diff --git a/apps/server/src/preview/ServerBrowser.test.ts b/apps/server/src/preview/ServerBrowser.test.ts
index c32947b30b995..61af124efb55c 100644
--- a/apps/server/src/preview/ServerBrowser.test.ts
+++ b/apps/server/src/preview/ServerBrowser.test.ts
@@ -2525,3 +2525,237 @@ it.live("a full server takes back the slot of the longest idle agent tab nobody
     }),
   ).pipe(Effect.provide(layer)),
 );
+
+/** Makes the tab's next color scheme change hang in Playwright; resolves once it has started. */
+const stallColorScheme = (page: ReturnType<typeof makeContext>["page"]) => {
+  const started = Promise.withResolvers<void>();
+  page.emulateMedia.mockImplementationOnce(() => {
+    started.resolve();
+    return new Promise<void>(() => {});
+  });
+  return Effect.promise(() => started.promise);
+};
+
+it.effect("a stuck agent action times out on the server and later calls on its tab fail fast", () =>
+  Effect.scoped(
+    Effect.gen(function* () {
+      const { broker, tabId } = yield* ready;
+      const stalled = stallColorScheme(contexts[0]!.page);
+      const stuck = yield* broker
+        .invoke<void>({
+          scope,
+          tabId,
+          operation: "setColorScheme",
+          input: { colorScheme: "dark" },
+          timeoutMs: 1_000,
+        })
+        .pipe(Effect.flip, Effect.forkScoped);
+      yield* stalled;
+      // The broker evicts a host that has not answered by timeoutMs plus its one second grace.
+      yield* TestClock.adjust("2 seconds");
+      expect(yield* Fiber.join(stuck)).toMatchObject({ _tag: "PreviewAutomationTimeoutError" });
+      // The server answered in time, so the host every tab shares stays connected.
+      const status = yield* broker
+        .invoke({ scope, tabId, operation: "status", input: {} })
+        .pipe(Effect.exit);
+      expect(status).toMatchObject({ _tag: "Success" });
+      const busy = yield* broker
+        .invoke<void>({ scope, tabId, operation: "click", input: { ref: "e1" }, timeoutMs: 1_000 })
+        .pipe(Effect.flip);
+      expect(busy.message).toMatch(/busy|still running/i);
+    }),
+  ).pipe(Effect.provide(layer)),
+);
+
+it.effect("a tab stuck in an agent action does not hold back setting changes for other tabs", () =>
+  Effect.scoped(
+    Effect.gen(function* () {
+      const { broker, tabId } = yield* ready;
+      const manager = yield* Manager.PreviewManager;
+      const other = yield* broker.invoke<PreviewAutomationStatus>({
+        scope,
+        operation: "open",
+        input: { reuseExistingTab: false, show: false },
+      });
+      const stalled = stallColorScheme(contexts[0]!.page);
+      const applied = Promise.withResolvers<void>();
+      contexts[1]!.page.emulateMedia.mockImplementationOnce(async () => applied.resolve());
+      yield* broker
+        .invoke({
+          scope,
+          tabId,
+          operation: "setColorScheme",
+          input: { colorScheme: "dark" },
+          timeoutMs: 1_000,
+        })
+        .pipe(Effect.ignore, Effect.forkScoped);
+      // The stuck action has already published its own tab's resized event.
+      yield* stalled;
+      yield* manager.adjust({
+        threadId: scope.thread.threadId,
+        tabId: PreviewTabId.make(other.tabId!),
+        colorScheme: "dark",
+      });
+      yield* Effect.promise(() => applied.promise);
+      expect(contexts[1]!.page.emulateMedia).toHaveBeenCalledWith({ colorScheme: "dark" });
+    }),
+  ).pipe(Effect.provide(layer)),
+);
+
+it.effect("a dialog on a stuck tab is reported before the tab is busy", () =>
+  Effect.scoped(
+    Effect.gen(function* () {
+      const { broker, tabId } = yield* ready;
+      const page = contexts[0]!.page;
+      yield* broker.invoke({ scope, tabId, operation: "recordingStart", input: {} });
+      const stalled = stallColorScheme(page);
+      const stuck = yield* broker
+        .invoke<void>({
+          scope,
+          tabId,
+          operation: "setColorScheme",
+          input: { colorScheme: "dark" },
+          timeoutMs: 1_000,
+        })
+        .pipe(Effect.flip, Effect.forkScoped);
+      yield* stalled;
+      yield* TestClock.adjust("2 seconds");
+      expect(yield* Fiber.join(stuck)).toMatchObject({ _tag: "PreviewAutomationTimeoutError" });
+      page.emit("dialog", {
+        type: () => "alert",
+        message: () => "Saved",
+        defaultValue: () => "",
+        accept: vi.fn(async () => {}),
+        dismiss: vi.fn(async () => {}),
+      });
+      const blocked = yield* broker
+        .invoke<void>({
+          scope,
+          tabId,
+          operation: "click",
+          input: { ref: "e1" },
+          timeoutMs: 1_000,
+        })
+        .pipe(Effect.flip);
+      expect(blocked).toMatchObject({
+        _tag: "PreviewAutomationControlInterruptedError",
+        reason: "dialogPending",
+      });
+      const stop = yield* broker
+        .invoke<void>({ scope, tabId, operation: "recordingStop", input: {}, timeoutMs: 1_000 })
+        .pipe(Effect.flip);
+      expect(stop).toMatchObject({
+        _tag: "PreviewAutomationControlInterruptedError",
+        reason: "dialogPending",
+      });
+    }),
+  ).pipe(Effect.provide(layer)),
+);
+
+it.effect("an action that fails just after its timeout still reports its own failure", () =>
+  Effect.scoped(
+    Effect.gen(function* () {
