diff --git a/sdk/api/handlers/handlers_interceptors.go b/sdk/api/handlers/handlers_interceptors.go
--- a/sdk/api/handlers/handlers_interceptors.go
+++ b/sdk/api/handlers/handlers_interceptors.go
@@ -1,6 +1,7 @@
 package handlers
 
 import (
+	stdcontext "context"
 	"net/http"
 	"sync"
 	"time"
@@ -66,6 +67,9 @@ type requestLifecycleSkipHost interface {
 
 type requestLifecycleTracker struct {
 	once         sync.Once
+	cancelMu     sync.Mutex
+	cancelStop   func() bool
+	completed    bool
 	ctx          context.Context
 	host         PluginInterceptorHost
 	skipPluginID string
@@ -76,7 +80,7 @@ func (h *BaseAPIHandler) newRequestLifecycleTracker(ctx context.Context, sourceF
 	requestID := uuid.NewString()
 	traceID := logging.GetRequestID(ctx)
 	ctx = coreusage.WithRequestIdentity(ctx, requestID, traceID)
-	return &requestLifecycleTracker{
+	tracker := &requestLifecycleTracker{
 		ctx:          ctx,
 		host:         h.interceptorHost(),
 		skipPluginID: skipPluginID,
@@ -91,6 +95,39 @@ func (h *BaseAPIHandler) newRequestLifecycleTracker(ctx context.Context, sourceF
 			Metadata:       metadata,
 		},
 	}
+	tracker.watchCancellation(ctx)
+	return tracker
+}
+
+func (t *requestLifecycleTracker) watchCancellation(ctx context.Context) {
+	if t == nil || ctx == nil || ctx.Done() == nil {
+		return
+	}
+	stop := stdcontext.AfterFunc(ctx, func() {
+		t.complete(pluginapi.RequestCompletionCanceled, 0, ctx.Err())
+	})
+	t.cancelMu.Lock()
+	if t.completed {
+		t.cancelMu.Unlock()
+		stop()
+		return
+	}
+	t.cancelStop = stop
+	t.cancelMu.Unlock()
+}
+
+func (t *requestLifecycleTracker) stopCancellationWatch() {
+	if t == nil {
+		return
+	}
+	t.cancelMu.Lock()
+	t.completed = true
+	stop := t.cancelStop
+	t.cancelStop = nil
+	t.cancelMu.Unlock()
+	if stop != nil {
+		stop()
+	}
 }
 
 func (t *requestLifecycleTracker) executionContext() context.Context {
@@ -112,6 +149,7 @@ func (t *requestLifecycleTracker) complete(outcome pluginapi.RequestCompletionOu
 		return
 	}
 	t.once.Do(func() {
+		t.stopCancellationWatch()
 		completion := t.completion
 		completion.Outcome = outcome
 		completion.StatusCode = statusCode
diff --git a/sdk/api/handlers/handlers_interceptors_test.go b/sdk/api/handlers/handlers_interceptors_test.go
--- a/sdk/api/handlers/handlers_interceptors_test.go
+++ b/sdk/api/handlers/handlers_interceptors_test.go
@@ -516,6 +516,78 @@ func TestHandlerLifecycleCompletesCanceledStream(t *testing.T) {
 	}
 }
 
+func TestHandlerLifecycleCompletesCanceledStreamWhileExecutorIsStarting(t *testing.T) {
+	model := "handler-interceptor-lifecycle-canceled-before-stream-start"
+	executorStarted := make(chan struct{})
+	releaseExecutor := make(chan struct{})
+	executor := &interceptorCaptureExecutor{
+		stream: func(context.Context, *coreauth.Auth, coreexecutor.Request, coreexecutor.Options) (*coreexecutor.StreamResult, error) {
+			close(executorStarted)
+			<-releaseExecutor
+			chunks := make(chan coreexecutor.StreamChunk)
+			close(chunks)
+			return &coreexecutor.StreamResult{Chunks: chunks}, nil
+		},
+	}
+	handler := newInterceptorHandler(t, model, executor, &sdkconfig.SDKConfig{})
+	completions := make(chan pluginapi.RequestCompletion, 2)
+	handler.SetPluginHost(&handlerInterceptorTestHost{
+		completeRequest: func(_ context.Context, completion pluginapi.RequestCompletion) {
+			completions <- completion
+		},
+	})
+
+	ctx, cancel := context.WithCancel(context.Background())
+	type streamResult struct {
+		data <-chan []byte
+		errs <-chan *interfaces.ErrorMessage
+	}
+	result := make(chan streamResult, 1)
+	go func() {
+		data, _, errs := handler.ExecuteStreamWithAuthManager(ctx, "openai", model, []byte(`{"model":"`+model+`","stream":true}`), "")
+		result <- streamResult{data: data, errs: errs}
+	}()
+
+	select {
+	case <-executorStarted:
+	case <-time.After(time.Second):
+		close(releaseExecutor)
+		t.Fatal("executor did not start")
+	}
+	cancel()
+
+	select {
+	case completion := <-completions:
+		if completion.Outcome != pluginapi.RequestCompletionCanceled || completion.StatusCode != 0 || completion.Error == "" {
+			close(releaseExecutor)
+			t.Fatalf("completion = %#v", completion)
+		}
+	case <-time.After(time.Second):
+		close(releaseExecutor)
+		t.Fatal("missing cancellation completion while executor startup was blocked")
+	}
+
+	close(releaseExecutor)
+	channels := <-result
+	for channels.data != nil || channels.errs != nil {
+		select {
+		case _, ok := <-channels.data:
+			if !ok {
+				channels.data = nil
+			}
+		case _, ok := <-channels.errs:
+			if !ok {
+				channels.errs = nil
+			}
+		}
+	}
+	select {
+	case duplicate := <-completions:
+		t.Fatalf("duplicate stream completion = %#v", duplicate)
+	default:
+	}
+}
+
 func TestHandlerRequestInterceptorRewritesExecutorRequest(t *testing.T) {
 	model := "handler-interceptor-request-model"
 	executor := &interceptorCaptureExecutor{}