Fix Codex replacement turn idle flash

packages/app/src/contexts/session-stream-lifecycle.test.ts

@@ -13,10 +13,17 @@ const turnFailedEvent: AgentStreamEventPayload = {
   error: "failed",
 };
 
+const turnCanceledEvent: AgentStreamEventPayload = {
+  type: "turn_canceled",
+  provider: "codex",
+  reason: "interrupted",
+};
+
 describe("session stream lifecycle helpers", () => {
   it("derives optimistic terminal lifecycle only when current status is running", () => {
     expect(deriveOptimisticLifecycleStatus("running", turnCompletedEvent)).toBe("idle");
     expect(deriveOptimisticLifecycleStatus("running", turnFailedEvent)).toBe("error");
+    expect(deriveOptimisticLifecycleStatus("running", turnCanceledEvent)).toBe(null);
     expect(deriveOptimisticLifecycleStatus("initializing", turnCompletedEvent)).toBe(null);
     expect(deriveOptimisticLifecycleStatus("idle", turnFailedEvent)).toBe(null);
   });

packages/app/src/contexts/session-stream-lifecycle.ts

@@ -10,10 +10,13 @@ export function deriveOptimisticLifecycleStatus(
   }
   switch (event.type) {
     case "turn_completed":
-    case "turn_canceled":
       return "idle";
     case "turn_failed":
       return "error";
+    case "turn_canceled":
+      // A canceled turn can be either a final user cancel or an interrupt before
+      // a replacement turn starts. The daemon snapshot is authoritative here.
+      return null;
     default:
       return null;
   }

packages/app/src/contexts/session-stream-reducers.test.ts

@@ -602,6 +602,28 @@ describe("processAgentStreamEvent", () => {
     expect(result.agent!.status).toBe("error");
   });
 
+  it("does not derive optimistic idle status on turn_canceled for running agent", () => {
+    const turnCanceledEvent: AgentStreamEventPayload = {
+      type: "turn_canceled",
+      provider: "codex",
+      reason: "interrupted",
+    };
+
+    const result = processAgentStreamEvent({
+      ...baseStreamInput,
+      event: turnCanceledEvent,
+      currentAgent: {
+        status: "running",
+        updatedAt: new Date(1000),
+        lastActivityAt: new Date(1000),
+      },
+      timestamp: new Date(2000),
+    });
+
+    expect(result.agentChanged).toBe(false);
+    expect(result.agent).toBe(null);
+  });
+
   it("does not change agent when status is not running", () => {
     const turnCompletedEvent: AgentStreamEventPayload = {
       type: "turn_completed",