+      const { broker, tabId } = yield* ready;
+      const failing = Promise.withResolvers<void>();
+      const started = Promise.withResolvers<void>();
+      contexts[0]!.page.emulateMedia.mockImplementationOnce(() => {
+        started.resolve();
+        return failing.promise;
+      });
+      const failed = yield* broker
+        .invoke<void>({
+          scope,
+          tabId,
+          operation: "setColorScheme",
+          input: { colorScheme: "dark" },
+          timeoutMs: 1_000,
+        })
+        .pipe(Effect.flip, Effect.forkScoped);
+      yield* Effect.promise(() => started.promise);
+      // Operations time themselves out a little after the request arrives; their reason wins.
+      yield* TestClock.adjust("1100 millis");
+      failing.reject(new Error("Target page crashed"));
+      expect(yield* Fiber.join(failed)).toMatchObject({ _tag: "PreviewAutomationExecutionError" });
+    }),
+  ).pipe(Effect.provide(layer)),
+);
+
+it.effect("a recording stop that never finishes leaves its tab busy", () =>
+  Effect.scoped(
+    Effect.gen(function* () {
+      const { broker, tabId } = yield* ready;
+      const context = contexts[0]!;
+      yield* broker.invoke({ scope, tabId, operation: "recordingStart", input: {} });
+      const started = Promise.withResolvers<void>();
+      context.sessions.at(-1)!.detach.mockImplementationOnce(() => {
+        started.resolve();
+        return new Promise<void>(() => {});
+      });
+      const stuck = yield* broker
+        .invoke<void>({ scope, tabId, operation: "recordingStop", input: {}, timeoutMs: 1_000 })
+        .pipe(Effect.flip, Effect.forkScoped);
+      yield* Effect.promise(() => started.promise);
+      yield* TestClock.adjust("2 seconds");
+      expect(yield* Fiber.join(stuck)).toMatchObject({ _tag: "PreviewAutomationTimeoutError" });
+      const busy = yield* broker
+        .invoke<void>({ scope, tabId, operation: "click", input: { ref: "e1" }, timeoutMs: 1_000 })
+        .pipe(Effect.flip, Effect.forkScoped);
+      // The host answers in arrival order, so a refused click has its answer before this one.
+      yield* broker.invoke({ scope, tabId, operation: "status", input: {} });
+      // A click that queued instead of being refused would time out here.
+      yield* TestClock.adjust("2 seconds");
+      expect(yield* Fiber.join(busy)).toMatchObject({
+        _tag: "PreviewAutomationControlInterruptedError",
+        reason: "busy",
+      });
+    }),
+  ).pipe(Effect.provide(layer)),
+);
+
+it.effect("a queued action whose timeout passes while it waits never runs", () =>
+  Effect.scoped(
+    Effect.gen(function* () {
+      const { broker, tabId } = yield* ready;
+      const page = contexts[0]!.page;
+      yield* broker.invoke({ scope, tabId, operation: "recordingStart", input: {} });
+      const stopRan = Promise.withResolvers<void>();
+      contexts[0]!.sessions.at(-1)!.detach.mockImplementationOnce(() => {
+        stopRan.resolve();
+        return new Promise<void>(() => {});
+      });
+      const started = Promise.withResolvers<void>();
+      const finish = Promise.withResolvers<void>();
+      page.emulateMedia.mockImplementationOnce(() => {
+        started.resolve();
+        return finish.promise;
+      });
+      const slow = yield* broker
+        .invoke<void>({
+          scope,
+          tabId,
+          operation: "setColorScheme",
+          input: { colorScheme: "dark" },
+          timeoutMs: 3_000,
+        })
+        .pipe(Effect.forkScoped);
+      yield* Effect.promise(() => started.promise);
+      const queued = yield* broker
+        .invoke<void>({ scope, tabId, operation: "click", input: { ref: "e1" }, timeoutMs: 1_000 })
+        .pipe(Effect.flip, Effect.forkScoped);
+      // The host answers in arrival order, so the click is queued once this answers.
+      yield* broker.invoke({ scope, tabId, operation: "status", input: {} });
+      yield* broker
+        .invoke({ scope, tabId, operation: "recordingStop", input: {}, timeoutMs: 1_000 })
+        .pipe(Effect.ignore, Effect.forkScoped);
+      yield* broker.invoke({ scope, tabId, operation: "status", input: {} });
+      // Past the click's timeout but before the server answers it.
+      yield* TestClock.adjust("1200 millis");
+      finish.resolve();
+      yield* Fiber.join(slow);
+      expect(yield* Fiber.join(queued)).toMatchObject({ _tag: "PreviewAutomationTimeoutError" });
+      expect(page.locator).not.toHaveBeenCalled();
+      // A late stop still runs, or capture would keep going with no recording to stop.
+      yield* Effect.promise(() => stopRan.promise);
+    }),
+  ).pipe(Effect.provide(layer)),
+);
diff --git a/apps/server/src/preview/ServerBrowser.ts b/apps/server/src/preview/ServerBrowser.ts
index ee31b17cc5889..7071a0d40c3d2 100644
--- a/apps/server/src/preview/ServerBrowser.ts
+++ b/apps/server/src/preview/ServerBrowser.ts
@@ -127,6 +127,8 @@ const EDITABLE_AT_POINT_SCRIPT = `(x, y) => {
 }`;
 const UNATTACHED_FILL_VIEWPORT = { width: 1280, height: 800 } as const;
 const NAVIGATION_TIMEOUT_MS = 15_000;
+// Must stay under the broker's 1 s host response grace.
+const HOST_ANSWER_MARGIN_MS = 500;
 const VIEWER_NAVIGATION_OPTIONS = { waitUntil: "commit", timeout: NAVIGATION_TIMEOUT_MS } as const;
 const ACTION_TIMELINE_LIMIT = 50;
 
