From 37e82781b141e3eacdcdc310ba639424c58ed0e3 Mon Sep 17 00:00:00 2001
From: Ed Zynda <ezynda3@gmail.com>
Date: Mon, 2 Mar 2026 14:49:51 +0300
Subject: [PATCH] feat: add OnModelChange event and ctx.Exit(); remove Gap/Pi
 references from comments

---
 cmd/root.go                            |  7 ++++
 examples/extensions/auto-commit.go     |  3 +-
 examples/extensions/bookmark.go        |  2 +-
 examples/extensions/inline-bash.go     |  2 +-
 examples/extensions/minimal.go         |  2 +-
 examples/extensions/notify.go          |  3 +-
 examples/extensions/permission-gate.go |  2 +-
 examples/extensions/pirate.go          |  2 +-
 examples/extensions/plan-mode.go       |  3 +-
 examples/extensions/project-rules.go   |  2 +-
 examples/extensions/protected-paths.go |  2 +-
 examples/extensions/summarize.go       |  2 +-
 internal/app/app.go                    | 16 ++++++++
 internal/compaction/compaction.go      | 30 +++++++-------
 internal/compaction/compaction_test.go |  4 +-
 internal/core/tools.go                 |  8 ++--
 internal/extensions/api.go             | 54 ++++++++++++++++++--------
 internal/extensions/events.go          |  6 ++-
 internal/extensions/events_test.go     |  5 ++-
 internal/extensions/loader.go          |  6 +++
 internal/extensions/runner.go          |  2 +-
 internal/extensions/symbols.go         |  1 +
 internal/extensions/wrapper.go         |  3 +-
 internal/session/entry.go              |  4 +-
 internal/session/store.go              |  2 +-
 internal/session/tree_manager.go       |  6 +--
 internal/skills/skills.go              |  2 +-
 internal/ui/model.go                   |  4 +-
 internal/ui/tree_selector.go           |  3 +-
 pkg/kit/compaction.go                  |  4 +-
 pkg/kit/config.go                      |  2 +-
 pkg/kit/kit.go                         | 28 +++++++++++--
 32 files changed, 147 insertions(+), 75 deletions(-)

diff --git a/cmd/root.go b/cmd/root.go
index 9a27cbb2..2689263d 100644
--- a/cmd/root.go
+++ b/cmd/root.go
@@ -616,6 +616,7 @@ func runNormalMode(ctx context.Context) error {
 			PrintBlock:    appInstance.PrintBlockFromExtension,
 			SendMessage:   func(text string) { appInstance.Run(text) },
 			CancelAndSend: func(text string) { appInstance.Steer(text) },
+			Exit:          func() { appInstance.QuitFromExtension() },
 			SetWidget: func(config extensions.WidgetConfig) {
 				kitInstance.SetExtensionWidget(config)
 				appInstance.NotifyWidgetUpdate()
@@ -746,6 +747,8 @@ func runNormalMode(ctx context.Context) error {
 				kitInstance.SetExtensionOption(name, value)
 			},
 			SetModel: func(modelString string) error {
+				// Capture previous model for the ModelChange event.
+				previousModel := kitInstance.GetExtensionContext().Model
 				err := kitInstance.SetModel(context.Background(), modelString)
 				if err != nil {
 					return err
@@ -753,6 +756,10 @@ func runNormalMode(ctx context.Context) error {
 				// 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.UpdateExtensionContextModel(modelString)
+				// Fire OnModelChange event to extensions.
+				kitInstance.EmitModelChange(modelString, previousModel, "extension")
 				return nil
 			},
 			GetAvailableModels: func() []extensions.ModelInfoEntry {
diff --git a/examples/extensions/auto-commit.go b/examples/extensions/auto-commit.go
index ccab929f..39e7765c 100644
--- a/examples/extensions/auto-commit.go
+++ b/examples/extensions/auto-commit.go
@@ -10,8 +10,7 @@ import (
 )
 
 // Init automatically commits staged changes when the session shuts down,
-// using the last assistant message as the commit message. Inspired by
-// Pi's auto-commit-on-exit.ts.
+// using the last assistant message as the commit message.
 //
 // Only commits if:
 //   - There are staged changes (git diff --cached is non-empty)
diff --git a/examples/extensions/bookmark.go b/examples/extensions/bookmark.go
index 75fd6ef2..ef4e50a2 100644
--- a/examples/extensions/bookmark.go
+++ b/examples/extensions/bookmark.go
@@ -13,7 +13,7 @@ import (
 
 // Init adds bookmark commands for marking and recalling important points in
 // a conversation. Bookmarks are persisted in the session tree and survive
-// restarts. Inspired by Pi's bookmark.ts.
+// restarts.
 //
 // Commands:
 //
diff --git a/examples/extensions/inline-bash.go b/examples/extensions/inline-bash.go
index a9c6f94a..52e4c7f5 100644
--- a/examples/extensions/inline-bash.go
+++ b/examples/extensions/inline-bash.go
@@ -12,7 +12,7 @@ import (
 
