fix: verify lazy native provider launch during restored model selection

packages/paperclip-runner/runner/crates/runner-core/src/acpx_sidecar_transport.rs

@@ -642,6 +642,9 @@ fn response_error_classification(error: &ResponseError) -> &'static str {
         _ => {}
     }
     match error.message.as_str() {
+        "ACPX provider spawned after ownership admission was sealed" => {
+            "provider_spawn_after_ownership_seal"
+        }
         "ACPX recovery identity conflicts with the immutable session configuration" => {
             "recovery_configuration_mismatch"
         }

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

@@ -1311,6 +1311,36 @@ describe("Codex ACPX runtime adapter", () => {
     });
   });
 
+  it("verifies a lazy recovered provider spawned by model selection before returning", async () => {
+    const runtime = fakeRuntime();
+    const command = fakeCommand();
+    vi.mocked(command.spawn).mockReturnValue(fakeChild());
+    let runtimeOptions: AcpRuntimeOptions | undefined;
+    let acknowledgeOwnership!: () => void;
+    const ownership = new Promise<void>((resolve) => { acknowledgeOwnership = resolve; });
+    vi.mocked(runtime.setConfigOption!).mockImplementation(async () => {
+      await Promise.resolve();
+      runtimeOptions?.spawnAgent?.({ command: "ignored", args: ["--stdio"], options: {} });
+    });
+    const port = await openCodexAcpxRuntime(openOptions(command), {
+      createRegistry: () => registry(), createStore: () => store(),
+      awaitProviderOwnership: () => ownership,
+      awaitProviderExit: providerOwnershipEstablished,
+      createRuntime: (options) => { runtimeOptions = options; return runtime; },
+    });
+    let admitted = false;
+    const selection = port.setModel!("gpt-5.6-sol").then(() => { admitted = true; });
+    void selection.catch(() => undefined);
+    await vi.waitFor(() => expect(command.spawn).toHaveBeenCalledOnce());
+    expect(admitted).toBe(false);
+    acknowledgeOwnership();
+    await selection;
+    expect(admitted).toBe(true);
+    expect(() => runtimeOptions?.spawnAgent?.({ command: "ignored", args: [], options: {} }))
+      .toThrow("provider spawned after ownership admission was sealed");
+    await port.close({ reason: "test complete" });
+  });
+
   it("admits a verified provider that starts with the first recovered turn", async () => {
     const runtime = fakeRuntime();
     const child = fakeChild();

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

@@ -1156,11 +1156,20 @@ function runtimePort(
     ...(runtime.setConfigOption
       ? {
           async setModel(model: string) {
-            await runtime.setConfigOption?.({
-              handle,
-              key: "model",
-              value: model,
-            });
+            // A restored handle can be lazy: selecting the pinned model may
+            // launch its first provider before any prompt. Admit that spawn
+            // only for this control call, and verify ownership before return.
+            const finishOwnershipAdmission =
+              children.beginLifetimeOwnershipAdmission();
+            try {
+              await runtime.setConfigOption?.({
+                handle,
+                key: "model",
+                value: model,
+              });
+            } finally {
+              await finishOwnershipAdmission();
+            }
           },
         }
       : {}),