@@ -1928,6 +1930,15 @@ const make = Effect.gen(function* () {
     );
   };
 
+  // Work blocked by a dialog outlives its request; point the agent at the dialog, not at `busy`.
+  const requireNoOverdueDialog = (tab: ServerTab, agentSessionId: string) => {
+    if (tab.dialog && tab.control.overdue && tab.control.agentCanActNow(agentSessionId))
+      throw new BrowserControlInterrupted({
+        message: "A browser dialog is pending. Read preview_status and use preview_dialog first.",
+        reason: "dialogPending",
+      });
+  };
+
   const requireTab = async (request: PreviewAutomationRequest) => {
     if (!request.agentSessionId)
       throw new BrowserControlInterrupted({
@@ -1976,7 +1987,7 @@ const make = Effect.gen(function* () {
 
   const runOperation = async (
     request: PreviewAutomationRequest,
-    deadlineMs: number,
+    signal: AbortSignal,
   ): Promise<unknown> => {
     const input = request.input;
     switch (request.operation) {
@@ -2101,7 +2112,7 @@ const make = Effect.gen(function* () {
             }
             return statusWithTitle(tab, request.agentSessionId);
           },
-          deadlineMs,
+          signal,
         );
       }
       case "recordingStop": {
@@ -2135,9 +2146,12 @@ const make = Effect.gen(function* () {
             message: "No recording is active for this thread.",
           });
         }
+        requireNoOverdueDialog(tab, request.agentSessionId ?? "");
         // A stop cannot act twice (a retry finds no recording);
         // dropping a late one would leave capture running.
-        return tab.control.agent(request.agentSessionId ?? "", () => stopRecording(tab));
+        return tab.control.agent(request.agentSessionId ?? "", () => stopRecording(tab), signal, {
+          runIfExpired: true,
+        });
       }
     }
     const tab = await requireTab(request);
@@ -2184,6 +2198,7 @@ const make = Effect.gen(function* () {
         return executeTabOperation(tab, request);
       });
     }
+    requireNoOverdueDialog(tab, agentSessionId);
     return tab.control.agent(
       agentSessionId,
       async () => {
@@ -2204,7 +2219,7 @@ const make = Effect.gen(function* () {
           if (generation !== tab.control.generation) ServerBrowserPage.invalidateRefs(tab.page);
         }
       },
-      deadlineMs,
+      signal,
     );
   };
 
@@ -2302,14 +2317,35 @@ const make = Effect.gen(function* () {
     }
   };
 
