fix(runner): restore ACPX backend identity fallback

packages/paperclip-runner/src/drivers/acpx/codex-runtime-adapter.test.ts

@@ -1428,26 +1428,45 @@ describe("Codex ACPX runtime adapter", () => {
     },
   );
 
-  it("fails closed and closes the session when ACPX omits recovery identity", async () => {
+  it("uses the real ACP session when no second agent identity is advertised", async () => {
     const runtime = fakeRuntime({ ...HANDLE, agentSessionId: undefined });
-    await expect(
-      openCodexAcpxRuntime(openOptions(fakeCommand()), {
-        createRegistry: () => registry(),
-        createStore: () => store(),
-        createRuntime: () => runtime,
-      }),
-    ).rejects.toThrow("ACPX runtime omitted agentSessionId");
-    expect(runtime.close).toHaveBeenCalledWith({
-      handle: { ...HANDLE, agentSessionId: undefined },
-      reason: "ACPX runtime identity validation failed",
-      discardPersistentState: false,
+    const durableStore: AcpSessionStore = {
+      load: vi.fn(async () =>
+        structuredClone({
+          acpxRecordId: "record-1",
+          acpSessionId: "backend-1",
+          acpx: { current_model_id: "gpt-5.6-sol" },
+        } as never),
+      ),
+      save: vi.fn(),
+    };
+    const port = await openCodexAcpxRuntime(openOptions(fakeCommand()), {
+      createRegistry: () => registry(),
+      createStore: () => durableStore,
+      createRuntime: () => runtime,
     });
+
+    await expect(port.identity()).resolves.toEqual({
+      acpxRecordId: "record-1",
+      backendSessionId: "backend-1",
+      agentSessionId: "backend-1",
+    });
+    await expect(port.getStatus()).resolves.toMatchObject({
+      backendSessionId: "backend-1",
+      agentSessionId: "backend-1",
+      models: { currentModelId: "gpt-5.6-sol" },
+    });
+    expect(runtime.close).not.toHaveBeenCalled();
   });
 
   it("bounds invalid-identity cleanup before terminating the provider", async () => {
     vi.useFakeTimers();
     try {
-      const runtime = fakeRuntime({ ...HANDLE, agentSessionId: undefined });
+      const runtime = fakeRuntime({
+        ...HANDLE,
+        backendSessionId: undefined,
+        agentSessionId: undefined,
+      });
       vi.mocked(runtime.close).mockImplementation(
         () => new Promise<void>(() => undefined),
       );
@@ -1466,7 +1485,11 @@ describe("Codex ACPX runtime adapter", () => {
               args: ["--stdio"],
               options: {},
             });
-            return { ...HANDLE, agentSessionId: undefined };
+            return {
+              ...HANDLE,
+              backendSessionId: undefined,
+              agentSessionId: undefined,
+            };
           }),
         }),
       });

packages/paperclip-runner/src/drivers/acpx/codex-runtime-adapter.ts

@@ -1048,10 +1048,12 @@ async function persistedRuntimeStatus(
       { code: "ACPX_PERSISTED_SESSION_MISSING" },
     );
   }
+  const persistedAgentSessionId =
+    nonEmptyRuntimeIdentity(record.agentSessionId) ?? record.acpSessionId;
   if (
     record.acpxRecordId !== identity.acpxRecordId ||
     record.acpSessionId !== identity.backendSessionId ||
-    record.agentSessionId !== identity.agentSessionId
+    persistedAgentSessionId !== identity.agentSessionId
   ) {
     throw Object.assign(
       new Error("The persisted ACPX session identity changed after admission"),
@@ -1064,12 +1066,12 @@ async function persistedRuntimeStatus(
     summary: [
       `session=${record.acpxRecordId}`,
       `backendSessionId=${record.acpSessionId}`,
-      `agentSessionId=${record.agentSessionId}`,
+      `agentSessionId=${persistedAgentSessionId}`,
       record.closed === true ? "closed" : "open",
     ].join(" "),
     acpxRecordId: record.acpxRecordId,
     backendSessionId: record.acpSessionId,
-    agentSessionId: record.agentSessionId,
+    agentSessionId: persistedAgentSessionId,
     ...(currentModelId === undefined && !availableModelIds?.length
       ? {}
       : {
@@ -1491,17 +1493,21 @@ function pushUnique(errors: unknown[], error: unknown): void {
 }
 
 function requireIdentity(handle: AcpRuntimeHandle): AcpxRuntimePortIdentity {
-  const identity = {
-    acpxRecordId: handle.acpxRecordId,
-    backendSessionId: handle.backendSessionId,
-    agentSessionId: handle.agentSessionId,
-  };
-  for (const [name, value] of Object.entries(identity)) {
-    if (typeof value !== "string" || value.length === 0) {
-      throw new Error(`ACPX runtime omitted ${name}`);
-    }
+  const acpxRecordId = nonEmptyRuntimeIdentity(handle.acpxRecordId);
+  if (!acpxRecordId) throw new Error("ACPX runtime omitted acpxRecordId");
+  const backendSessionId = nonEmptyRuntimeIdentity(handle.backendSessionId);
+  if (!backendSessionId) {
+    throw new Error("ACPX runtime omitted backendSessionId");
   }
-  return identity as AcpxRuntimePortIdentity;
+  return {
+    acpxRecordId,
+    backendSessionId,
+    // ACPX agents do not all advertise a distinct native thread identity.
+    // In that case the backend ID is the real ACP protocol session, so retain
+    // it explicitly rather than inventing a Paperclip-owned identifier.
+    agentSessionId:
+      nonEmptyRuntimeIdentity(handle.agentSessionId) ?? backendSessionId,
+  };
 }
 
 function definedEnvironment(