaxTokens_DoesNotLowerCurrentValue(t *testing.T) {
+	cleanup := bindMaxTokensFlag(t, nil)
+	defer cleanup()
+
+	// User (via per-model settings, applied earlier) already bumped MaxTokens
+	// above the cap — we must not clobber their choice.
+	config := &ProviderConfig{MaxTokens: 100000}
+	modelInfo := &ModelInfo{
+		ID:    "devstral-medium-latest",
+		Limit: Limit{Context: 262144, Output: 262144},
+	}
+
+	rightSizeMaxTokens(config, modelInfo)
+
+	if config.MaxTokens != 100000 {
+		t.Errorf("expected MaxTokens preserved at 100000, got %d", config.MaxTokens)
+	}
+}
+
+func TestRightSizeMaxTokens_RespectsExplicitFlag(t *testing.T) {
+	// Simulate `--max-tokens 4096` on the command line.
+	cleanup := bindMaxTokensFlag(t, []string{"--max-tokens", "4096"})
+	defer cleanup()
+
+	config := &ProviderConfig{MaxTokens: 4096}
+	modelInfo := &ModelInfo{
+		ID:    "claude-sonnet-4-5",
+		Limit: Limit{Context: 200000, Output: 64000},
+	}
+
+	rightSizeMaxTokens(config, modelInfo)
+
+	if config.MaxTokens != 4096 {
+		t.Errorf("expected explicit --max-tokens to be preserved (4096), got %d", config.MaxTokens)
+	}
+}
+
+func TestRightSizeMaxTokens_NilModelInfo(t *testing.T) {
+	cleanup := bindMaxTokensFlag(t, nil)
+	defer cleanup()
+
+	config := &ProviderConfig{MaxTokens: 8192}
+	// Custom model / Ollama / unknown provider → no model info.
+	rightSizeMaxTokens(config, nil)
+
+	if config.MaxTokens != 8192 {
+		t.Errorf("expected MaxTokens unchanged with nil modelInfo, got %d", config.MaxTokens)
+	}
+}
+
+func TestRightSizeMaxTokens_ZeroOutputLimit(t *testing.T) {
+	cleanup := bindMaxTokensFlag(t, nil)
+	defer cleanup()
+
+	config := &ProviderConfig{MaxTokens: 8192}
+	// Model present in catalog but with no known output limit.
+	modelInfo := &ModelInfo{
+		ID:    "unknown-model",
+		Limit: Limit{Context: 0, Output: 0},
+	}
+
+	rightSizeMaxTokens(config, modelInfo)
+
+	if config.MaxTokens != 8192 {
+		t.Errorf("expected MaxTokens unchanged with zero output limit, got %d", config.MaxTokens)
+	}
+}

internal/models/sdk_defaults.go

@@ -0,0 +1,170 @@
+package models
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"regexp"
+	"strings"
+
+	"charm.land/fantasy/providers/google"
+)
+
+// templatePlaceholderRe matches "${NAME}" placeholders in URL templates from
+// models.dev (e.g. "https://${DATABRICKS_HOST}/ai-gateway/mlflow/v1").
+var templatePlaceholderRe = regexp.MustCompile(`\$\{([A-Z0-9_]+)\}`)
+
+// templateEnvVarOverrides supplies fallback environment variable names for
+// placeholders that providers commonly use under non-obvious env names.
+// The placeholder name itself is always tried first; this map adds extra
+// names to try when the placeholder doesn't match the canonical env var.
+var templateEnvVarOverrides = map[string][]string{
+	"CLOUDFLARE_ACCOUNT_ID":   {"CF_ACCOUNT_ID"},
+	"CLOUDFLARE_GATEWAY_NAME": {"CF_GATEWAY", "CLOUDFLARE_GATEWAY"},
+	"DATABRICKS_HOST":         {"DATABRICKS_WORKSPACE_URL"},
+	"SNOWFLAKE_ACCOUNT":       {"SNOWFLAKE_ACCOUNT_ID"},
+}
+
+// resolveTemplatedAPIURL substitutes "${VAR}" placeholders in apiURL with the
+// values of the named environment variables. Returns:
+//   - ("", nil) when apiURL contains no placeholders (caller keeps current URL),
+//   - (resolved, nil) when every placeholder was resolved,
+//   - ("", error) when one or more placeholders are unset, with a message that
+//     names the missing env vars and points at the relevant provider.
+//
+// The info parameter is used purely for error messaging (provider name).
+func resolveTemplatedAPIURL(apiURL string, info *ProviderInfo) (string, error) {
+	if apiURL == "" || !strings.Contains(apiURL, "${") {
+		return "", nil
+	}
+
+	var missing []string
+	resolved := templatePlaceholderRe.ReplaceAllStringFunc(apiURL, func(match string) string {
+		// match is "${NAME}". Extract NAME.
+		name := match[2 : len(match)-1]
+		if v := os.Getenv(name); v != "" {
+			return v
+		}
+		for _, alt := range templateEnvVarOverrides[name] {
+			if v := os.Getenv(alt); v != "" {
+				return v
+			}
+		}
+		missing = append(missing, name)
+		return match
+	})
+
+	if len(missing) > 0 {
+		providerName := info.ID
+		if info.Name != "" {
+			providerName = info.Name
+		}
+		return "", fmt.Errorf(
+			"provider %s requires environment variable(s) %s to construct its API URL (%s); "+
+				"set them or pass --provider-url to override",
+			providerName, strings.Join(missing, ", "), apiURL,
+		)
+	}
+	return resolved, nil
+}
+
+// ResolveProviderBaseURL returns the base API URL kit will use when talking to
+// the given provider, applying the same resolution order as CreateProvider:
+//
+//  1. The provider's `api` field from the models.dev registry.
+//  2. The hard-coded default base URL of its npm SDK package (e.g.
+//     @ai-sdk/groq → https://api.groq.com/openai/v1).
+//  3. Template substitution against the current process environment when the
+//     URL contains "${VAR}" placeholders (e.g. cloudflare-workers-ai needs
+//     CLOUDFLARE_ACCOUNT_ID).
+//
+// It returns an error when the provider is unknown, when no URL can be derived,
+// or when a templated URL has unset placeholders. The error message is suitable
+// for direct display to end users.
+//
+// Note: providers handled by bespoke auth schemes (amazon-bedrock SigV4,
+// azure resource URLs, google-vertex project/location, sap-ai-core customer
+// deployments) may return either an empty URL or a regional/templated URL —
+// the actual endpoint is finalised inside their native handlers and depends on
+// runtime credentials.
+func ResolveProviderBaseURL(providerID string) (string, error) {
+	registry := GetGlobalRegistry()
+	info := registry.GetProviderInfo(providerID)
+	if info == nil {
+		return "", fmt.Errorf("unknown provider: %s", providerID)
+	}
+
+	apiURL := info.API
+	if apiURL == "" {
+		if defaultURL, ok := sdkDefaultBaseURL[info.NPM]; ok {
+			apiURL = defaultURL
+		}
+	}
+
+	if apiURL == "" {
+		return "", fmt.Errorf(
+			"provider %s has no default API URL: its npm package %q does not "+
+				"ship a built-in baseURL (likely Bedrock SigV4, Azure deployment, "+
+				"Vertex project/location, or a customer-hosted endpoint). "+
+				"Pass --provider-url or set the provider's URL env var",
+			providerID, info.NPM,
+		)
+	}
+
+	if strings.Contains(apiURL, "${") {
+		resolved, err := resolveTemplatedAPIURL(apiURL, info)
+		if err != nil {
+			return apiURL, err
+		}
+		return resolved, nil
+	}
+	return apiURL, nil
+}
+
+// createGoogleVertexProvider creates a Google Gemini provider that targets the
+// Vertex AI backend (rather than the public generativelanguage.googleapis.com
+// endpoint). It requires the same project/region environment variables as
+// google-vertex-anthropic.
+func createGoogleVertexProvider(ctx context.Context, config *ProviderConfig, modelName string) (*ProviderResult, error) {
+	projectID := firstNonEmpty(
+		os.Getenv("GOOGLE_VERTEX_PROJECT"),
+		os.Getenv("GOOGLE_CLOUD_PROJECT"),
+		os.Getenv("GCLOUD_PROJECT"),
+		os.Getenv("CLOUDSDK_CORE_PROJECT"),
+	)
+	if projectID == "" {
+		return nil, fmt.Errorf(
+			"google Vertex project ID not provided, set GOOGLE_VERTEX_PROJECT, " +
+				"GOOGLE_CLOUD_PROJECT, or GCLOUD_PROJECT environment variable",
+		)
+	}
+
+	region := firstNonEmpty(
+		os.Getenv("GOOGLE_VERTEX_LOCATION"),
+		os.Getenv("CLOUD_ML_REGION"),
+	)
+	if region == "" {
+		region = "global"
+	}
+
+	opts := []google.Option{
+		google.WithVertex(projectID, region),
+		google.WithName("google-vertex"),
+	}
+
+	if config.TLSSkipVerify {
+		opts = append(opts, google.WithHTTPClient(createHTTPClientWithTLSConfig(true)))
+	}
+
+	provider, err := google.New(opts...)
+	if err != nil {
+		return nil, wrapProviderErr("Google Vertex", "provider", err)
+	}
+
+	model, err := provider.LanguageModel(ctx, modelName)
+	if err != nil {
+		return nil, wrapProviderErr("Google Vertex", "model", err)
+	}
+
+	return &ProviderResult{Model: model}, nil
+}