-  const handleRequest = (connectionId: string, request: PreviewAutomationRequest) =>
-    Effect.tryPromise({
-      try: () =>
-        runOperation(request, performance.now() + request.timeoutMs).finally(() =>
-          markUsed(request),
-        ),
+  const handleRequest = Effect.fn("ServerBrowser.handleRequest")(function* (
+    connectionId: string,
+    request: PreviewAutomationRequest,
+  ) {
+    // Queued work expires at the request's timeout; a started run past it marks its tab busy.
+    const expiry = yield* Scope.make();
+    const signal = yield* Effect.abortSignal.pipe(Scope.provide(expiry));
+    yield* Effect.sleep(request.timeoutMs).pipe(
+      Effect.andThen(Scope.close(expiry, Exit.void)),
+      Effect.forkChild,
+    );
+    const outcome = yield* Effect.tryPromise({
+      try: () => runOperation(request, signal).finally(() => markUsed(request)),
       catch: ServerBrowserPage.toOperationError,
     }).pipe(
+      // Lets the operation report its own timeout first, yet answers before the broker's
+      // response grace evicts this host. The abandoned run still holds its tab's queue.
+      Effect.timeoutOrElse({
+        duration: request.timeoutMs + HOST_ANSWER_MARGIN_MS,
+        orElse: () =>
+          Effect.fail(
+            new ServerBrowserPage.ServerBrowserOperationError({
+              tag: "PreviewAutomationTimeoutError",
+              message: `The browser did not finish ${request.operation} in ${request.timeoutMs}ms.`,
+              detail: { timeoutMs: request.timeoutMs },
+            }),
+          ),
+      }),
+      Effect.ensuring(Scope.close(expiry, Exit.void)),
       Effect.match({
         onSuccess: (result) => ({ ok: true as const, result }),
         onFailure: (error) => ({
@@ -2321,16 +2357,16 @@ const make = Effect.gen(function* () {
           },
         }),
       }),
-      Effect.flatMap((outcome) =>
-        broker.respond({
-          clientId: SERVER_HOST_CLIENT_ID,
-          connectionId,
-          requestId: request.requestId,
-          ...outcome,
-        }),
-      ),
-      Effect.ignore,
     );
+    yield* broker
+      .respond({
+        clientId: SERVER_HOST_CLIENT_ID,
+        connectionId,
+        requestId: request.requestId,
+        ...outcome,
+      })
+      .pipe(Effect.ignore);
+  });
 
   const mirrorManagerEvent = (event: PreviewEvent) =>
     Effect.promise(async () => {
@@ -2350,8 +2386,9 @@ const make = Effect.gen(function* () {
         closedSessions.delete(key);
         return;
       }
+      // Queued per tab at call time, so one stuck tab cannot hold back the others.
       if (event.type === "resized") {
-        await tab.control
+        void tab.control
           .system(async () => {
             if (
               !viewportSettingsEqual(event.snapshot.viewport ?? FILL_PREVIEW_VIEWPORT, tab.setting)
diff --git a/apps/server/src/preview/SessionControl.test.ts b/apps/server/src/preview/SessionControl.test.ts
index f47be6319cb81..c92d31b1c7702 100644
--- a/apps/server/src/preview/SessionControl.test.ts
+++ b/apps/server/src/preview/SessionControl.test.ts
@@ -344,6 +344,17 @@ describe("SessionControl", () => {
     expect(control.generation).toBe(1);
   });
 
+  it("rejects system work on a closed tab instead of throwing", async () => {
+    const control = new SessionControl("agent");
+    await control.close();
+    // Callers handle failures on the returned promise; a synchronous throw escapes them.
+    let applied: Promise<void> | undefined;
+    expect(() => {
+      applied = control.system(async () => {});
+    }).not.toThrow();
+    await expect(applied).rejects.toThrow("closed");
+  });
+
   it("lets any agent act on a tab no agent opened, but only while no human controls it", async () => {
     const control = new SessionControl(null);
     await expect(control.agent("agent-a", async () => "acted")).resolves.toBe("acted");
diff --git a/apps/server/src/preview/SessionControl.ts b/apps/server/src/preview/SessionControl.ts
index f19f566ed30bb..8f6cc0aebf6f6 100644
--- a/apps/server/src/preview/SessionControl.ts
+++ b/apps/server/src/preview/SessionControl.ts
@@ -34,6 +34,7 @@ export class SessionControl {
   private pending: Promise<void> = Promise.resolve();
   private readonly handoff = new Set<Promise<void>>();
   private owner: string | null = null;
+  private running: AbortSignal | undefined;
   private epoch = 0;
   private closed = false;
 
@@ -88,7 +89,17 @@ export class SessionControl {
     this.onGenerationChange();
   }
 
-  private async action<A>(allowed: () => boolean, run: () => Promise<A>, deadlineMs?: number) {
+  /** An agent run outlived its request; it keeps the queue until it settles. */
+  get overdue() {
+    return this.running?.aborted === true;
+  }
+
+  private async action<A>(
+    allowed: () => boolean,
+    run: () => Promise<A>,
+    signal?: AbortSignal,
+    runIfExpired = false,
+  ) {
     this.assertOpen();
     if (!allowed())
       throw new BrowserControlInterrupted({ message: "You do not control this browser tab." });
@@ -96,13 +107,18 @@ export class SessionControl {
     return this.enqueue(async () => {
       this.assertOpen();
       if (epoch !== this.epoch || !allowed()) throw new BrowserControlInterrupted();
-      if (deadlineMs !== undefined && performance.now() >= deadlineMs) {
+      if (signal?.aborted && !runIfExpired) {
         throw new DOMException(
           "This browser request expired before it could start.",
           "TimeoutError",
         );
       }
-      return run();
+      this.running = signal;
+      try {
+        return await run();
+      } finally {
+        this.running = undefined;
+      }
     });
   }
 