 // 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. Inspired by Pi's inline-bash.ts.
+// stdout.
 //
 // Examples:
 //
diff --git a/examples/extensions/minimal.go b/examples/extensions/minimal.go
index 416eaba5..bef59efd 100644
--- a/examples/extensions/minimal.go
+++ b/examples/extensions/minimal.go
@@ -10,7 +10,7 @@ import (
 	"kit/ext"
 )
 
-// Init demonstrates a minimal-chrome extension — a port of Pi's minimal.ts.
+// Init demonstrates a minimal-chrome extension.
 // Hides the startup banner, status bar, separator, and input hint, replacing
 // them with a compact footer showing model name and a context usage bar:
 //
diff --git a/examples/extensions/notify.go b/examples/extensions/notify.go
index 56bd92d3..c399ecfa 100644
--- a/examples/extensions/notify.go
+++ b/examples/extensions/notify.go
@@ -11,8 +11,7 @@ import (
 
 // Init sends a desktop notification when the agent finishes responding.
 // Useful for long-running tasks — get notified without watching the terminal.
-// Inspired by Pi's notify.ts.
-//
+
 // Supports: Linux (notify-send), macOS (osascript).
 //
 // Usage: kit -e examples/extensions/notify.go
diff --git a/examples/extensions/permission-gate.go b/examples/extensions/permission-gate.go
index 5d17ddc2..773bb058 100644
--- a/examples/extensions/permission-gate.go
+++ b/examples/extensions/permission-gate.go
@@ -10,7 +10,7 @@ import (
 )
 
 // Init intercepts potentially dangerous bash commands and asks the user for
-// confirmation before allowing execution. Inspired by Pi's permission-gate.ts.
+// confirmation before allowing execution.
 //
 // Dangerous patterns: rm -rf, sudo, chmod 777, mkfs, dd, > /dev/
 //
diff --git a/examples/extensions/pirate.go b/examples/extensions/pirate.go
index ff49f42a..45f98591 100644
--- a/examples/extensions/pirate.go
+++ b/examples/extensions/pirate.go
@@ -6,7 +6,7 @@ import "kit/ext"
 
 // Init injects a pirate persona into the system prompt, causing the LLM to
 // respond in pirate-speak. Demonstrates OnBeforeAgentStart system prompt
-// injection. Inspired by Pi's pirate.ts.
+// injection.
 //
 // Usage: kit -e examples/extensions/pirate.go
 func Init(api ext.API) {
diff --git a/examples/extensions/plan-mode.go b/examples/extensions/plan-mode.go
index c1c1dacf..fd8e2ef7 100644
--- a/examples/extensions/plan-mode.go
+++ b/examples/extensions/plan-mode.go
@@ -10,8 +10,7 @@ import (
 
 // Init implements a plan/explore mode that restricts the agent to read-only
 // tools. Toggle with /plan (or start in plan mode via KIT_OPT_PLAN=true).
-// Inspired by Pi's plan-mode extension.
-//
+
 // In plan mode the agent can only use read, grep, find, and ls — it cannot
 // write files, run bash, or make edits. This is useful for exploring a
 // codebase, reviewing architecture, or generating plans before executing.
diff --git a/examples/extensions/project-rules.go b/examples/extensions/project-rules.go
index 30f0f10d..aaa51c52 100644
--- a/examples/extensions/project-rules.go
+++ b/examples/extensions/project-rules.go
@@ -14,7 +14,7 @@ import (
 // Init loads project-specific rules from .kit/rules/ into the system prompt.
 // Each .md file in the rules directory is injected as additional context,
 // giving projects a way to customise LLM behaviour without editing the
-// main system prompt. Inspired by Pi's claude-rules.ts.
+// main system prompt.
 //
 // Place rule files in:
 //
diff --git a/examples/extensions/protected-paths.go b/examples/extensions/protected-paths.go
index 2cc630b3..497cbe2e 100644
--- a/examples/extensions/protected-paths.go
+++ b/examples/extensions/protected-paths.go
@@ -10,7 +10,7 @@ import (
 )
 
 // Init blocks tool calls that attempt to write, edit, or delete files in
-// protected paths. Inspired by Pi's protected-paths.ts.
+// protected paths.
 //
 // Protected: .env*, .git/, secrets/, credentials*, *.pem, *.key
 //
diff --git a/examples/extensions/summarize.go b/examples/extensions/summarize.go
index 5fface19..c2ef3b7f 100644
--- a/examples/extensions/summarize.go
+++ b/examples/extensions/summarize.go
@@ -11,7 +11,7 @@ import (
 
