diff --git a/internal/extensions/runner.go b/internal/extensions/runner.go
index 023c5112..1564e0b4 100644
--- a/internal/extensions/runner.go
+++ b/internal/extensions/runner.go
@@ -104,6 +104,7 @@ type Runner struct {
 	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
 }
@@ -770,6 +771,11 @@ func (r *Runner) GetMessageRenderer(name string) *MessageRendererConfig {
 // 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 {
@@ -779,7 +785,9 @@ func (r *Runner) SetState(key, value string) {
 	saver := r.stateSaver
 	r.stateMu.Unlock()
 	if saver != nil {
+		r.saverMu.Lock()
 		saver()
+		r.saverMu.Unlock()
 	}
 }
 
@@ -793,7 +801,8 @@ func (r *Runner) GetState(key string) (string, bool) {
 }
 
 // DeleteState removes a key from the state store. No-op if the key is
-// missing. Thread-safe.
+// 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]
@@ -803,7 +812,9 @@ func (r *Runner) DeleteState(key string) {
 	saver := r.stateSaver
 	r.stateMu.Unlock()
 	if existed && saver != nil {
+		r.saverMu.Lock()
 		saver()
+		r.saverMu.Unlock()
 	}
 }
 
@@ -847,17 +858,25 @@ func (r *Runner) SnapshotState() map[string]string {
 
 // 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" (not an error). Malformed JSON returns the parse error
-// without touching the existing store. Thread-safe.
+// "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
diff --git a/internal/extensions/state_test.go b/internal/extensions/state_test.go
index 59c8ee1c..c570848e 100644
--- a/internal/extensions/state_test.go
+++ b/internal/extensions/state_test.go
@@ -101,15 +101,37 @@ func TestRunner_State_SaveAndLoadRoundTrip(t *testing.T) {
 	}
 }
 
-func TestRunner_State_LoadMissingFileIsNoop(t *testing.T) {
+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)
 	}
-	// Existing in-memory state is left alone when file doesn't exist.
-	if v, ok := r.GetState("a"); !ok || v != "1" {
-		t.Errorf("expected pre-existing state preserved, got (%q,%v)", v, ok)
+	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")
 	}
 }
 
diff --git a/pkg/kit/extensions_bridge.go b/pkg/kit/extensions_bridge.go
index 4e999674..36e069bb 100644
--- a/pkg/kit/extensions_bridge.go
+++ b/pkg/kit/extensions_bridge.go
@@ -5,6 +5,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/mark3labs/kit/internal/auth"
 	"github.com/mark3labs/kit/internal/extensions"
 	"github.com/mark3labs/kit/internal/models"
 )
@@ -24,6 +25,15 @@ func (m *Kit) bridgeExtensions(runner *extensions.Runner) {
 	// Per-turn aggregator: collects tool/LLM/usage signals between AgentStart
 	// and AgentEnd so the enriched AgentEndEvent can be populated without
 	// requiring extensions to maintain parallel bookkeeping.
+	//
+	// NOTE: this aggregator assumes a single in-flight turn per *Kit instance,
+	// which is the current contract — runTurn does not serialize callers and
+	// the SDK's TurnStartEvent/TurnEndEvent do not carry a turn ID, so two
+	// concurrent Prompt() calls on the same *Kit would clobber the counters.
+	// All current callers (TUI app layer, CLI runner, SDK examples) serialize
+	// turns above this layer. If concurrent turns become a supported use case,
+	// extend TurnStartEvent/TurnEndEvent with a turn ID and key this map per
+	// turn instead.
 	turnAgg := &turnAggregator{kit: m}
 	m.Subscribe(func(e Event) {
 		switch ev := e.(type) {
@@ -530,8 +540,12 @@ func (a *turnAggregator) consume() turnSnapshot {
 
 // llmUsageMeta returns the current provider, model id, and computed cost for
 // the given usage values using the Kit instance's active model. Cost is zero
-// when the model is not in the registry (e.g. custom fine-tunes, unknown
-// providers) or pricing fields are unset.
+// in any of the following cases:
+//   - the *Kit pointer is nil or has no active model;
+//   - the model is not in the registry (custom fine-tunes, unknown providers);
+//   - the model has no pricing fields set;
+//   - the active credential is an Anthropic OAuth token (matches the
+//     existing usage_tracker behavior of suppressing cost for OAuth users).
 func llmUsageMeta(m *Kit, usage LLMUsage) (provider, modelID string, cost float64) {
 	if m == nil {
 		return "", "", 0
@@ -549,6 +563,9 @@ func llmUsageMeta(m *Kit, usage LLMUsage) (provider, modelID string, cost float6
 	if info == nil {
 		return provider, modelID, 0
 	}
+	if isAnthropicOAuth(m, provider) {
+		return provider, modelID, 0
+	}
 	cost = float64(usage.InputTokens) * info.Cost.Input / 1_000_000
 	cost += float64(usage.OutputTokens) * info.Cost.Output / 1_000_000
 	if info.Cost.CacheRead != nil {
@@ -560,6 +577,21 @@ func llmUsageMeta(m *Kit, usage LLMUsage) (provider, modelID string, cost float6
 	return provider, modelID, cost
 }
 
+// isAnthropicOAuth reports whether the current Anthropic credential resolves
+// to a stored OAuth token (in which case the user is not billed per-token).
+// Mirrors the OAuth detection in cmd/extension_context.go's usage tracker
+// update path so OnLLMUsage cost reporting agrees with ctx.GetSessionUsage().
+func isAnthropicOAuth(m *Kit, provider string) bool {
+	if m == nil || provider != "anthropic" {
+		return false
+	}
+	_, source, err := auth.GetAnthropicAPIKey(m.v.GetString("provider-api-key"))
+	if err != nil {
+		return false
+	}
+	return strings.HasPrefix(source, "stored OAuth")
+}
+
 // llmToContextMessages converts a slice of LLM messages to extension
 // ContextMessage values, extracting plain text from each message.
 func llmToContextMessages(msgs []LLMMessage) []extensions.ContextMessage {
diff --git a/pkg/kit/extensions_bridge_test.go b/pkg/kit/extensions_bridge_test.go
index 29a5c18d..cf0dd976 100644
--- a/pkg/kit/extensions_bridge_test.go
+++ b/pkg/kit/extensions_bridge_test.go
@@ -108,6 +108,16 @@ func TestLLMUsageMeta_NilKit(t *testing.T) {
 	}
 }
 
+// TestIsAnthropicOAuth_NonAnthropic verifies the helper short-circuits for any
+// provider other than "anthropic" without touching the credential store.
+func TestIsAnthropicOAuth_NonAnthropic(t *testing.T) {
+	for _, provider := range []string{"openai", "google", "openrouter", ""} {
+		if isAnthropicOAuth(nil, provider) {
+			t.Errorf("isAnthropicOAuth(nil, %q) = true, want false", provider)
+		}
+	}
+}
+
 func TestExtStateSidecarPath(t *testing.T) {
 	tests := []struct {
 		name string