@@ -110,7 +126,13 @@ export class SessionControl {
     return this.agentId === null || this.agentId === agentId;
   }
 
-  agent<A>(agentId: string, run: () => Promise<A>, deadlineMs?: number) {
+  /** `runIfExpired` still starts work whose request expired while queued, for work that must not be dropped. */
+  agent<A>(
+    agentId: string,
+    run: () => Promise<A>,
+    signal?: AbortSignal,
+    { runIfExpired = false }: { readonly runIfExpired?: boolean } = {},
+  ) {
     if (!this.agentMayAct(agentId))
       return Promise.reject(
         new BrowserControlInterrupted({
@@ -125,7 +147,19 @@ export class SessionControl {
           reason: "humanControl",
         }),
       );
-    return this.action(() => this.agentMayAct(agentId) && this.owner === null, run, deadlineMs);
+    if (this.overdue)
+      return Promise.reject(
+        new BrowserControlInterrupted({
+          message: "An earlier action on this tab timed out and is still running.",
+          reason: "busy",
+        }),
+      );
+    return this.action(
+      () => this.agentMayAct(agentId) && this.owner === null,
+      run,
+      signal,
+      runIfExpired,
+    );
   }
 
   agentCanActNow(agentId: string) {
@@ -138,7 +172,7 @@ export class SessionControl {
     return run();
   }
 
-  system<A>(run: () => Promise<A>) {
+  async system<A>(run: () => Promise<A>) {
     this.assertOpen();
     return this.enqueue(async () => {
       this.assertOpen();
diff --git a/evals/mcp-tool-use/README.md b/evals/mcp-tool-use/README.md
index c5802a35af63b..ec3ab35266114 100644
--- a/evals/mcp-tool-use/README.md
+++ b/evals/mcp-tool-use/README.md
@@ -27,8 +27,9 @@ Cases are JSON lines in `cases/`, one file per area. The case schema in `run.ts`
 - `kind`: `bad` reproduces a mistake, `guard` protects correct behavior, `control` is ordinary work.
 - `experience`: the real mistake, in a sentence. `source`: where it happened.
 - `setup`: the fake thread, its `world` (repo, branch, files, PRs, pages), `priorTurns`, and `mocks` for tool results.
+- `setup.priorTurns[].toolCalls[].result`: a T3 preview error (`{"_tag": "PreviewAutomationControlInterruptedError", ...}`, every field the contracts class needs) replays as T3 sends it: an error result with the class's own message.
 - `setup.priorTurns[].toolCalls[].resultFile`: load a repository-relative file as the tool result instead of copying its contents into the case. An empty `prompt` resumes a top-level thread after its last prior tool result without adding a user message. Side-chat and subagent setups require a nonempty prompt.
-- `expect`: the checks, such as `calls`, `callsMustNot`, `maxCalls`, `order`, `shellMust`, `textMust`, `finalTextMust` (the last text, once the turn ended), and `linked`. A call's `args` match partially: `~text` is a case-insensitive substring, `"<=N"` / `">=N"` bound a number, `null` matches a missing key, and keys may be dotted paths. Device cases also use `device` to check every child brief, forbid raw UI commands, bound failed device actions per user turn, and require artifact paths in completed final answers. Secrets cases use `secret` (see [Secrets cases](#secrets-cases)).
+- `expect`: the checks, such as `calls`, `callsMustNot`, `maxCalls`, `order`, `firstCall` (the first call of any of its `tools` matches one of its `calls`), `shellMust`, `textMust`, `finalTextMust` (the last text, once the turn ended), and `linked`. A call's `args` match partially: `~text` is a case-insensitive substring, `"<=N"` / `">=N"` bound a number, `null` matches a missing key, and keys may be dotted paths. Device cases also use `device` to check every child brief, forbid raw UI commands, bound failed device actions per user turn, and require artifact paths in completed final answers. Secrets cases use `secret` (see [Secrets cases](#secrets-cases)).
 
 Check that the case builds with `--dry-run --case <id>`. Then run it on the text before your fix, where it must fail, and on the text after, where it must pass.
 