 // Init adds a /summarize command that generates a concise summary of the
 // current conversation using a direct LLM completion. Demonstrates the
-// ctx.Complete API (Gap 17). Inspired by Pi's summarize.ts.
+// ctx.Complete API.
 //
 // The summary is displayed in a styled block and can optionally be saved
 // to the session via AppendEntry for later retrieval.
diff --git a/internal/app/app.go b/internal/app/app.go
index 08acb460..6eda0d30 100644
--- a/internal/app/app.go
+++ b/internal/app/app.go
@@ -516,6 +516,22 @@ func (a *App) subscribeSDKEvents(sendFn func(tea.Msg)) func() {
 	}
 }
 
+// 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
+// step. Safe to call from any goroutine; idempotent.
+func (a *App) QuitFromExtension() {
+	a.mu.Lock()
+	prog := a.program
+	a.mu.Unlock()
+	if prog != nil {
+		prog.Send(tea.QuitMsg{})
+		return
+	}
+	// Non-interactive: cancel the root context.
+	a.rootCancel()
+}
+
 // PrintFromExtension outputs text from an extension to the user. The level
 // controls styling: "" for plain text, "info" for a system message block,
 // "error" for an error block. In interactive mode it sends an
diff --git a/internal/compaction/compaction.go b/internal/compaction/compaction.go
index fec92423..b64c3f42 100644
--- a/internal/compaction/compaction.go
+++ b/internal/compaction/compaction.go
@@ -1,7 +1,7 @@
 // Package compaction provides context window management with token estimation,
 // compaction triggers, and LLM-based conversation summarization.
 //
-// The algorithm mirrors Pi's approach: preserve a token budget of recent
+// The algorithm preserves a token budget of recent
 // messages (KeepRecentTokens, default 20 000) rather than a fixed message
 // count. Auto-compaction fires when estimated context usage exceeds
 // contextWindow − ReserveTokens.
@@ -50,8 +50,8 @@ func estimateSingleMessageTokens(msg fantasy.Message) int {
 // Auto-compact trigger
 // ---------------------------------------------------------------------------
 
-// ShouldCompact reports whether auto-compaction should fire. It uses Pi's
-// formula: contextTokens > contextWindow − reserveTokens.
+// ShouldCompact reports whether auto-compaction should fire.
+// Formula: contextTokens > contextWindow − reserveTokens.
 func ShouldCompact(messages []fantasy.Message, contextWindow int, reserveTokens int) bool {
 	if contextWindow <= 0 || reserveTokens <= 0 {
 		return false
@@ -72,8 +72,8 @@ type CompactionResult struct {
 	MessagesRemoved int    // Number of messages replaced by the summary
 }
 
-// CompactionOptions configures compaction behaviour. Pi-style token-based
-// defaults are applied for zero-value fields.
+// CompactionOptions configures compaction behaviour. Token-based defaults
+// are applied for zero-value fields.
 type CompactionOptions struct {
 	ContextWindow    int    // Model's context window size (tokens)
 	ReserveTokens    int    // Tokens to reserve for LLM response, default 16384
@@ -81,7 +81,7 @@ type CompactionOptions struct {
 	SummaryPrompt    string // Custom summary prompt (empty = use default)
 }
 
-// defaults fills zero-value fields with sensible Pi-style defaults.
+// defaults fills zero-value fields with sensible defaults.
 func (o *CompactionOptions) defaults() {
 	if o.ReserveTokens <= 0 {
 		o.ReserveTokens = 16384
@@ -92,13 +92,13 @@ func (o *CompactionOptions) defaults() {
 }
 
 // defaultSystemPrompt is the system prompt sent to the summarisation LLM.
-// Matches Pi's compaction system prompt.
+
 const defaultSystemPrompt = `You are a context summarization assistant. Your task is to read a conversation between a user and an AI coding assistant, then produce a structured summary following the exact format specified.
 
 Do NOT continue the conversation. Do NOT respond to any questions in the conversation. ONLY output the structured summary.`
 
 // defaultSummaryPrompt is the user prompt appended after the serialised
-// conversation. Matches Pi's initial-compaction format.
+// conversation.
 const defaultSummaryPrompt = `The messages above are a conversation to summarize. Create a structured context checkpoint summary that another LLM will use to continue the work.
 
 Use this EXACT format:
@@ -133,7 +133,7 @@ Use this EXACT format:
 Keep each section concise. Preserve exact file paths, function names, and error messages.`
 
 // ---------------------------------------------------------------------------
-// Cut point (token-based, Pi-style)
+// Cut point (token-based)
 // ---------------------------------------------------------------------------
 
 // isValidCutPoint returns true if the message at index i is a valid place to
@@ -208,11 +208,11 @@ func forceCutPoint(messages []fantasy.Message) int {
 }
 
 // ---------------------------------------------------------------------------
-// Message serialisation (Pi-style)
+// Message serialisation
 // ---------------------------------------------------------------------------
 