internal/models/sdk_defaults_test.go

@@ -0,0 +1,214 @@
+package models
+
+import (
+	"context"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+// TestSDKDefaultBaseURL_CoversAllWireMappedPackages enforces the invariant
+// that every npm package recognised by the auto-router has a corresponding
+// default base URL — otherwise a provider that omits its `api` field in the
+// registry would silently fail to route at runtime.
+func TestSDKDefaultBaseURL_CoversAllWireMappedPackages(t *testing.T) {
+	for npm := range npmToWireProtocol {
+		// @ai-sdk/openai-compatible is a wire family, not a single SDK with
+		// a default URL — providers using it always supply their own `api`.
+		if npm == "@ai-sdk/openai-compatible" {
+			continue
+		}
+		if _, ok := sdkDefaultBaseURL[npm]; !ok {
+			t.Errorf("npm %q is in npmToWireProtocol but has no sdkDefaultBaseURL entry — "+
+				"providers using this npm with no `api` field cannot be routed", npm)
+		}
+	}
+}
+
+// TestSDKDefaultBaseURL_AllURLsAreAbsolute sanity-checks that every default
+// URL is a well-formed absolute https endpoint (catches typos in the table).
+func TestSDKDefaultBaseURL_AllURLsAreAbsolute(t *testing.T) {
+	for npm, url := range sdkDefaultBaseURL {
+		if !strings.HasPrefix(url, "https://") {
+			t.Errorf("sdkDefaultBaseURL[%q] = %q is not an absolute https URL", npm, url)
+		}
+	}
+}
+
+// TestResolveProviderBaseURL_RegistryFirst verifies that the registry's `api`
+// field wins over any SDK default.
+func TestResolveProviderBaseURL_RegistryFirst(t *testing.T) {
+	// xai is in the registry with no `api` field — its URL comes from the
+	// SDK default. Use a synthetic registry-backed provider to test the
+	// priority via the public registry instead.
+	url, err := ResolveProviderBaseURL("openai")
+	if err != nil {
+		t.Fatalf("ResolveProviderBaseURL(openai): %v", err)
+	}
+	if url != "https://api.openai.com/v1" {
+		t.Errorf("openai URL = %q, want https://api.openai.com/v1", url)
+	}
+}
+
+// TestResolveProviderBaseURL_SDKDefaultFallback verifies that providers
+// without an `api` field (groq, cerebras, xai, …) resolve to their SDK
+// hard-coded default URL.
+func TestResolveProviderBaseURL_SDKDefaultFallback(t *testing.T) {
+	tests := map[string]string{
+		"groq":       "https://api.groq.com/openai/v1",
+		"cerebras":   "https://api.cerebras.ai/v1",
+		"xai":        "https://api.x.ai/v1",
+		"mistral":    "https://api.mistral.ai/v1",
+		"perplexity": "https://api.perplexity.ai",
+		"togetherai": "https://api.together.xyz/v1",
+		"deepinfra":  "https://api.deepinfra.com/v1/openai",
+		"cohere":     "https://api.cohere.com/compatibility/v1",
+		"v0":         "https://api.v0.dev/v1",
+		"aihubmix":   "https://aihubmix.com/v1",
+		"venice":     "https://api.venice.ai/api/v1",
+		"openrouter": "https://openrouter.ai/api/v1",
+	}
+	for providerID, wantURL := range tests {
+		t.Run(providerID, func(t *testing.T) {
+			got, err := ResolveProviderBaseURL(providerID)
+			if err != nil {
+				t.Fatalf("ResolveProviderBaseURL(%s): %v", providerID, err)
+			}
+			if got != wantURL {
+				t.Errorf("%s URL = %q, want %q", providerID, got, wantURL)
+			}
+		})
+	}
+}
+
+// TestResolveProviderBaseURL_TemplatedURL_MissingEnv verifies that providers
+// whose URL contains "${VAR}" placeholders surface a targeted error when the
+// environment variables are unset.
+func TestResolveProviderBaseURL_TemplatedURL_MissingEnv(t *testing.T) {
+	// cloudflare-workers-ai's api URL contains ${CLOUDFLARE_ACCOUNT_ID}.
+	// Ensure the variable is unset for this test.
+	t.Setenv("CLOUDFLARE_ACCOUNT_ID", "")
+	t.Setenv("CF_ACCOUNT_ID", "")
+
+	_, err := ResolveProviderBaseURL("cloudflare-workers-ai")
+	if err == nil {
+		t.Fatal("expected error for unset CLOUDFLARE_ACCOUNT_ID, got nil")
+	}
+	if !strings.Contains(err.Error(), "CLOUDFLARE_ACCOUNT_ID") {
+		t.Errorf("error should name the missing env var, got: %v", err)
+	}
+	if !strings.Contains(err.Error(), "--provider-url") {
+		t.Errorf("error should suggest --provider-url override, got: %v", err)
+	}
+}
+
+// TestResolveProviderBaseURL_TemplatedURL_Resolved verifies env-var
+// substitution succeeds when the placeholder is set.
+func TestResolveProviderBaseURL_TemplatedURL_Resolved(t *testing.T) {
+	t.Setenv("CLOUDFLARE_ACCOUNT_ID", "test-acct-123")
+	got, err := ResolveProviderBaseURL("cloudflare-workers-ai")
+	if err != nil {
+		t.Fatalf("ResolveProviderBaseURL: %v", err)
+	}
+	if !strings.Contains(got, "test-acct-123") {
+		t.Errorf("resolved URL %q should contain test-acct-123", got)
+	}
+	if strings.Contains(got, "${") {
+		t.Errorf("resolved URL %q still contains template placeholder", got)
+	}
+}
+
+// TestResolveProviderBaseURL_UnknownProvider verifies the not-in-registry error.
+func TestResolveProviderBaseURL_UnknownProvider(t *testing.T) {
+	_, err := ResolveProviderBaseURL("does-not-exist")
+	if err == nil {
+		t.Fatal("expected error for unknown provider, got nil")
+	}
+	if !strings.Contains(err.Error(), "unknown provider") {
+		t.Errorf("error should say 'unknown provider', got: %v", err)
+	}
+}
+
+// TestAutoRouteProvider_SDKDefaultURLFallback verifies that providers whose
+// registry entry omits the `api` field (groq, mistral, xai, etc.) are still
+// auto-routed by falling back to the SDK's hard-coded default URL.
+func TestAutoRouteProvider_SDKDefaultURLFallback(t *testing.T) {
+	tests := []struct {
+		name       string
+		npmPackage string
+		wantInURL  string
+	}{
+		{"groq", "@ai-sdk/groq", "groq.com"},
+		{"cerebras", "@ai-sdk/cerebras", "cerebras.ai"},
+		{"xai", "@ai-sdk/xai", "x.ai"},
+		{"mistral", "@ai-sdk/mistral", "mistral.ai"},
+		{"v0", "@ai-sdk/vercel", "v0.dev"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			reg := &ModelsRegistry{
+				providers: map[string]ProviderInfo{
+					"testfallback": {
+						ID:   "testfallback",
+						Name: "Test Fallback",
+						Env:  []string{"TESTFALLBACK_API_KEY"},
+						NPM:  tt.npmPackage,
+						// API intentionally omitted — must fall back to SDK default.
+						Models: map[string]ModelInfo{
+							"any-model": {ID: "any-model", Name: "any-model"},
+						},
+					},
+				},
+			}
+			config := &ProviderConfig{ProviderAPIKey: "test-key"}
+
+			result, err := autoRouteProvider(context.Background(), config, "testfallback", "any-model", reg)
+			if err != nil {
+				t.Fatalf("autoRouteProvider returned error: %v", err)
+			}
+			if result == nil || result.Model == nil {
+				t.Fatal("autoRouteProvider returned nil model")
+			}
+			// Verify the SDK default URL was picked up.
+			if !strings.Contains(config.ProviderURL, tt.wantInURL) {
+				t.Errorf("config.ProviderURL = %q, want substring %q (SDK default)",
+					config.ProviderURL, tt.wantInURL)
+			}
+			// All these wrappers route through the openai-compat wire.
+			gotType := reflect.TypeOf(result.Model).String()
+			if gotType != "openai.languageModel" {
+				t.Errorf("model type = %q, want openai.languageModel", gotType)
+			}
+		})
+	}
+}
+
+// TestResolveTemplatedAPIURL_NoPlaceholders verifies that URLs without
+// placeholders are returned as-is (the caller keeps using the original).
+func TestResolveTemplatedAPIURL_NoPlaceholders(t *testing.T) {
+	got, err := resolveTemplatedAPIURL("https://api.example.com/v1", &ProviderInfo{ID: "x"})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if got != "" {
+		t.Errorf("got %q, want empty string for URL with no placeholders", got)
+	}
+}
+
+// TestResolveTemplatedAPIURL_AltEnvVar verifies that the alternative env-var
+// names (e.g. CF_ACCOUNT_ID for CLOUDFLARE_ACCOUNT_ID) are honoured.
+func TestResolveTemplatedAPIURL_AltEnvVar(t *testing.T) {
+	t.Setenv("CLOUDFLARE_ACCOUNT_ID", "")
+	t.Setenv("CF_ACCOUNT_ID", "alt-name-123")
+
+	got, err := resolveTemplatedAPIURL(
+		"https://api.cloudflare.com/client/v4/accounts/${CLOUDFLARE_ACCOUNT_ID}/ai/v1",
+		&ProviderInfo{ID: "cloudflare-workers-ai"},
+	)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if !strings.Contains(got, "alt-name-123") {
+		t.Errorf("resolved URL %q should have picked up CF_ACCOUNT_ID alternative", got)
+	}
+}