diff --git a/evals/mcp-tool-use/cases/visual-preview.ndjson b/evals/mcp-tool-use/cases/visual-preview.ndjson
index edd66f26966e9..33854939c12cb 100644
--- a/evals/mcp-tool-use/cases/visual-preview.ndjson
+++ b/evals/mcp-tool-use/cases/visual-preview.ndjson
@@ -34,3 +34,5 @@
 {"id": "browser-status-reuses-existing-tab", "area": "visual-preview", "kind": "control", "privacy": "commit-safe", "pattern": "usable Browser tab needs no new tab", "experience": "An already paired, usable tab should be reused during verification.", "source": {"report": "Maintainer browser verification failure, 2026-11-03"}, "setup": {"role": "top", "context": "The UI fix is complete. The test server is healthy at http://localhost:5173 and its browser profile is already paired. The current tab can be automated.", "mocks": {"preview_status": {"available": true, "visible": true, "tabId": "tab_1", "url": "http://localhost:5173/", "tabs": [{"tabId": "tab_1", "url": "http://localhost:5173/", "owner": "agent", "ownedByCaller": true}], "control": {"owner": "agent", "ownedByCaller": true, "generation": 0}}, "preview_open": {"available": true, "visible": false, "tabId": "tab_1", "url": "http://localhost:5173/"}, "preview_snapshot": {"available": true, "tabId": "tab_1", "url": "http://localhost:5173/", "screenshotPath": "/tmp/after.png"}}, "priorTurns": [{"role": "user", "text": "Use the Browser panel to verify the UI fix and capture an after screenshot."}, {"role": "assistant", "toolCalls": [{"tool": "Bash", "input": {"command": "cat .agents/skills/test-t3-app/SKILL.md"}, "result": "", "resultFile": ".agents/skills/test-t3-app/SKILL.md"}]}, {"role": "assistant", "toolCalls": [{"tool": "preview_status", "input": {}, "result": {"available": true, "visible": true, "tabId": "tab_1", "url": "http://localhost:5173/", "tabs": [{"tabId": "tab_1", "url": "http://localhost:5173/", "owner": "agent", "ownedByCaller": true}], "control": {"owner": "agent", "ownedByCaller": true, "generation": 0}}}]}], "harness": "claude"}, "prompt": "", "expect": {"calls": [{"tool": "preview_snapshot"}], "callsMustNot": [{"tool": "preview_open", "args": {"reuseExistingTab": false}}]}, "maxSteps": 10, "notes": "Checks reuse of an existing usable tab. Screenshot saving is outside this control. Prior tool-result replay is validated on the Claude harness; Codex output wrapping needs separate validation."}
 {"id": "browser-resume-after-unavailable-status", "area": "visual-preview", "kind": "bad", "privacy": "commit-safe", "pattern": "closed Browser panel treated as unavailable automation", "experience": "The agent read test-t3-app, saw available false with no tab, and asked the user to open the panel instead of calling preview_open. Opening a tab succeeded on the next turn.", "source": {"report": "Maintainer browser verification failure, 2026-11-03; fresh skill read followed by unavailable status. Opus replay reproduces the stop on the old skill."}, "setup": {"role": "top", "context": "Repository instructions: Before web or desktop UI verification, read .agents/skills/test-t3-app/SKILL.md and follow it. If a skill causes you to stop or leave requested work unfinished, link the SKILL.md, quote the relevant instruction, and explain how it applies.", "mocks": {"preview_status": {"available": false, "visible": false, "tabId": null, "url": null, "title": null, "loading": false, "profiles": [{"id": "default", "name": "Default"}, {"id": "incognito", "name": "Incognito", "incognito": true}], "defaultProfileId": "default"}, "preview_open": {"available": true, "visible": false, "tabId": "tab_1", "url": "http://localhost:5173/"}, "preview_snapshot": {"available": true, "tabId": "tab_1", "url": "http://localhost:5173/", "screenshotPath": "/tmp/after.png"}}, "world": {"repo": "acme/app", "branch": "fix/permissions-popup", "status": "", "files": {"apps/web/src/components/ComposerContextPermissions.tsx": "export function ComposerContextPermissions() { return <PermissionsMenu side=\"top\" align=\"end\" />; }\n"}, "description": "The requested fix is already committed as a1b2c3d on fix/permissions-popup, ahead of main. It changes ComposerContextPermissions.tsx to open the permissions popup above its trigger. Dependencies are installed. No development server is running and no .t3 directory exists.", "commits": [{"sha": "a1b2c3d", "subject": "fix(web): open composer permissions above its button"}]}, "priorTurns": [{"role": "user", "text": "When I click Full access in the composer, open its permissions menu directly above that button."}, {"role": "assistant", "toolCalls": [{"tool": "Bash", "input": {"command": "git commit -am \"fix(web): open composer permissions above its button\""}, "result": "[fix/permissions-popup a1b2c3d] fix(web): open composer permissions above its button\n 1 file changed, 3 insertions(+), 1 deletion(-)"}]}, {"role": "assistant", "text": "Implemented. Clicking Full access opens the permissions menu above the button. Focused checks pass. Full-client visual verification remains undone."