 // roleLabel returns a human-readable label for a fantasy message role,
-// matching Pi's serialisation format.
+
 func roleLabel(role fantasy.MessageRole) string {
 	switch role {
 	case fantasy.MessageRoleUser:
@@ -230,7 +230,7 @@ func roleLabel(role fantasy.MessageRole) string {
 
 // serializeMessages converts a slice of fantasy messages into a plain-text
 // representation suitable for sending to the summarisation LLM. The format
-// mirrors Pi's compaction serialisation.
+
 func serializeMessages(messages []fantasy.Message) string {
 	var sb strings.Builder
 	for _, msg := range messages {
@@ -277,8 +277,8 @@ func Compact(
 	cutPoint := FindCutPoint(messages, opts.KeepRecentTokens)
 	if cutPoint == 0 {
 		// All messages fit within the keep budget. Force a cut that
-		// keeps only the last non-tool message — matching Pi, which
-		// always compacts when the user explicitly requests it.
+		// keeps only the last non-tool message — always compact when
+		// the user explicitly requests it.
 		cutPoint = forceCutPoint(messages)
 		if cutPoint == 0 {
 			return nil, messages, nil
@@ -289,7 +289,7 @@ func Compact(
 	recentMessages := messages[cutPoint:]
 	originalTokens := EstimateMessageTokens(messages)
 
-	// Serialise old messages to text, matching Pi's format.
+	// Serialise old messages to text.
 	conversationText := serializeMessages(oldMessages)
 
 	// Build the user-facing prompt: conversation text + summary instructions.
diff --git a/internal/compaction/compaction_test.go b/internal/compaction/compaction_test.go
index d54b4ed9..d75e8b6f 100644
--- a/internal/compaction/compaction_test.go
+++ b/internal/compaction/compaction_test.go
@@ -63,7 +63,7 @@ func TestEstimateMessageTokens_Empty(t *testing.T) {
 }
 
 // ---------------------------------------------------------------------------
-// ShouldCompact (Pi-style: contextTokens > contextWindow - reserveTokens)
+// ShouldCompact (contextTokens > contextWindow - reserveTokens)
 // ---------------------------------------------------------------------------
 
 func TestShouldCompact(t *testing.T) {
@@ -94,7 +94,7 @@ func TestShouldCompact(t *testing.T) {
 }
 
 // ---------------------------------------------------------------------------
-// FindCutPoint (token-based, Pi-style)
+// FindCutPoint (token-based)
 // ---------------------------------------------------------------------------
 
 func TestFindCutPoint_TokenBased(t *testing.T) {
diff --git a/internal/core/tools.go b/internal/core/tools.go
index 6336a616..5edcb300 100644
--- a/internal/core/tools.go
+++ b/internal/core/tools.go
@@ -1,7 +1,7 @@
 // Package core provides the built-in core tools for KIT's coding agent.
 // These tools are direct fantasy.AgentTool implementations — no MCP layer,
-// no JSON-RPC, no serialization overhead. They match the pi coding agent's
-// core tool set: bash, read, write, edit, grep, find, ls.
+// no JSON-RPC, no serialization overhead. Core tool set: bash, read, write,
+// edit, grep, find, ls.
 package core
 
 import (
@@ -65,7 +65,7 @@ func parseArgs(input string, target any) error {
 }
 
 // CodingTools returns the default set of core tools for a coding agent:
-// bash, read, write, edit. This matches pi's codingTools collection.
+// bash, read, write, edit.
 func CodingTools(opts ...ToolOption) []fantasy.AgentTool {
 	return []fantasy.AgentTool{
 		NewBashTool(opts...),
@@ -76,7 +76,7 @@ func CodingTools(opts ...ToolOption) []fantasy.AgentTool {
 }
 
 // ReadOnlyTools returns tools for read-only exploration:
-// read, grep, find, ls. This matches pi's readOnlyTools collection.
+// read, grep, find, ls.
 func ReadOnlyTools(opts ...ToolOption) []fantasy.AgentTool {
 	return []fantasy.AgentTool{
 		NewReadTool(opts...),
diff --git a/internal/extensions/api.go b/internal/extensions/api.go
index 7e4f204f..5ba8bd0e 100644
--- a/internal/extensions/api.go
+++ b/internal/extensions/api.go
@@ -245,8 +245,6 @@ type Context struct {
 	//   fmt.Sprintf("[%s%s] %d%%", strings.Repeat("#", pct/10), strings.Repeat("-", 10-pct/10), pct)
 	GetContextStats func() ContextStats
 
-	// --- Session Management (Gap 1) ---
-
 	// GetMessages returns the conversation messages on the current branch,
 	// ordered from root to leaf. This is a read-only view; extensions
 	// cannot modify messages directly.
@@ -265,8 +263,6 @@ type Context struct {
 	// file. Returns empty string for in-memory (ephemeral) sessions.
 	GetSessionPath func() string
 