internal/prompts/loader.go

@@ -2,11 +2,10 @@ package prompts
 
 import (
 	"fmt"
+	"log"
 	"os"
 	"path/filepath"
 	"strings"
-
-	"github.com/charmbracelet/log"
 )
 
 // LoadOptions configures how templates are discovered and loaded.
@@ -37,15 +36,17 @@ type Diagnostic struct {
 }
 
 // LoadAll discovers and loads all prompt templates from standard locations
-// and any extra paths. Templates are loaded in order of precedence (lowest
-// to highest), with later templates overriding earlier ones of the same name.
+// and any extra paths. Templates are loaded in order of precedence (highest
+// to lowest); the first source to define a given name wins, later definitions
+// of the same name are dropped with a diagnostic.
 //
 // Discovery paths searched in order:
 //  1. Default templates (if IncludeDefaults)
-//  2. ~/.kit/prompts/ (global user templates)
-//  3. .kit/prompts/ (project-local templates)
-//  4. ConfigPaths (from configuration)
-//  5. ExtraPaths (explicit paths, highest precedence)
+//  2. ~/.kit/prompts/                          (legacy global)
+//  3. $XDG_CONFIG_HOME/kit/prompts/            (XDG global, default ~/.config/kit/prompts/)
+//  4. <cwd>/.kit/prompts/                      (project-local templates)
+//  5. ConfigPaths                              (from configuration)
+//  6. ExtraPaths                               (explicit paths, lowest precedence)
 func LoadAll(opts LoadOptions) ([]*PromptTemplate, []Diagnostic, error) {
 	if opts.Cwd == "" {
 		opts.Cwd, _ = os.Getwd()
@@ -74,10 +75,7 @@ func LoadAll(opts LoadOptions) ([]*PromptTemplate, []Diagnostic, error) {
 					DroppedPath: tpl.FilePath,
 					Reason:      fmt.Sprintf("template from %s overridden by %s", source, existing.Source),
 				})
-				log.Debug("template collision",
-					"name", tpl.Name,
-					"dropped", tpl.FilePath,
-					"kept", existing.FilePath)
+				log.Printf("DEBUG template collision: name=%s dropped=%s kept=%s", tpl.Name, tpl.FilePath, existing.FilePath)
 			} else {
 				tpl.Source = source
 				seen[tpl.Name] = tpl
@@ -92,13 +90,21 @@ func LoadAll(opts LoadOptions) ([]*PromptTemplate, []Diagnostic, error) {
 		addTemplates(defaults, "default")
 	}
 
-	// 2. Global user templates: ~/.kit/prompts/
-	globalDir := filepath.Join(opts.HomeDir, ".kit", "prompts")
-	if templates, err := LoadFromDir(globalDir); err == nil {
+	// 2. Legacy global user templates: ~/.kit/prompts/
+	legacyGlobalDir := filepath.Join(opts.HomeDir, ".kit", "prompts")
+	if templates, err := LoadFromDir(legacyGlobalDir); err == nil {
 		addTemplates(templates, "global")
 	}
 
-	// 3. Project-local templates: .kit/prompts/
+	// 3. XDG global user templates: $XDG_CONFIG_HOME/kit/prompts/
+	//    Default: ~/.config/kit/prompts/. Aligns with extensions and skills.
+	if xdgDir := GlobalDir(); xdgDir != "" && xdgDir != legacyGlobalDir {
+		if templates, err := LoadFromDir(xdgDir); err == nil {
+			addTemplates(templates, "global")
+		}
+	}
+
+	// 4. Project-local templates: .kit/prompts/
 	localDir := filepath.Join(opts.Cwd, ".kit", "prompts")
 	if templates, err := LoadFromDir(localDir); err == nil {
 		addTemplates(templates, "local")
@@ -183,31 +189,6 @@ func LoadFromDir(dir string) ([]*PromptTemplate, error) {
 	return templates, nil
 }
 
