Cover child completion before create_agent returns (#6008)

Adds a regression test for #5928: a child that finishes its initial turn before its turn start returns still sends one finish notification to its parent. The fix itself is on main from 0009a7e.

Co-authored-by: Ahmet İzgi <ahmetizgi@gmail.com>

packages/server/src/server/agent/mcp-server.test.ts

@@ -3296,6 +3296,50 @@ describe("create_agent MCP tool", () => {
     );
   });
 
+  it("notifies the caller when a created child finishes its initial turn before the turn start returns", async () => {
+    const workdir = await mkdtemp(join(tmpdir(), "mcp-create-fast-child-"));
+    const storage = new AgentStorage(join(workdir, "agents"), logger);
+    const parentClient = new HeldTurnAgentClient("claude", false);
+    const childClient = new HeldTurnAgentClient("codex", false, { finishTurnsDuringStart: true });
+    const agentManager = new AgentManager({
+      clients: { claude: parentClient, codex: childClient },
+      registry: storage,
+      logger,
+    });
+
+    try {
+      const parent = await agentManager.createAgent(
+        { provider: "claude", cwd: existingCwd },
+        undefined,
+        { workspaceId: "wks_parent" },
+      );
+      const server = await createAgentMcpServer({
+        agentManager,
+        agentStorage: storage,
+        callerAgentId: parent.id,
+        providerSnapshotManager: createOpenCodeManager().manager,
+        logger,
+      });
+
+      const response = await invokeToolWithParsedInput(registeredTool(server, "create_agent"), {
+        ...subagentCurrentWorkspace(),
+        title: "Fast Child",
+        provider: "codex/gpt-5.4",
+        initialPrompt: "Finish immediately",
+      });
+      const childId = z.object({ agentId: z.string() }).parse(response.structuredContent).agentId;
+
+      await vi.waitFor(() => {
+        const parentPrompts = parentClient.sessions[0]!.prompts;
+        expect(parentPrompts).toHaveLength(1);
+        expect(parentPrompts[0]).toContain(childId);
+        expect(parentPrompts[0]).toContain("finished");
+      });
+    } finally {
+      await removeAgentStateDir(agentManager, storage, workdir);
+    }
+  });
+
   it("creates detached caller agents without a parent label", async () => {
     const { agentManager, agentStorage, spies } = createTestDeps();
     spies.agentManager.getAgent.mockReturnValue({
@@ -3770,6 +3814,11 @@ const HELD_TURN_CAPABILITIES = {
   supportsToolInvocations: false,
 } as const;
 
+interface HeldTurnOptions {
+  /** Run the whole turn before the turn-start request returns. */
+  finishTurnsDuringStart?: boolean;
+}
+
 /**
  * Provider session that records every prompt it receives. With `holdTurns`, a started
  * turn stays running until `finishTurn()` so a caller's bounded wait can run out first.
@@ -3786,6 +3835,7 @@ class HeldTurnAgentSession implements AgentSession {
   constructor(
     readonly provider: AgentProvider,
     private readonly holdTurns: boolean,
+    private readonly options: HeldTurnOptions = {},
   ) {}
 
   /** Hold the next turn's acknowledgment, as a provider awaiting its turn-start request does. */
@@ -3819,6 +3869,13 @@ class HeldTurnAgentSession implements AgentSession {
       throw error;
     }
     const turnId = randomUUID();
+    if (this.options.finishTurnsDuringStart) {
+      // The whole turn runs before the turn-start request returns, as a provider whose
+      // start acknowledgment arrives together with the turn's completion does.
+      this.pushEvent({ type: "turn_started", provider: this.provider, turnId });
+      this.pushEvent({ type: "turn_completed", provider: this.provider, turnId });
+      return { turnId };
+    }
     this.activeTurnId = turnId;
     this.pushEvent({
       type: "timeline",
@@ -3926,6 +3983,7 @@ class HeldTurnAgentClient implements AgentClient {
   constructor(
     readonly provider: AgentProvider,
     private readonly holdTurns: boolean,
+    private readonly options: HeldTurnOptions = {},
   ) {}
 
   async isAvailable(): Promise<boolean> {
@@ -3933,7 +3991,7 @@ class HeldTurnAgentClient implements AgentClient {
   }
 
   async createSession(): Promise<AgentSession> {
-    const session = new HeldTurnAgentSession(this.provider, this.holdTurns);
+    const session = new HeldTurnAgentSession(this.provider, this.holdTurns, this.options);
     this.sessions.push(session);
     return session;
   }