-	// --- Session Persistence (Gap 2) ---
-
 	// AppendEntry persists custom extension data in the session tree.
 	// The data survives across session restarts and can be retrieved via
 	// GetEntries. Use entryType to namespace your data (e.g. "myext:state").
@@ -299,8 +295,6 @@ type Context struct {
 	//   ctx.SetEditorText("Please review the changes in src/main.go")
 	SetEditorText func(text string)
 
-	// --- Keyed Status Bar (Gap M3) ---
-
 	// SetStatus places or updates a keyed entry in the TUI status bar.
 	// Multiple entries from different extensions coexist; each is identified
 	// by a unique key. Lower priority values render further left.
@@ -314,8 +308,6 @@ type Context struct {
 	// does not exist.
 	RemoveStatus func(key string)
 
-	// --- Extension Options (Gap 7) ---
-
 	// GetOption returns the value of a named extension option. Options are
 	// resolved in priority order:
 	//   1. Runtime override (via SetOption)
@@ -338,8 +330,6 @@ type Context struct {
 	// persisting user choices during a session.
 	SetOption func(name string, value string)
 
-	// --- Model Management (Gap 2) ---
-
 	// SetModel changes the active LLM model at runtime. The model string
 	// should be in "provider/model" format (e.g. "anthropic/claude-sonnet-4-5-20250929").
 	// Existing tools, system prompt, and session are preserved. Returns an
@@ -365,8 +355,6 @@ type Context struct {
 	//   }
 	GetAvailableModels func() []ModelInfoEntry
 
-	// --- Inter-Extension Event Bus (Gap 13) ---
-
 	// EmitCustomEvent publishes a named event that other extensions can
 	// subscribe to via api.OnCustomEvent(). Data is an arbitrary string
 	// (JSON-encode complex payloads). Handlers run synchronously in
@@ -377,8 +365,6 @@ type Context struct {
 	//   ctx.EmitCustomEvent("plan-mode:toggled", `{"active":true}`)
 	EmitCustomEvent func(name string, data string)
 
-	// --- Tool Management (Gap 3) ---
-
 	// GetAllTools returns information about all tools available to the agent,
 	// including core tools (bash, read, write, etc.), MCP server tools, and
 	// extension-registered tools. Each entry includes the tool's enabled status.
@@ -402,7 +388,21 @@ type Context struct {
 	//   ctx.SetActiveTools([]string{"Read", "Glob", "Grep", "LS"})
 	SetActiveTools func(names []string)
 
-	// --- Direct LLM Completion (Gap 17) ---
+	// Exit triggers a graceful application shutdown. In interactive mode
+	// this sends a quit signal to the TUI; in non-interactive mode it
+	// cancels the current operation. Safe to call from any goroutine.
+	//
+	// Example:
+	//
+	//   api.RegisterCommand(ext.CommandDef{
+	//       Name:        "quit",
+	//       Description: "Exit the application",
+	//       Execute: func(args string, ctx ext.Context) (string, error) {
+	//           ctx.Exit()
+	//           return "", nil
+	//       },
+	//   })
+	Exit func()
 
 	// Complete makes a standalone LLM completion call, bypassing the agent
 	// tool loop. Use this for summarisation, question extraction, or any
@@ -579,6 +579,7 @@ type API struct {
 	registerToolFn         func(ToolDef)
 	registerCmdFn          func(CommandDef)
 	registerToolRendererFn func(ToolRenderConfig)
+	onModelChange          func(func(ModelChangeEvent, Context))
 	onCustomEvent          func(name string, handler func(string))
 	registerOption         func(OptionDef)
 }
@@ -651,6 +652,13 @@ func (a *API) OnSessionShutdown(handler func(SessionShutdownEvent, Context)) {
 	a.onSessionShutdown(handler)
 }
 
+// OnModelChange registers a handler that fires after the active model is
+// changed via ctx.SetModel(). The handler receives the new and previous model
+// strings plus the source of the change.
+func (a *API) OnModelChange(handler func(ModelChangeEvent, Context)) {
+	a.onModelChange(handler)
+}
+
 // RegisterTool adds a custom tool that the LLM can invoke.
 func (a *API) RegisterTool(tool ToolDef) {
 	a.registerToolFn(tool)
@@ -1126,8 +1134,7 @@ type EditorKeyAction struct {
 // submit) and/or modify the rendered output (add mode indicators, apply visual
 // effects).
 //
-// This follows Pi's extension editor pattern (modal editor, rainbow editor)
-// but uses concrete function fields instead of interfaces for Yaegi safety.
+// Uses concrete function fields instead of interfaces for Yaegi safety.
 //
 // IMPORTANT (Yaegi limitation): Function fields MUST be set using anonymous
 // function literals (closures), NOT bare function references. Yaegi does not
@@ -1289,4 +1296,17 @@ func (e SessionStartEvent) Type() EventType { return SessionStart }
 // SessionShutdownEvent fires when the application is closing.
 type SessionShutdownEvent struct{}
 
