fix: relay ACP sessions_spawn parent streaming (#34310) (thanks @vincentkoc) (#34310)

Co-authored-by: Onur Solmaz <2453968+osolmaz@users.noreply.github.com>

src/agents/tools/sessions-spawn-tool.test.ts

@@ -16,6 +16,7 @@ vi.mock("../subagent-spawn.js", () => ({
 
 vi.mock("../acp-spawn.js", () => ({
   ACP_SPAWN_MODES: ["run", "session"],
+  ACP_SPAWN_STREAM_TARGETS: ["parent"],
   spawnAcpDirect: (...args: unknown[]) => hoisted.spawnAcpDirectMock(...args),
 }));
 
@@ -94,6 +95,7 @@ describe("sessions_spawn tool", () => {
       cwd: "/workspace",
       thread: true,
       mode: "session",
+      streamTo: "parent",
     });
 
     expect(result.details).toMatchObject({
@@ -108,6 +110,7 @@ describe("sessions_spawn tool", () => {
         cwd: "/workspace",
         thread: true,
         mode: "session",
+        streamTo: "parent",
       }),
       expect.objectContaining({
         agentSessionKey: "agent:main:main",
@@ -165,6 +168,26 @@ describe("sessions_spawn tool", () => {
     expect(hoisted.spawnSubagentDirectMock).not.toHaveBeenCalled();
   });
 
+  it('rejects streamTo when runtime is not "acp"', async () => {
+    const tool = createSessionsSpawnTool({
+      agentSessionKey: "agent:main:main",
+    });
+
+    const result = await tool.execute("call-3b", {
+      runtime: "subagent",
+      task: "analyze file",
+      streamTo: "parent",
+    });
+
+    expect(result.details).toMatchObject({
+      status: "error",
+    });
+    const details = result.details as { error?: string };
+    expect(details.error).toContain("streamTo is only supported for runtime=acp");
+    expect(hoisted.spawnAcpDirectMock).not.toHaveBeenCalled();
+    expect(hoisted.spawnSubagentDirectMock).not.toHaveBeenCalled();
+  });
+
   it("keeps attachment content schema unconstrained for llama.cpp grammar safety", () => {
     const tool = createSessionsSpawnTool();
     const schema = tool.parameters as {

src/agents/tools/sessions-spawn-tool.ts

@@ -1,6 +1,6 @@
 import { Type } from "@sinclair/typebox";
 import type { GatewayMessageChannel } from "../../utils/message-channel.js";
-import { ACP_SPAWN_MODES, spawnAcpDirect } from "../acp-spawn.js";
+import { ACP_SPAWN_MODES, ACP_SPAWN_STREAM_TARGETS, spawnAcpDirect } from "../acp-spawn.js";
 import { optionalStringEnum } from "../schema/typebox.js";
 import { SUBAGENT_SPAWN_MODES, spawnSubagentDirect } from "../subagent-spawn.js";
 import type { AnyAgentTool } from "./common.js";
@@ -34,6 +34,7 @@ const SessionsSpawnToolSchema = Type.Object({
   mode: optionalStringEnum(SUBAGENT_SPAWN_MODES),
   cleanup: optionalStringEnum(["delete", "keep"] as const),
   sandbox: optionalStringEnum(SESSIONS_SPAWN_SANDBOX_MODES),
+  streamTo: optionalStringEnum(ACP_SPAWN_STREAM_TARGETS),
 
   // Inline attachments (snapshot-by-value).
   // NOTE: Attachment contents are redacted from transcript persistence by sanitizeToolCallInputs.
@@ -97,6 +98,7 @@ export function createSessionsSpawnTool(opts?: {
       const cleanup =
         params.cleanup === "keep" || params.cleanup === "delete" ? params.cleanup : "keep";
       const sandbox = params.sandbox === "require" ? "require" : "inherit";
+      const streamTo = params.streamTo === "parent" ? "parent" : undefined;
       // Back-compat: older callers used timeoutSeconds for this tool.
       const timeoutSecondsCandidate =
         typeof params.runTimeoutSeconds === "number"
@@ -118,6 +120,13 @@ export function createSessionsSpawnTool(opts?: {
           }>)
         : undefined;
 
+      if (streamTo && runtime !== "acp") {
+        return jsonResult({
+          status: "error",
+          error: `streamTo is only supported for runtime=acp; got runtime=${runtime}`,
+        });
+      }
+
       if (runtime === "acp") {
         if (Array.isArray(attachments) && attachments.length > 0) {
           return jsonResult({
@@ -135,6 +144,7 @@ export function createSessionsSpawnTool(opts?: {
             mode: mode && ACP_SPAWN_MODES.includes(mode) ? mode : undefined,
             thread,
             sandbox,
+            streamTo,
           },
           {
             agentSessionKey: opts?.agentSessionKey,