}, {"role": "user", "text": "before/after screenshot"}, {"role": "assistant", "toolCalls": [{"tool": "Bash", "input": {"command": "cat .agents/skills/test-t3-app/SKILL.md"}, "result": "", "resultFile": ".agents/skills/test-t3-app/SKILL.md"}]}, {"role": "assistant", "text": "I'll use the test-t3-app skill to capture the before and after states in the app."}, {"role": "assistant", "toolCalls": [{"tool": "preview_status", "input": {}, "result": {"available": false, "visible": false, "tabId": null, "url": null, "title": null, "loading": false, "profiles": [{"id": "default", "name": "Default"}, {"id": "incognito", "name": "Incognito", "incognito": true}], "defaultProfileId": "default"}}]}], "shellMocks": [{"match": "git status|ls .*\\.t3", "output": "ls: .t3: No such file or directory"}], "harness": "claude"}, "prompt": "", "expect": {"calls": [{"tool": "preview_open"}]}, "maxSteps": 8, "notes": "Resume at the failed decision after reading the skill and receiving available=false. Opus reproduces the premature stop; Sonnet did not reliably reproduce it. This checks recovery through preview_open, not completion of screenshots. Prior tool-result replay is validated on the Claude harness; Codex output wrapping needs separate validation."}
 {"id": "revise-page-resends-html", "area": "visual-preview", "kind": "bad", "pattern": "revising a page on disk resends its full HTML to html_preview and html_render", "experience": "The pages already existed as generated files on disk, but the agent retyped each one into html_preview and again into html_render, about four minutes per call. To save time it published fixes without previewing them again.", "source": {"discussion": "https://github.com/pingdotgg/t3code/discussions/23303"}, "rule": "edit the .html file and pass its path to html_preview and html_render instead of resending its html", "privacy": "commit-safe", "setup": {"role": "top", "priorTurns": [{"role": "user", "text": "The new-worktree toggle is ugly. Come up with four better options and mock them."}, {"role": "assistant", "text": "Here are four options.", "toolCalls": [{"tool": "Write", "input": {"file_path": "/tmp/design/index.html", "content": "<!doctype html><html><head><meta charset=\"utf-8\"><style>body{margin:0;font-family:var(--font-sans)}.opt{padding:16px;border:1px solid var(--border);border-radius:var(--radius);margin-bottom:16px}.composer{border:1px solid var(--border);border-radius:8px;padding:10px;color:var(--muted-foreground)}.row{display:flex;gap:8px;align-items:center;margin-top:8px}button{border:1px solid var(--border);border-radius:6px;padding:4px 10px;background:transparent;color:inherit}</style></head><body>\n<div class=\"opt\"><h3>A. Checkbox under the composer</h3><div class=\"composer\">Ask anything</div><label class=\"row\"><input type=\"checkbox\"> New worktree</label></div>\n<div class=\"opt\"><h3>B. Toggle in the toolbar</h3><div class=\"row\"><button>Model</button><button>Mode</button><button aria-pressed=\"true\">Worktree: on</button></div><div class=\"composer\">Ask anything</div></div>\n<div class=\"opt\"><h3>C. Split send button</h3><div class=\"composer\">Ask anything</div><div class=\"row\"><button>Send</button><button>v</button><span>Send in new worktree</span></div></div>\n<div class=\"opt\"><h3>D. Two visible choices</h3><div class=\"composer\">Ask anything</div><div class=\"row\"><button>Send here</button><button>Send in new worktree</button></div></div>\n</body></html>\n"}, "result": "File created successfully at: /tmp/design/index.html"}, {"tool": "html_preview", "input": {"html": "<!doctype html><html><head><meta charset=\"utf-8\"><style>body{margin:0;font-family:var(--font-sans)}.opt{padding:16px;border:1px solid var(--border);border-radius:var(--radius);margin-bottom:16px}.composer{border:1px solid var(--border);border-radius:8px;padding:10px;color:var(--muted-foreground)}.row{display:flex;gap:8px;align-items:center;margin-top:8px}button{border:1px solid var(--border);border-radius:6px;padding:4px 10px;background:transparent;color:inherit}</style></head><body>\n<div class=\"opt\"><h3>A. Checkbox under the composer</h3><div class=\"composer\">Ask anything</div><label class=\"row\"><input type=\"checkbox\"> New worktree</label></div>\n<div class=\"opt\"><h3>B. Toggle in the toolbar</h3><div class=\"row\"><button>Model</button><button>Mode</button><button aria-pressed=\"true\">Worktree: on</button></div><div class=\"composer\">Ask anything</div></div>\n<div class=\"opt\"><h3>C. Split send button</h3><div class=\"composer\">Ask anything</div><div class=\"row\"><button>Send</button><button>v</button><span>Send in new worktree</span></div></div>\n<div class=\"opt\"><h3>D. Two visible choices</h3><div class=\"composer\">Ask anything</div><div class=\"row\"><button>Send here</button><button>Send in new worktree</button></div></div>\n</body></html>\n"}, "result": "{\"width\":728,\"contentHeight\":1840,\"capturedHeight\":1840,\"consoleMessages\":[]}"}, {"tool": "html_render", "input": {"html": "<!doctype