+// ModelChangeEvent fires after the active model is changed via ctx.SetModel().
+type ModelChangeEvent struct {
+	// NewModel is the model string that was set (e.g. "anthropic/claude-sonnet-4-5-20250929").
+	NewModel string
+	// PreviousModel is the model string before the change.
+	PreviousModel string
+	// Source indicates what triggered the change: "extension" for ctx.SetModel(),
+	// "user" for interactive model selection.
+	Source string
+}
+
 func (e SessionShutdownEvent) Type() EventType { return SessionShutdown }
+
+func (e ModelChangeEvent) Type() EventType { return ModelChange }
diff --git a/internal/extensions/events.go b/internal/extensions/events.go
index 75190812..f8d82c24 100644
--- a/internal/extensions/events.go
+++ b/internal/extensions/events.go
@@ -1,4 +1,4 @@
-// Package extensions implements a Pi-style in-process extension system for KIT.
+// Package extensions implements an in-process extension system for KIT.
 // Extensions are plain Go files loaded at runtime via Yaegi (a Go interpreter).
 // They register event handlers using an API object, enabling tool interception,
 // input transformation, and lifecycle observation — all without recompilation.
@@ -48,6 +48,9 @@ const (
 
 	// SessionShutdown fires when the application is closing.
 	SessionShutdown EventType = "session_shutdown"
+
+	// ModelChange fires after the active model is changed via ctx.SetModel().
+	ModelChange EventType = "model_change"
 )
 
 // AllEventTypes returns every supported event type.
@@ -57,6 +60,7 @@ func AllEventTypes() []EventType {
 		Input, BeforeAgentStart, AgentStart, AgentEnd,
 		MessageStart, MessageUpdate, MessageEnd,
 		SessionStart, SessionShutdown,
+		ModelChange,
 	}
 }
 
diff --git a/internal/extensions/events_test.go b/internal/extensions/events_test.go
index d8c8973e..ede280fe 100644
--- a/internal/extensions/events_test.go
+++ b/internal/extensions/events_test.go
@@ -4,8 +4,8 @@ import "testing"
 
 func TestAllEventTypes_Count(t *testing.T) {
 	all := AllEventTypes()
-	if len(all) != 13 {
-		t.Fatalf("expected 13 event types, got %d", len(all))
+	if len(all) != 14 {
+		t.Fatalf("expected 14 event types, got %d", len(all))
 	}
 }
 
@@ -50,6 +50,7 @@ func TestEventType_TypeMethod(t *testing.T) {
 		{MessageEndEvent{Content: "done"}, MessageEnd},
 		{SessionStartEvent{SessionID: "abc"}, SessionStart},
 		{SessionShutdownEvent{}, SessionShutdown},
+		{ModelChangeEvent{NewModel: "a/b"}, ModelChange},
 	}
 
 	for _, tt := range tests {
diff --git a/internal/extensions/loader.go b/internal/extensions/loader.go
index 78199879..c22fe052 100644
--- a/internal/extensions/loader.go
+++ b/internal/extensions/loader.go
@@ -283,6 +283,12 @@ func loadSingleExtension(path string) (*LoadedExtension, error) {
 				return nil
 			})
 		},
+		onModelChange: func(h func(ModelChangeEvent, Context)) {
+			reg(ModelChange, func(e Event, c Context) Result {
+				h(e.(ModelChangeEvent), c)
+				return nil
+			})
+		},
 		registerToolFn: func(tool ToolDef) {
 			ext.Tools = append(ext.Tools, tool)
 		},
diff --git a/internal/extensions/runner.go b/internal/extensions/runner.go
index b6b8d50f..1149158e 100644
--- a/internal/extensions/runner.go
+++ b/internal/extensions/runner.go
@@ -12,7 +12,7 @@ import (
 )
 
 // Runner manages loaded extensions and dispatches events to their handlers
-// sequentially, mirroring Pi's ExtensionRunner. Handlers execute in extension
+// sequentially. Handlers execute in extension
 // load order; for cancellable events the first blocking result wins.
 type Runner struct {
 	extensions      []LoadedExtension
diff --git a/internal/extensions/symbols.go b/internal/extensions/symbols.go
index 46bc290b..e302140d 100644
--- a/internal/extensions/symbols.go
+++ b/internal/extensions/symbols.go
@@ -110,6 +110,7 @@ func Symbols() interp.Exports {
 			"MessageEndEvent":         reflect.ValueOf((*MessageEndEvent)(nil)),
 			"SessionStartEvent":       reflect.ValueOf((*SessionStartEvent)(nil)),
 			"SessionShutdownEvent":    reflect.ValueOf((*SessionShutdownEvent)(nil)),
+			"ModelChangeEvent":        reflect.ValueOf((*ModelChangeEvent)(nil)),
 		},
 	}
 }
diff --git a/internal/extensions/wrapper.go b/internal/extensions/wrapper.go
index 68cd9952..e581e97e 100644
--- a/internal/extensions/wrapper.go
+++ b/internal/extensions/wrapper.go
@@ -9,8 +9,7 @@ import (
 