-// Deduplicate removes duplicate templates by name, keeping the first occurrence.
-// It returns the deduplicated list and diagnostics for any collisions.
-// This is a standalone function for when you need to deduplicate an existing list.
-func Deduplicate(templates []*PromptTemplate) ([]*PromptTemplate, []Diagnostic) {
-	seen := make(map[string]*PromptTemplate)
-	var result []*PromptTemplate
-	var diagnostics []Diagnostic
-
-	for _, tpl := range templates {
-		if existing, ok := seen[tpl.Name]; ok {
-			diagnostics = append(diagnostics, Diagnostic{
-				Name:        tpl.Name,
-				KeptPath:    existing.FilePath,
-				DroppedPath: tpl.FilePath,
-				Reason:      "duplicate template name (first-match-wins)",
-			})
-		} else {
-			seen[tpl.Name] = tpl
-			result = append(result, tpl)
-		}
-	}
-
-	return result, diagnostics
-}
-
 // loadDefaultTemplates returns the built-in default templates.
 // These are embedded templates that ship with Kit.
 func loadDefaultTemplates() []*PromptTemplate {
@@ -215,3 +196,22 @@ func loadDefaultTemplates() []*PromptTemplate {
 	// For now, return an empty slice - users can define their own templates
 	return nil
 }
+
+// GlobalDir returns the XDG-aligned global prompts directory, respecting
+// $XDG_CONFIG_HOME. Defaults to ~/.config/kit/prompts/. Returns an empty
+// string if the user's home directory cannot be resolved.
+//
+// This is the canonical location for user-wide prompt templates and aligns
+// with the discovery paths used for extensions ($XDG_CONFIG_HOME/kit/extensions/)
+// and skills ($XDG_CONFIG_HOME/kit/skills/).
+func GlobalDir() string {
+	base := os.Getenv("XDG_CONFIG_HOME")
+	if base == "" {
+		home, err := os.UserHomeDir()
+		if err != nil {
+			return ""
+		}
+		base = filepath.Join(home, ".config")
+	}
+	return filepath.Join(base, "kit", "prompts")
+}

internal/prompts/template.go

@@ -7,10 +7,12 @@ import (
 	"regexp"
 	"strconv"
 	"strings"
+
+	"github.com/mark3labs/kit/internal/fences"
 )
 
 // PromptTemplate is a named prompt template with shell-style argument placeholders.
-// It supports Pi-style $1, $2, $@, $ARGUMENTS, ${@:N}, ${@:N:L} syntax.
+// It supports Pi-style $1, $2, $@, $+, $ARGUMENTS, ${@:N}, ${@:N:L} syntax.
 type PromptTemplate struct {
 	// Name is the human-readable identifier for this template.
 	Name string
@@ -68,7 +70,8 @@ func ParseTemplate(path string) (*PromptTemplate, error) {
 }
 
 // ParseCommandArgs splits a command line into arguments respecting quotes.
-// It handles single quotes, double quotes, and backslash escaping.
+// It handles single quotes, double quotes, backslash escaping, and splits on
+// spaces and tabs.
 func ParseCommandArgs(input string) []string {
 	var args []string
 	var current strings.Builder
@@ -76,7 +79,7 @@ func ParseCommandArgs(input string) []string {
 	inDoubleQuote := false
 	escaped := false
 
-	for i, r := range input {
+	for _, r := range input {
 		if escaped {
 			current.WriteRune(r)
 			escaped = false
@@ -99,7 +102,7 @@ func ParseCommandArgs(input string) []string {
 			continue
 		}
 
-		if r == ' ' && !inSingleQuote && !inDoubleQuote {
+		if (r == ' ' || r == '\t') && !inSingleQuote && !inDoubleQuote {
 			if current.Len() > 0 {
 				args = append(args, current.String())
 				current.Reset()
@@ -108,7 +111,6 @@ func ParseCommandArgs(input string) []string {
 		}
 
 		current.WriteRune(r)
-		_ = i // silence unused warning when we need position later
 	}
 
 	if current.Len() > 0 {
@@ -120,19 +122,28 @@ func ParseCommandArgs(input string) []string {
 
 // argPlaceholder matches shell-style argument placeholders:
 //   - $1, $2, etc. - positional arguments
-//   - $@ - all arguments
+//   - $@ - all arguments (zero or more)
+//   - $+ - all arguments (one or more required)
 //   - $ARGUMENTS - all arguments (alias for $@)
 //   - ${@:N} - arguments from N onwards
 //   - ${@:N:L} - L arguments starting from N
-var argPlaceholder = regexp.MustCompile(`\$\{(\d+)\}|\$\{(\d+):(\d+)\}|\$\{ARGUMENTS\}|\$\{@(:\d+)?(:\d+)?\}|\$(\d+)|\$@|\$ARGUMENTS`)
+var argPlaceholder = regexp.MustCompile(`\$\{(\d+)\}|\$\{(\d+):(\d+)\}|\$\{ARGUMENTS\}|\$\{@(:\d+)?(:\d+)?\}|\$(\d+)|\$@|\$\+|\$ARGUMENTS`)
 
 // SubstituteArgs replaces argument placeholders in content with values from args.
 // Supported placeholders:
 //   - $N, ${N} - the Nth argument (1-indexed)
-//   - $@, $ARGUMENTS, ${ARGUMENTS} - all arguments joined with spaces
+//   - $@, $+, $ARGUMENTS, ${ARGUMENTS} - all arguments joined with spaces
 //   - ${@:N} - arguments from index N onwards (0-indexed)
 //   - ${@:N:L} - L arguments starting from index N (0-indexed)
 func SubstituteArgs(content string, args []string) string {
+	return fences.ReplaceOutside(content, func(segment string) string {
+		return substituteArgsInSegment(segment, args)
+	})
+}
+
+// substituteArgsInSegment performs argument substitution on a single text
+// segment that is known to be outside fenced code blocks.
+func substituteArgsInSegment(content string, args []string) string {
 	return argPlaceholder.ReplaceAllStringFunc(content, func(match string) string {
 		// Check for ${N} or ${N:M} format
 		if strings.HasPrefix(match, "${") && strings.Contains(match, "}") {
@@ -191,8 +202,8 @@ func SubstituteArgs(content string, args []string) string {
 		if strings.HasPrefix(match, "$") && !strings.HasPrefix(match, "${") {
 			suffix := match[1:]
 
-			// $@ or $ARGUMENTS
-			if suffix == "@" || suffix == "ARGUMENTS" {
+			// $@, $+, or $ARGUMENTS
+			if suffix == "@" || suffix == "+" || suffix == "ARGUMENTS" {
 				return strings.Join(args, " ")
 			}
 
@@ -266,14 +277,51 @@ func joinArgsRange(args []string, start, length int) string {
 	return strings.Join(args[start:end], " ")
 }
 
