fix(sessions_spawn): silently strip ACP-only fields for runtime=subagent

skernelx · skernelx · commit e5c2067d2fc0 · 2026-04-20T18:33:58.000+08:00
Schema-following LLMs (notably gpt-5.4) auto-fill every advertised tool parameter, including streamTo and resumeSessionId, regardless of the runtime value they also set. Under runtime="subagent" the executor rejected those fields with a hard error, making subagent spawn effectively unusable for these models (see #43556, #56193, #63120, Since the subagent code path never reads either field, treat them as no-ops when runtime!=="acp" instead of failing. ACP semantics are unchanged: when runtime==="acp" the fields still flow through unmodified and the existing ACP spawn logic validates them. Tests updated to match the new silently-drop behavior.
diff --git a/src/agents/tools/sessions-spawn-tool.test.ts b/src/agents/tools/sessions-spawn-tool.test.ts
@@ -328,7 +328,7 @@ describe("sessions_spawn tool", () => {
     );
   });
 
-  it("rejects resumeSessionId without runtime=acp", async () => {
+  it("silently drops resumeSessionId for runtime=subagent", async () => {
     const tool = createSessionsSpawnTool({
       agentSessionKey: "agent:main:main",
     });
@@ -338,9 +338,13 @@ describe("sessions_spawn tool", () => {
       resumeSessionId: "7f4a78e0-f6be-43fe-855c-c1c4fd229bc4",
     });
 
-    expect(JSON.stringify(result)).toContain("resumeSessionId is only supported for runtime=acp");
-    expect(hoisted.spawnSubagentDirectMock).not.toHaveBeenCalled();
+    expect(JSON.stringify(result)).not.toContain("resumeSessionId is only supported");
     expect(hoisted.spawnAcpDirectMock).not.toHaveBeenCalled();
+    expect(hoisted.spawnSubagentDirectMock).toHaveBeenCalledTimes(1);
+    const [spawnArgs] = hoisted.spawnSubagentDirectMock.mock.calls[0] as [
+      Record<string, unknown>,
+    ];
+    expect(spawnArgs).not.toHaveProperty("resumeSessionId");
   });
 
   it("rejects attachments for ACP runtime", async () => {
@@ -367,7 +371,7 @@ describe("sessions_spawn tool", () => {
     expect(hoisted.spawnSubagentDirectMock).not.toHaveBeenCalled();
   });
 
-  it('rejects streamTo when runtime is not "acp"', async () => {
+  it('silently drops streamTo when runtime is not "acp"', async () => {
     const tool = createSessionsSpawnTool({
       agentSessionKey: "agent:main:main",
     });
@@ -378,13 +382,13 @@ describe("sessions_spawn tool", () => {
       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(JSON.stringify(result)).not.toContain("streamTo is only supported");
     expect(hoisted.spawnAcpDirectMock).not.toHaveBeenCalled();
-    expect(hoisted.spawnSubagentDirectMock).not.toHaveBeenCalled();
+    expect(hoisted.spawnSubagentDirectMock).toHaveBeenCalledTimes(1);
+    const [spawnArgs] = hoisted.spawnSubagentDirectMock.mock.calls[0] as [
+      Record<string, unknown>,
+    ];
+    expect(spawnArgs).not.toHaveProperty("streamTo");
   });
 
   it("keeps attachment content schema unconstrained for llama.cpp grammar safety", () => {
diff --git a/src/agents/tools/sessions-spawn-tool.ts b/src/agents/tools/sessions-spawn-tool.ts
@@ -101,7 +101,7 @@ const SessionsSpawnToolSchema = Type.Object({
   resumeSessionId: Type.Optional(
     Type.String({
       description:
-        'Resume an existing agent session by its ID (e.g. a Codex session UUID from ~/.codex/sessions/). Requires runtime="acp". The agent replays conversation history via session/load instead of starting fresh.',
+        'Resume an existing agent session by its ID (e.g. a Codex session UUID from ~/.codex/sessions/). Only applies when runtime="acp"; silently ignored for runtime="subagent". The agent replays conversation history via session/load instead of starting fresh.',
     }),
   ),
   model: Type.Optional(Type.String()),
@@ -176,7 +176,13 @@ export function createSessionsSpawnTool(
       const label = readStringParam(params, "label") ?? "";
       const runtime = params.runtime === "acp" ? "acp" : "subagent";
       const requestedAgentId = readStringParam(params, "agentId");
-      const resumeSessionId = readStringParam(params, "resumeSessionId");
+      // ACP-only fields: schema-strict LLMs (e.g. gpt-5.4) auto-fill these on
+      // every call regardless of the runtime they also set. The subagent code
+      // path never consumes either field, so silently drop them when
+      // runtime!=="acp" instead of failing the whole spawn. ACP semantics are
+      // preserved: fields still flow through unchanged when runtime==="acp".
+      const resumeSessionId =
+        runtime === "acp" ? readStringParam(params, "resumeSessionId") : undefined;
       const modelOverride = readStringParam(params, "model");
       const thinkingOverrideRaw = readStringParam(params, "thinking");
       const cwd = readStringParam(params, "cwd");
@@ -185,7 +191,8 @@ export function createSessionsSpawnTool(
         params.cleanup === "keep" || params.cleanup === "delete" ? params.cleanup : "keep";
       const expectsCompletionMessage = params.expectsCompletionMessage !== false;
       const sandbox = params.sandbox === "require" ? "require" : "inherit";
-      const streamTo = params.streamTo === "parent" ? "parent" : undefined;
+      const streamTo =
+        runtime === "acp" && params.streamTo === "parent" ? "parent" : undefined;
       const lightContext = params.lightContext === true;
       if (runtime === "acp" && lightContext) {
         throw new Error("lightContext is only supported for runtime='subagent'.");
@@ -213,22 +220,6 @@ export function createSessionsSpawnTool(
 
       const roleContext = requestedAgentId ? { role: requestedAgentId } : {};
 
-      if (streamTo && runtime !== "acp") {
-        return jsonResult({
-          status: "error",
-          error: `streamTo is only supported for runtime=acp; got runtime=${runtime}`,
-          ...roleContext,
-        });
-      }
-
-      if (resumeSessionId && runtime !== "acp") {
-        return jsonResult({
-          status: "error",
-          error: `resumeSessionId is only supported for runtime=acp; got runtime=${runtime}`,
-          ...roleContext,
-        });
-      }
-
       if (runtime === "acp") {
         const { isSpawnAcpAcceptedResult, spawnAcpDirect } = await loadAcpSpawnModule();
         if (Array.isArray(attachments) && attachments.length > 0) {