 // WrapToolsWithExtensions wraps each tool so that ToolCall and ToolResult
 // events are emitted through the extension runner before and after execution.
-// This is the Go equivalent of Pi's wrapper.ts pattern.
-//
+
 // If the runner has no relevant handlers the original tools are returned
 // unchanged (zero overhead).
 func WrapToolsWithExtensions(tools []fantasy.AgentTool, runner *Runner) []fantasy.AgentTool {
diff --git a/internal/session/entry.go b/internal/session/entry.go
index 46dd39b2..ae41f490 100644
--- a/internal/session/entry.go
+++ b/internal/session/entry.go
@@ -11,8 +11,8 @@ import (
 )
 
 // EntryType identifies the kind of entry stored in a JSONL session file.
-// Following pi's design, sessions are append-only JSONL files where each line
-// is a typed entry linked by id/parent_id to form a tree structure.
+// Sessions are append-only JSONL files where each line is a typed entry
+// linked by id/parent_id to form a tree structure.
 type EntryType string
 
 const (
diff --git a/internal/session/store.go b/internal/session/store.go
index 8468a3a3..506948de 100644
--- a/internal/session/store.go
+++ b/internal/session/store.go
@@ -12,7 +12,7 @@ import (
 )
 
 // SessionInfo contains metadata about a discovered session, used for listing
-// and session picker display. Follows pi's SessionInfo design.
+// and session picker display.
 type SessionInfo struct {
 	// Path is the absolute path to the JSONL session file.
 	Path string
diff --git a/internal/session/tree_manager.go b/internal/session/tree_manager.go
index fc4f65dd..b140f699 100644
--- a/internal/session/tree_manager.go
+++ b/internal/session/tree_manager.go
@@ -16,7 +16,7 @@ import (
 )
 
 // TreeNode represents a node in the session tree for display purposes.
-// It mirrors pi's SessionTreeNode design.
+
 type TreeNode struct {
 	Entry    any         // the underlying entry (*MessageEntry, *ModelChangeEntry, etc.)
 	ID       string      // entry ID
@@ -25,7 +25,7 @@ type TreeNode struct {
 }
 
 // TreeManager manages a tree-structured JSONL session. It is the replacement
-// for the linear session.Manager, following pi's design decisions:
+// for the linear session.Manager:
 //
 //   - JSONL append-only format (one JSON object per line)
 //   - Tree structure via id/parent_id on every entry
@@ -717,7 +717,7 @@ func (tm *TreeManager) buildTreeNode(id string) *TreeNode {
 // --- Path conventions ---
 
 // DefaultSessionDir returns the default session storage directory for a cwd.
-// Following pi's convention: ~/.kit/sessions/--<cwd-path>--/
+// Convention: ~/.kit/sessions/--<cwd-path>--/
 func DefaultSessionDir(cwd string) string {
 	home, err := os.UserHomeDir()
 	if err != nil {
diff --git a/internal/skills/skills.go b/internal/skills/skills.go
index 1eae7bb6..a653554a 100644
--- a/internal/skills/skills.go
+++ b/internal/skills/skills.go
@@ -202,7 +202,7 @@ func LoadSkills(cwd string) ([]*Skill, error) {
 // FormatForPrompt formats skills as metadata-only XML for inclusion in a
 // system prompt. Only the name, description, and file location are included;
 // the agent reads the full skill file on demand using the read tool. This
-// matches the Pi SDK's formatSkillsForPrompt convention.
+
 func FormatForPrompt(skills []*Skill) string {
 	if len(skills) == 0 {
 		return ""
diff --git a/internal/ui/model.go b/internal/ui/model.go
index 4a45f898..846623e0 100644
--- a/internal/ui/model.go
+++ b/internal/ui/model.go
@@ -1722,7 +1722,7 @@ func (m *AppModel) printCompactResult(evt app.CompactCompleteEvent) tea.Cmd {
 // and on step completion.
 //
 // After flushing, a ClearScreen is issued to force a full terminal redraw.
-// This is the bubbletea equivalent of pi's "clearOnShrink" mechanism: when
+// When
 // the stream content is moved to scrollback the view height shrinks, and
 // bubbletea's inline renderer doesn't clear the orphaned terminal rows
 // below the managed region. ClearScreen ensures a clean redraw.
@@ -1935,7 +1935,7 @@ func (m *AppModel) handleNameCommand() tea.Cmd {
 	}
 	// For now, prompt user to provide name via input. We print instructions
 	// and the next non-command input starting with "name:" will be captured.
-	// TODO: inline input dialog like pi's implementation.
+	// TODO: inline input dialog.
 	currentName := ts.GetSessionName()
 	if currentName != "" {
 		return m.printSystemMessage(fmt.Sprintf("Current session name: %q\nTo rename, type: `/name <new name>` (not yet implemented — use the session file directly).", currentName))
diff --git a/internal/ui/tree_selector.go b/internal/ui/tree_selector.go
index 9d84cde4..5bbb49b1 100644
--- a/internal/ui/tree_selector.go
+++ b/internal/ui/tree_selector.go
@@ -52,8 +52,7 @@ type FlatNode struct {
 }
 