+// HasArgPlaceholders reports whether the template content contains any
+// argument placeholders ($1, $@, $ARGUMENTS, ${@:...}, etc.).
+// Placeholders inside fenced code blocks and inline code spans are ignored.
+func (t *PromptTemplate) HasArgPlaceholders() bool {
+	return argPlaceholder.MatchString(fences.StripCode(t.Content))
+}
+
+// RequiredArgs returns the number of positional arguments the template
+// expects. This is determined by the highest $N or ${N} placeholder found
+// in the content (1-indexed, so $2 means 2 args required). The $+
+// placeholder (required variadic) ensures at least 1. Optional wildcards
+// ($@, $ARGUMENTS) do not contribute to the count.
+func (t *PromptTemplate) RequiredArgs() int {
+	content := fences.StripCode(t.Content)
+	maxN := 0
+	hasRequiredVariadic := strings.Contains(content, "$+")
+	for _, match := range argPlaceholder.FindAllStringSubmatch(content, -1) {
+		// Group 1: ${N} format — the N value.
+		if match[1] != "" {
+			if n, err := strconv.Atoi(match[1]); err == nil && n > maxN {
+				maxN = n
+			}
+		}
+		// Group 2: ${N:M} format — the N value (start index).
+		if match[2] != "" {
+			if n, err := strconv.Atoi(match[2]); err == nil && n > maxN {
+				maxN = n
+			}
+		}
+		// Group 6: $N format (no braces) — the N value.
+		if match[6] != "" {
+			if n, err := strconv.Atoi(match[6]); err == nil && n > maxN {
+				maxN = n
+			}
+		}
+	}
+	if hasRequiredVariadic && maxN < 1 {
+		maxN = 1
+	}
+	return maxN
+}
+
 // Expand substitutes arguments into the template content and returns the result.
 // It first parses args from the input string, then substitutes them into the template.
 func (t *PromptTemplate) Expand(argsInput string) string {
 	args := ParseCommandArgs(argsInput)
 	return SubstituteArgs(t.Content, args)
 }
-
-// ExpandWithArgs substitutes the provided arguments into the template content.
-func (t *PromptTemplate) ExpandWithArgs(args []string) string {
-	return SubstituteArgs(t.Content, args)
-}

internal/prompts/template_test.go

@@ -129,6 +129,48 @@ func TestSubstituteArgs(t *testing.T) {
 			args:     []string{},
 			expected: "Args: ",
 		},
+		{
+			name:     "$1 inside code block preserved",
+			content:  "Use $1 here\n```bash\necho $1\n```\ndone",
+			args:     []string{"foo"},
+			expected: "Use foo here\n```bash\necho $1\n```\ndone",
+		},
+		{
+			name:     "$@ inside code block preserved",
+			content:  "Run $@\n```\necho $@\n```\n",
+			args:     []string{"a", "b"},
+			expected: "Run a b\n```\necho $@\n```\n",
+		},
+		{
+			name:     "all placeholders inside code block",
+			content:  "Prompt\n```\n$1 $2 $@\n```\n",
+			args:     []string{"x"},
+			expected: "Prompt\n```\n$1 $2 $@\n```\n",
+		},
+		{
+			name:     "$1 inside inline code preserved",
+			content:  "Use `$1` here and $1 outside",
+			args:     []string{"foo"},
+			expected: "Use `$1` here and foo outside",
+		},
+		{
+			name:     "$+ required variadic",
+			content:  "Args: $+",
+			args:     []string{"a", "b", "c"},
+			expected: "Args: a b c",
+		},
+		{
+			name:     "$+ with empty args",
+			content:  "Args: $+",
+			args:     []string{},
+			expected: "Args: ",
+		},
+		{
+			name:     "all placeholders in inline code",
+			content:  "Use `$1` and `$@` for args",
+			args:     []string{"x"},
+			expected: "Use `$1` and `$@` for args",
+		},
 	}
 
 	for _, tt := range tests {
@@ -213,3 +255,78 @@ func TestPromptTemplateExpand(t *testing.T) {
 		})
 	}
 }
+
+func TestHasArgPlaceholders(t *testing.T) {
+	tests := []struct {
+		name    string
+		content string
+		want    bool
+	}{
+		{"no placeholders", "Just a plain prompt with no args", false},
+		{"$1 placeholder", "Create a $1 component", true},
+		{"$@ placeholder", "Run with args: $@", true},
+		{"$ARGUMENTS placeholder", "Features: $ARGUMENTS", true},
+		{"${1} placeholder", "Name: ${1}", true},
+		{"${ARGUMENTS} placeholder", "All: ${ARGUMENTS}", true},
+		{"${@:1} placeholder", "Rest: ${@:1}", true},
+		{"${@:1:2} placeholder", "Slice: ${@:1:2}", true},
+		{"dollar in text", "Cost is one hundred dollars", false},
+		{"empty content", "", false},
+		{"$1 inside code block only", "Prompt\n```\necho $1\n```\n", false},
+		{"$1 outside and inside code block", "Use $1 here\n```\necho $1\n```\n", true},
+		{"$@ inside code block only", "Prompt\n```bash\necho $@\n```\n", false},
+		{"$+ placeholder", "Run with args: $+", true},
+		{"$+ inside inline code only", "Use `$+` for required args", false},
+		{"$1 inside inline code only", "Use `$1` for positional args", false},
+		{"$1 outside and in inline code", "Create $1 (see `$1` syntax)", true},
+		{"$@ outside $1 in inline code", "Run $@ with `$1` syntax", true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			tpl := &PromptTemplate{Content: tt.content}
+			if got := tpl.HasArgPlaceholders(); got != tt.want {
+				t.Errorf("HasArgPlaceholders() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestRequiredArgs(t *testing.T) {
+	tests := []struct {
+		name    string
+		content string
+		want    int
+	}{
+		{"no placeholders", "Just a plain prompt", 0},
+		{"$1 only", "Create a $1 component", 1},
+		{"$1 and $2", "Create $1 with $2", 2},
+		{"$3 skipping $2", "Use $1 and $3", 3},
+		{"${1} braced", "Name: ${1}", 1},
+		{"${2} braced", "Name: ${1} Desc: ${2}", 2},
+		{"$@ only", "Run with: $@", 0},
+		{"$ARGUMENTS only", "Features: $ARGUMENTS", 0},
+		{"${ARGUMENTS} only", "All: ${ARGUMENTS}", 0},
+		{"$1 and $@", "Create $1 with extras: $@", 1},
+		{"${@:1} slice only", "Rest: ${@:1}", 0},
+		{"${@:1:2} slice only", "Slice: ${@:1:2}", 0},
+		{"mixed $1 $2 and $@", "Create $1 named $2: $@", 2},
+		{"empty content", "", 0},
+		{"$2 inside code block only", "Prompt\n```\n$1 $2\n```\n", 0},
+		{"$1 outside $2 inside code block", "Use $1\n```\n$2 inside\n```\n", 1},
+		{"$+ only", "Run with: $+", 1},
+		{"$+ and $2", "Create $2 with: $+", 2},
+		{"$+ inside inline code only", "Use `$+` for required args", 0},
+		{"$1 and $2 in inline code only", "Use `$1` and `$2` for args", 0},
+		{"$1 outside $2 in inline code", "Create $1 (see `$2`)", 1},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			tpl := &PromptTemplate{Content: tt.content}
+			if got := tpl.RequiredArgs(); got != tt.want {
+				t.Errorf("RequiredArgs() = %d, want %d", got, tt.want)
+			}
+		})
+	}
+}

