diff --git a/src/agents/tools/sessions-spawn-tool.test.ts b/src/agents/tools/sessions-spawn-tool.test.ts
index d406ccb1759..4fe106a7ebd 100644
--- a/src/agents/tools/sessions-spawn-tool.test.ts
+++ b/src/agents/tools/sessions-spawn-tool.test.ts
@@ -185,6 +185,21 @@ describe("sessions_spawn tool", () => {
     );
   });
 
+  it("rejects resumeSessionId without runtime=acp", async () => {
+    const tool = createSessionsSpawnTool({
+      agentSessionKey: "agent:main:main",
+    });
+
+    const result = await tool.execute("call-guard", {
+      task: "resume prior work",
+      resumeSessionId: "7f4a78e0-f6be-43fe-855c-c1c4fd229bc4",
+    });
+
+    expect(JSON.stringify(result)).toContain("resumeSessionId is only supported for runtime=acp");
+    expect(hoisted.spawnSubagentDirectMock).not.toHaveBeenCalled();
+    expect(hoisted.spawnAcpDirectMock).not.toHaveBeenCalled();
+  });
+
   it("rejects attachments for ACP runtime", async () => {
     const tool = createSessionsSpawnTool({
       agentSessionKey: "agent:main:main",
diff --git a/src/agents/tools/sessions-spawn-tool.ts b/src/agents/tools/sessions-spawn-tool.ts
index 30ca08bc306..b735084d2b0 100644
--- a/src/agents/tools/sessions-spawn-tool.ts
+++ b/src/agents/tools/sessions-spawn-tool.ts
@@ -28,7 +28,7 @@ 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/). 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/). Requires runtime="acp". The agent replays conversation history via session/load instead of starting fresh.',
     }),
   ),
   model: Type.Optional(Type.String()),
@@ -134,6 +134,13 @@ export function createSessionsSpawnTool(
         });
       }
 
+      if (resumeSessionId && runtime !== "acp") {
+        return jsonResult({
+          status: "error",
+          error: `resumeSessionId is only supported for runtime=acp; got runtime=${runtime}`,
+        });
+      }
+
       if (runtime === "acp") {
         if (Array.isArray(attachments) && attachments.length > 0) {
           return jsonResult({