 // TreeSelectorComponent is a Bubble Tea component that renders the session
-// tree as an ASCII art list with navigation and selection. It follows pi's
-// tree selector design.
+// tree as an ASCII art list with navigation and selection.
 type TreeSelectorComponent struct {
 	tm         *session.TreeManager
 	flatNodes  []FlatNode
diff --git a/pkg/kit/compaction.go b/pkg/kit/compaction.go
index b2061399..08e26386 100644
--- a/pkg/kit/compaction.go
+++ b/pkg/kit/compaction.go
@@ -23,8 +23,8 @@ func (m *Kit) EstimateContextTokens() int {
 }
 
 // ShouldCompact reports whether the conversation is near the model's context
-// limit and should be compacted. Uses Pi's formula:
-// contextTokens > contextWindow − reserveTokens.
+// limit and should be compacted.
+// Formula: contextTokens > contextWindow − reserveTokens.
 // Returns false if the model's context limit is unknown.
 func (m *Kit) ShouldCompact() bool {
 	info := m.GetModelInfo()
diff --git a/pkg/kit/config.go b/pkg/kit/config.go
index 0363e2e6..3b180a69 100644
--- a/pkg/kit/config.go
+++ b/pkg/kit/config.go
@@ -11,7 +11,7 @@ import (
 
 // defaultSystemPrompt is the built-in system prompt used when no custom
 // prompt is configured. It describes the available core tools and provides
-// usage guidelines, matching the Pi SDK's default prompt style.
+// usage guidelines.
 const defaultSystemPrompt = `You are an expert coding assistant operating inside kit, a coding agent harness. You help users by reading files, executing commands, editing code, and writing new files.
 
 Available tools:
diff --git a/pkg/kit/kit.go b/pkg/kit/kit.go
index bbd55b9d..0b7c03c6 100644
--- a/pkg/kit/kit.go
+++ b/pkg/kit/kit.go
@@ -146,6 +146,17 @@ func (m *Kit) GetExtensionContext() extensions.Context {
 	return extensions.Context{}
 }
 
+// UpdateExtensionContextModel updates the Model field on the extension
+// context so subsequent event handlers see the new model. This is a
+// targeted update that avoids replacing the entire Context struct.
+func (m *Kit) UpdateExtensionContextModel(model string) {
+	if m.extRunner != nil {
+		ctx := m.extRunner.GetContext()
+		ctx.Model = model
+		m.extRunner.SetContext(ctx)
+	}
+}
+
 // EmitSessionStart fires the SessionStart event for extensions.
 // No-op if extensions are disabled or no handlers are registered.
 func (m *Kit) EmitSessionStart() {
@@ -513,6 +524,18 @@ func (m *Kit) SetExtensionOption(name, value string) {
 	}
 }
 
+// EmitModelChange fires the ModelChange event for extensions.
+// No-op if extensions are disabled or no handlers are registered.
+func (m *Kit) EmitModelChange(newModel, previousModel, source string) {
+	if m.extRunner != nil && m.extRunner.HasHandlers(extensions.ModelChange) {
+		_, _ = m.extRunner.Emit(extensions.ModelChangeEvent{
+			NewModel:      newModel,
+			PreviousModel: previousModel,
+			Source:        source,
+		})
+	}
+}
+
 // EmitExtensionCustomEvent dispatches a named event to all extension handlers.
 // No-op if extensions are disabled.
 func (m *Kit) EmitExtensionCustomEvent(name, data string) {
@@ -757,8 +780,7 @@ func New(ctx context.Context, opts *Options) (*Kit, error) {
 	}
 
 	// Always compose the system prompt with runtime context: base prompt +
-	// AGENTS.md context + skills metadata + date/cwd. This matches Pi's
-	// buildSystemPrompt() convention.
+	// AGENTS.md context + skills metadata + date/cwd.
 	{
 		basePrompt := viper.GetString("system-prompt")
 		pb := skills.NewPromptBuilder(basePrompt)
@@ -891,7 +913,7 @@ func loadContextFiles(cwd string) []*ContextFile {
 // so, re-reads the skill file, strips its YAML frontmatter, wraps the body in
 // a <skill> block with baseDir metadata, and appends any trailing user args.
 // Returns the original text unchanged when the prefix is absent or the skill is
-// not found. This matches Pi's _expandSkillCommand() convention.
+// not found.
 func (m *Kit) expandSkillCommand(prompt string) string {
 	if !strings.HasPrefix(prompt, "/skill:") {
 		return prompt