internal/session/compaction_cycle_test.go

@@ -0,0 +1,66 @@
+package session
+
+import (
+	"testing"
+
+	"github.com/mark3labs/kit/internal/message"
+)
+
+// TestCompactionParentCycleRegression tests that after multiple compactions,
+// newly appended messages always have a valid parent chain and BuildContext
+// returns the correct messages.
+func TestCompactionParentCycleRegression(t *testing.T) {
+	tm := InMemoryTreeSession("/test")
+
+	// Simulate a long conversation with multiple compactions.
+	msg1, _ := tm.AppendMessage(message.Message{Role: message.RoleUser, Parts: []message.ContentPart{message.TextContent{Text: "msg1"}}})
+	msg2, _ := tm.AppendMessage(message.Message{Role: message.RoleAssistant, Parts: []message.ContentPart{message.TextContent{Text: "msg2"}}})
+
+	// First compaction
+	comp1, _ := tm.AppendCompaction("Summary 1", msg1, 1000, 500, 1, []string{}, []string{})
+
+	msg3, _ := tm.AppendMessage(message.Message{Role: message.RoleUser, Parts: []message.ContentPart{message.TextContent{Text: "msg3"}}})
+	msg4, _ := tm.AppendMessage(message.Message{Role: message.RoleAssistant, Parts: []message.ContentPart{message.TextContent{Text: "msg4"}}})
+
+	// Second compaction
+	comp2, _ := tm.AppendCompaction("Summary 2", msg3, 1000, 500, 1, []string{}, []string{})
+
+	msg5, _ := tm.AppendMessage(message.Message{Role: message.RoleUser, Parts: []message.ContentPart{message.TextContent{Text: "msg5"}}})
+	msg6, _ := tm.AppendMessage(message.Message{Role: message.RoleAssistant, Parts: []message.ContentPart{message.TextContent{Text: "msg6"}}})
+
+	// Verify parent chain integrity
+	for _, id := range []string{msg1, msg2, comp1, msg3, msg4, comp2, msg5, msg6} {
+		entry := tm.GetEntry(id)
+		if entry == nil {
+			t.Fatalf("entry %s not found in index", id)
+		}
+	}
+
+	// Walk parent chain from msg6 — must reach root without cycles
+	visited := make(map[string]bool)
+	current := msg6
+	for current != "" {
+		if visited[current] {
+			t.Fatalf("cycle detected at entry %s", current)
+		}
+		visited[current] = true
+		entry := tm.GetEntry(current)
+		if entry == nil {
+			t.Fatalf("entry %s missing from index during parent walk", current)
+		}
+		parent := ""
+		switch e := entry.(type) {
+		case *MessageEntry:
+			parent = e.ParentID
+		case *CompactionEntry:
+			parent = e.ParentID
+		}
+		current = parent
+	}
+
+	// BuildContext should return: Summary2 + msg6 + msg5 + msg3 + msg4 = 5 messages
+	msgs, _, _ := tm.BuildContext()
+	if len(msgs) != 5 {
+		t.Fatalf("expected 5 messages, got %d: %+v", len(msgs), msgs)
+	}
+}

internal/session/compaction_test.go

@@ -0,0 +1,326 @@
+package session
+
+import (
+	"slices"
+	"testing"
+
+	"charm.land/fantasy"
+	"github.com/mark3labs/kit/internal/message"
+)
+
+// TestCompactionCreatesNewLeaf verifies that after compaction, the compaction
+// entry has no parent (creating a new root), and BuildContext returns only
+// the summary and kept messages, not the old compacted messages.
+func TestCompactionCreatesNewLeaf(t *testing.T) {
+	tm := InMemoryTreeSession("/test")
+
+	// Add some messages: M1, M2 (old, will be compacted), M3, M4 (kept)
+	msg1 := message.Message{Role: message.RoleUser, Parts: []message.ContentPart{message.TextContent{Text: "Message 1 - old"}}}
+	msg2 := message.Message{Role: message.RoleAssistant, Parts: []message.ContentPart{message.TextContent{Text: "Message 2 - old"}}}
+	msg3 := message.Message{Role: message.RoleUser, Parts: []message.ContentPart{message.TextContent{Text: "Message 3 - kept"}}}
+	msg4 := message.Message{Role: message.RoleAssistant, Parts: []message.ContentPart{message.TextContent{Text: "Message 4 - kept"}}}
+
+	_, _ = tm.AppendMessage(msg1)
+	_, _ = tm.AppendMessage(msg2)
+	id3, _ := tm.AppendMessage(msg3)
+	id4, _ := tm.AppendMessage(msg4)
+
+	// Verify initial state - all messages should be in context
+	messages, _, _ := tm.BuildContext()
+	if len(messages) != 4 {
+		t.Fatalf("expected 4 messages before compaction, got %d", len(messages))
+	}
+
+	// Verify entry IDs
+	entryIDs := tm.GetContextEntryIDs()
+	if len(entryIDs) != 4 {
+		t.Fatalf("expected 4 entry IDs before compaction, got %d", len(entryIDs))
+	}
+
+	// Now add a compaction entry, simulating that M3 is the first kept entry
+	summary := "Summary of old messages"
+	compactionID, err := tm.AppendCompaction(summary, id3, 1000, 500, 2, []string{}, []string{})
+	if err != nil {
+		t.Fatalf("failed to append compaction: %v", err)
+	}
+
+	// Verify the compaction entry has no parent (empty ParentID)
+	compactionEntry := tm.GetEntry(compactionID).(*CompactionEntry)
+	if compactionEntry.ParentID != "" {
+		t.Errorf("compaction entry should have no parent, got %q", compactionEntry.ParentID)
+	}
+
+	// Verify the leaf is now the compaction entry
+	if tm.GetLeafID() != compactionID {
+		t.Errorf("leaf should be compaction entry %q, got %q", compactionID, tm.GetLeafID())
+	}
+
+	// Now BuildContext should return: [summary] + [M3, M4]
+	messages, _, _ = tm.BuildContext()
+	if len(messages) != 3 {
+		t.Fatalf("expected 3 messages after compaction (summary + 2 kept), got %d", len(messages))
+	}
+
+	// First message should be the summary
+	if messages[0].Role != fantasy.MessageRoleSystem {
+		t.Errorf("first message should be system summary, got %s", messages[0].Role)
+	}
+	summaryText := messages[0].Content[0].(fantasy.TextPart).Text
+	if summaryText != "[Conversation summary — earlier messages were compacted]\n\n"+summary {
+		t.Errorf("unexpected summary text: %s", summaryText)
+	}
+
+	// Second message should be M3 (kept)
+	if messages[1].Role != fantasy.MessageRoleUser {
+		t.Errorf("second message should be user (M3), got %s", messages[1].Role)
+	}
+	m3Text := messages[1].Content[0].(fantasy.TextPart).Text
+	if m3Text != "Message 3 - kept" {
+		t.Errorf("unexpected M3 text: %s", m3Text)
+	}
+
+	// Third message should be M4 (kept)
+	if messages[2].Role != fantasy.MessageRoleAssistant {
+		t.Errorf("third message should be assistant (M4), got %s", messages[2].Role)
+	}
+	m4Text := messages[2].Content[0].(fantasy.TextPart).Text
+	if m4Text != "Message 4 - kept" {
+		t.Errorf("unexpected M4 text: %s", m4Text)
+	}
+
+	// Verify GetContextEntryIDs returns correct IDs
+	entryIDs = tm.GetContextEntryIDs()
+	if len(entryIDs) != 3 {
+		t.Fatalf("expected 3 entry IDs after compaction (empty for summary + 2 kept), got %d: %v", len(entryIDs), entryIDs)
+	}
+
+	// First entry ID should be empty (summary has no entry)
+	if entryIDs[0] != "" {
+		t.Errorf("first entry ID should be empty (summary), got %q", entryIDs[0])
+	}
+
+	// Second and third should be id3 and id4 (the kept messages)
+	if entryIDs[1] != id3 {
+		t.Errorf("second entry ID should be %q (M3), got %q", id3, entryIDs[1])
+	}
+	if entryIDs[2] != id4 {
+		t.Errorf("third entry ID should be %q (M4), got %q", id4, entryIDs[2])
+	}
+}
+
+// TestCompactionWithNewMessagesAfterCompaction verifies that messages appended
+// after compaction are correctly included in the context.
+func TestCompactionWithNewMessagesAfterCompaction(t *testing.T) {
+	tm := InMemoryTreeSession("/test")
+
+	// Add initial messages
+	msg1 := message.Message{Role: message.RoleUser, Parts: []message.ContentPart{message.TextContent{Text: "Message 1"}}}
+	msg2 := message.Message{Role: message.RoleAssistant, Parts: []message.ContentPart{message.TextContent{Text: "Message 2"}}}
+	msg3 := message.Message{Role: message.RoleUser, Parts: []message.ContentPart{message.TextContent{Text: "Message 3 - kept"}}}
+
+	_, _ = tm.AppendMessage(msg1)
+	_, _ = tm.AppendMessage(msg2)
+	id3, _ := tm.AppendMessage(msg3)
+
+	// Compact, keeping only M3
+	_, _ = tm.AppendCompaction("Summary", id3, 1000, 500, 2, []string{}, []string{})
+
+	// Add a new message after compaction
+	msg4 := message.Message{Role: message.RoleAssistant, Parts: []message.ContentPart{message.TextContent{Text: "Message 4 - after compaction"}}}
+	_, _ = tm.AppendMessage(msg4)
+
+	// BuildContext should return: [summary] + [M3 (kept)] + [M4 (new after compaction)]
+	// Kept messages must appear BEFORE post-compaction messages so the LLM
+	// sees the conversation in chronological order. Otherwise the latest
+	// post-compaction user message would be followed by an older kept user
+	// message, breaking user/assistant alternation and causing the model to
+	// respond as if the post-compaction turn never happened.
+	messages, _, _ := tm.BuildContext()
+	if len(messages) != 3 {
+		t.Fatalf("expected 3 messages (summary + M3 + M4), got %d: %+v", len(messages), messages)
+	}
+
+	// Verify order: summary, M3 (kept), M4 (new)
+	if messages[0].Role != fantasy.MessageRoleSystem {
+		t.Errorf("first message should be summary, got %s", messages[0].Role)
+	}
+	if messages[1].Role != fantasy.MessageRoleUser {
+		t.Errorf("second message should be user (M3 kept), got %s", messages[1].Role)
+	}
+	m3Text := messages[1].Content[0].(fantasy.TextPart).Text
+	if m3Text != "Message 3 - kept" {
+		t.Errorf("unexpected M3 text: %s", m3Text)
+	}
+	if messages[2].Role != fantasy.MessageRoleAssistant {
+		t.Errorf("third message should be assistant (M4 post-compact), got %s", messages[2].Role)
+	}
+	m4Text := messages[2].Content[0].(fantasy.TextPart).Text
+	if m4Text != "Message 4 - after compaction" {
+		t.Errorf("unexpected M4 text: %s", m4Text)
+	}
+
+	// Verify that M1 is NOT in the context
+	for i, msg := range messages {
+		if msg.Role == fantasy.MessageRoleUser {
+			text := msg.Content[0].(fantasy.TextPart).Text
+			if text == "Message 1" {
+				t.Errorf("Message 1 (compacted) should not be in context at index %d", i)
+			}
+		}
+	}
+}
+
+// TestCompactionWithNoKeptMessages verifies compaction when all messages are compacted.
+func TestCompactionWithNoKeptMessages(t *testing.T) {
+	tm := InMemoryTreeSession("/test")
+
+	// Add messages that will all be compacted
+	msg1 := message.Message{Role: message.RoleUser, Parts: []message.ContentPart{message.TextContent{Text: "Message 1"}}}
+	msg2 := message.Message{Role: message.RoleAssistant, Parts: []message.ContentPart{message.TextContent{Text: "Message 2"}}}
+
+	if _, err := tm.AppendMessage(msg1); err != nil {
+		t.Fatalf("failed to append message: %v", err)
+	}
+	if _, err := tm.AppendMessage(msg2); err != nil {
+		t.Fatalf("failed to append message: %v", err)
+	}
+
+	// Compact with no kept messages (empty firstKeptEntryID)
+	summary := "All messages summarized"
+	compactionID, _ := tm.AppendCompaction(summary, "", 1000, 100, 2, []string{}, []string{})
+
+	// Verify the compaction entry has no parent
+	compactionEntry := tm.GetEntry(compactionID).(*CompactionEntry)
+	if compactionEntry.ParentID != "" {
+		t.Errorf("compaction entry should have no parent, got %q", compactionEntry.ParentID)
+	}
+
+	// BuildContext should return only the summary
+	messages, _, _ := tm.BuildContext()
+	if len(messages) != 1 {
+		t.Fatalf("expected 1 message (summary only), got %d: %+v", len(messages), messages)
+	}
+	if messages[0].Role != fantasy.MessageRoleSystem {
+		t.Errorf("message should be system summary, got %s", messages[0].Role)
+	}
+}
+
+// TestMultipleCompactions verifies that multiple compactions work correctly.
+func TestMultipleCompactions(t *testing.T) {
+	tm := InMemoryTreeSession("/test")
+
+	// First batch of messages
+	msg1 := message.Message{Role: message.RoleUser, Parts: []message.ContentPart{message.TextContent{Text: "Batch 1 - User"}}}
+	msg2 := message.Message{Role: message.RoleAssistant, Parts: []message.ContentPart{message.TextContent{Text: "Batch 1 - Assistant"}}}
+	id1, _ := tm.AppendMessage(msg1)
+	id2, _ := tm.AppendMessage(msg2)
+
+	// First compaction
+	_, _ = tm.AppendCompaction("Summary 1", id1, 1000, 500, 1, []string{}, []string{})
+
+	// Second batch
+	msg3 := message.Message{Role: message.RoleUser, Parts: []message.ContentPart{message.TextContent{Text: "Batch 2 - User"}}}
+	msg4 := message.Message{Role: message.RoleAssistant, Parts: []message.ContentPart{message.TextContent{Text: "Batch 2 - Assistant"}}}
+	id3, _ := tm.AppendMessage(msg3)
+	id4, _ := tm.AppendMessage(msg4)
+
+	// Second compaction (compacting the first compaction + batch 2)
+	// Note: id3 is the first kept entry, so id3 and id4 should be preserved
+	compactionID2, _ := tm.AppendCompaction("Summary 2", id3, 1000, 500, 3, []string{}, []string{})
+
+	// Verify second compaction has no parent
+	compactionEntry2 := tm.GetEntry(compactionID2).(*CompactionEntry)
+	if compactionEntry2.ParentID != "" {
+		t.Errorf("second compaction entry should have no parent, got %q", compactionEntry2.ParentID)
+	}
+
+	// Add final message
+	msg5 := message.Message{Role: message.RoleUser, Parts: []message.ContentPart{message.TextContent{Text: "Final message"}}}
+	id5, _ := tm.AppendMessage(msg5)
+
+	// BuildContext should include:
+	// - Summary 2 (from second compaction)
+	// - msg5 (final message)
+	// - msg3, msg4 (kept from second compaction)
+	// But NOT Summary 1 or msg1, msg2 (they're before the first kept entry of compaction 2)
+	messages, _, _ := tm.BuildContext()
+
+	// Should have: Summary 2 + msg5 + msg3 + msg4 = 4 messages
+	if len(messages) != 4 {
+		t.Fatalf("expected 4 messages (Summary 2 + msg5 + msg3 + msg4), got %d: %+v", len(messages), messages)
+	}
+
+	// First should be Summary 2
+	if messages[0].Role != fantasy.MessageRoleSystem {
+		t.Errorf("first message should be system (Summary 2), got %s", messages[0].Role)
+	}
+	summaryText := messages[0].Content[0].(fantasy.TextPart).Text
+	if summaryText != "[Conversation summary — earlier messages were compacted]\n\nSummary 2" {
+		t.Errorf("unexpected summary: %s", summaryText)
+	}
+
+	// Verify msg5 is included
+	foundFinal := false
+	for _, msg := range messages {
+		if msg.Role == fantasy.MessageRoleUser {
+			text := msg.Content[0].(fantasy.TextPart).Text
+			if text == "Final message" {
+				foundFinal = true
+				break
+			}
+		}
+	}
+	if !foundFinal {
+		t.Error("Final message (msg5) should be in context")
+	}
+
+	// Verify msg1, msg2 are NOT included (compacted by first compaction, then second)
+	for _, msg := range messages {
+		if msg.Role == fantasy.MessageRoleUser || msg.Role == fantasy.MessageRoleAssistant {
+			text := msg.Content[0].(fantasy.TextPart).Text
+			if text == "Batch 1 - User" || text == "Batch 1 - Assistant" {
+				t.Errorf("Batch 1 messages should not be in context, found: %s", text)
+			}
+		}
+	}
+
+	// Verify entry IDs
+	entryIDs := tm.GetContextEntryIDs()
+	if len(entryIDs) != 4 {
+		t.Fatalf("expected 4 entry IDs, got %d: %v", len(entryIDs), entryIDs)
+	}
+
+	// First should be empty (summary)
+	if entryIDs[0] != "" {
+		t.Errorf("first entry ID should be empty (summary), got %q", entryIDs[0])
+	}
+
+	// Check that id5 is in the list
+	if !slices.Contains(entryIDs, id5) {
+		t.Errorf("id5 (final message) should be in entry IDs, got %v", entryIDs)
+	}
+
+	// Verify id3 and id4 ARE in the list (they were kept)
+	foundID3, foundID4 := false, false
+	for _, id := range entryIDs {
+		if id == id3 {
+			foundID3 = true
+		}
+		if id == id4 {
+			foundID4 = true
+		}
+	}
+	if !foundID3 {
+		t.Errorf("id3 (kept message) should be in entry IDs, got %v", entryIDs)
+	}
+	if !foundID4 {
+		t.Errorf("id4 (kept message) should be in entry IDs, got %v", entryIDs)
+	}
+
+	// Verify id1 and id2 are NOT in the list (they were compacted away)
+	for _, id := range entryIDs {
+		if id == id1 || id == id2 {
+			t.Errorf("id1 or id2 (compacted) should not be in entry IDs, found %q in %v", id, entryIDs)
+		}
+	}
+}

internal/session/session_dir_test.go

@@ -0,0 +1,70 @@
+package session
+
+import (
+	"strings"
+	"testing"
+)
+
+// TestEncodeCwdForDir verifies the working-directory → session-directory
+// name encoding strips characters that are illegal on Windows (notably the
+// drive-letter colon, see issue #18) while preserving the previous output
+// for the typical Unix paths.
+func TestEncodeCwdForDir(t *testing.T) {
+	tests := []struct {
+		name string
+		cwd  string
+		want string
+	}{
+		{
+			name: "unix absolute path",
+			cwd:  "/home/user/proj",
+			want: "home--user--proj",
+		},
+		{
+			name: "unix relative path",
+			cwd:  "proj/sub",
+			want: "proj--sub",
+		},
+		{
+			name: "windows drive root",
+			cwd:  `C:\test`,
+			want: "C--test",
+		},
+		{
+			name: "windows nested path",
+			cwd:  `C:\Users\User\code`,
+			want: "C--Users--User--code",
+		},
+		{
+			name: "windows secondary drive",
+			cwd:  `S:\work\repo`,
+			want: "S--work--repo",
+		},
+		{
+			name: "windows mixed separators",
+			cwd:  `C:\Users/User\code`,
+			want: "C--Users--User--code",
+		},
+		{
+			name: "windows other illegal chars stripped",
+			cwd:  `C:\a<b>c|d?e*f"g`,
+			want: "C--abcdefg",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got := encodeCwdForDir(tc.cwd)
+			if got != tc.want {
+				t.Errorf("encodeCwdForDir(%q) = %q, want %q", tc.cwd, got, tc.want)
+			}
+			// Encoded directory must never contain characters that are
+			// illegal in Windows directory names.
+			for _, bad := range []string{":", "<", ">", "\"", "|", "?", "*", "\\", "/"} {
+				if strings.Contains(got, bad) {
+					t.Errorf("encodeCwdForDir(%q) = %q contains illegal char %q", tc.cwd, got, bad)
+				}
+			}
+		})
+	}
+}

internal/session/store.go

@@ -6,8 +6,10 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"runtime"
 	"sort"
 	"strings"
+	"sync"
 	"time"
 )
 
@@ -97,6 +99,11 @@ func ListAllSessions() ([]SessionInfo, error) {
 
 // listSessionsInDir reads all .jsonl files in a directory and extracts session info.
 // Empty sessions (no messages) are automatically cleaned up and not returned.
+//
+// Per-file extraction is parallelized across a small worker pool because each
+// file requires a full JSONL scan to compute MessageCount and FirstMessage —
+// for users with many sessions this is the dominant cost of opening the
+// session picker.
 func listSessionsInDir(dir string) ([]SessionInfo, error) {
 	if _, err := os.Stat(dir); os.IsNotExist(err) {
 		return nil, nil
@@ -107,20 +114,47 @@ func listSessionsInDir(dir string) ([]SessionInfo, error) {
 		return nil, fmt.Errorf("failed to read directory %s: %w", dir, err)
 	}
 
-	var sessions []SessionInfo
+	// Collect candidate paths first so we can parallelize the heavy work.
+	paths := make([]string, 0, len(entries))
 	for _, entry := range entries {
 		if entry.IsDir() || !strings.HasSuffix(entry.Name(), ".jsonl") {
 			continue
 		}
+		paths = append(paths, filepath.Join(dir, entry.Name()))
+	}
 
-		path := filepath.Join(dir, entry.Name())
-		info, err := extractSessionInfo(path)
-		if err != nil {
-			continue // skip malformed session files
+	results := make([]*SessionInfo, len(paths))
+
+	// Worker pool sized to GOMAXPROCS, capped to avoid thrashing for tiny lists.
+	workers := max(min(runtime.GOMAXPROCS(0), len(paths)), 1)
+
+	var wg sync.WaitGroup
+	jobs := make(chan int, len(paths))
+	for range workers {
+		wg.Go(func() {
+			for i := range jobs {
+				info, err := extractSessionInfo(paths[i])
+				if err != nil {
+					continue // skip malformed session files
+				}
+				results[i] = info
+			}
+		})
+	}
+	for i := range paths {
+		jobs <- i
+	}
+	close(jobs)
+	wg.Wait()
+
+	sessions := make([]SessionInfo, 0, len(results))
+	for i, info := range results {
+		if info == nil {
+			continue
 		}
-		// Clean up and skip empty sessions (no messages)
+		// Clean up and skip empty sessions (no messages).
 		if info.MessageCount == 0 {
-			_ = os.Remove(path)
+			_ = os.Remove(paths[i])
 			continue
 		}
 		sessions = append(sessions, *info)