html><html><head><meta charset=\"utf-8\"><style>body{margin:0;font-family:var(--font-sans)}.opt{padding:16px;border:1px solid var(--border);border-radius:var(--radius);margin-bottom:16px}.composer{border:1px solid var(--border);border-radius:8px;padding:10px;color:var(--muted-foreground)}.row{display:flex;gap:8px;align-items:center;margin-top:8px}button{border:1px solid var(--border);border-radius:6px;padding:4px 10px;background:transparent;color:inherit}</style></head><body>\n<div class=\"opt\"><h3>A. Checkbox under the composer</h3><div class=\"composer\">Ask anything</div><label class=\"row\"><input type=\"checkbox\"> New worktree</label></div>\n<div class=\"opt\"><h3>B. Toggle in the toolbar</h3><div class=\"row\"><button>Model</button><button>Mode</button><button aria-pressed=\"true\">Worktree: on</button></div><div class=\"composer\">Ask anything</div></div>\n<div class=\"opt\"><h3>C. Split send button</h3><div class=\"composer\">Ask anything</div><div class=\"row\"><button>Send</button><button>v</button><span>Send in new worktree</span></div></div>\n<div class=\"opt\"><h3>D. Two visible choices</h3><div class=\"composer\">Ask anything</div><div class=\"row\"><button>Send here</button><button>Send in new worktree</button></div></div>\n</body></html>\n", "title": "New worktree: four options", "height": 1840}, "result": "{\"htmlRender\":{\"attachmentId\":\"att-1\",\"title\":\"New worktree: four options\",\"height\":1840},\"message\":\"Shown to the reader above your reply. Don't mention or describe the page; reply with only what it doesn't already say.\"}"}]}], "mocks": {"html_preview": {"width": 728, "contentHeight": 1840, "capturedHeight": 1840, "consoleMessages": []}, "html_render": {"htmlRender": {"attachmentId": "att-1", "title": "New worktree: four options", "height": 1840}, "message": "Shown to the reader above your reply. Don't mention or describe the page; reply with only what it doesn't already say."}}, "world": {"files": {"/tmp/design/index.html": "<!doctype html><html><head><meta charset=\"utf-8\"><style>body{margin:0;font-family:var(--font-sans)}.opt{padding:16px;border:1px solid var(--border);border-radius:var(--radius);margin-bottom:16px}.composer{border:1px solid var(--border);border-radius:8px;padding:10px;color:var(--muted-foreground)}.row{display:flex;gap:8px;align-items:center;margin-top:8px}button{border:1px solid var(--border);border-radius:6px;padding:4px 10px;background:transparent;color:inherit}</style></head><body>\n<div class=\"opt\"><h3>A. Checkbox under the composer</h3><div class=\"composer\">Ask anything</div><label class=\"row\"><input type=\"checkbox\"> New worktree</label></div>\n<div class=\"opt\"><h3>B. Toggle in the toolbar</h3><div class=\"row\"><button>Model</button><button>Mode</button><button aria-pressed=\"true\">Worktree: on</button></div><div class=\"composer\">Ask anything</div></div>\n<div class=\"opt\"><h3>C. Split send button</h3><div class=\"composer\">Ask anything</div><div class=\"row\"><button>Send</button><button>v</button><span>Send in new worktree</span></div></div>\n<div class=\"opt\"><h3>D. Two visible choices</h3><div class=\"composer\">Ask anything</div><div class=\"row\"><button>Send here</button><button>Send in new worktree</button></div></div>\n</body></html>\n"}}}, "prompt": "Make the Send in new worktree button in option C blue, and show me the page again.", "expect": {"calls": [{"tool": "html_preview", "args": {"path": "/tmp/design/index.html", "html": null}}, {"tool": "html_render", "args": {"path": "/tmp/design/index.html", "html": null}}], "callsMustNot": [{"tool": "html_preview", "args": {"html": "~Ask anything"}}, {"tool": "html_render", "args": {"html": "~Ask anything"}}], "order": ["html_preview", "html_render"]}, "notes": "On main neither tool has a path parameter."}
+{"id": "preview-busy-tab-opens-new-tab", "area": "visual-preview", "kind": "bad", "pattern": "preview tab stuck behind a timed-out action; the next preview call retries the same tab", "experience": "A server-browser action never finished, so every later call on that tab timed out with the same generic error and the agent kept retrying the stuck tab.", "rule": "On a busy tab, continue in a new tab (preview_open with reuseExistingTab=false) or close it with t3_preview_close; do not retry it.", "privacy": "commit-safe", "source": {"issue": "pingdotgg/t3code#32849: a browser action that never finishes wedges its tab; the agent only sees a generic timeout and keeps retrying the same tab"}, "setup": {"role": "top", "context": "The dev server is healthy at http://localhost:5173. The agent opened preview tab tab_1 (owner agent) earlier in this turn.", "priorTurns": [{"role": "user", "text": "Open http://localhost:5173/settings in the browser, switch it to dark mode, click Save, and check that the settings-saved toast still reads well in dark mode."}, {"role": "assistant", "toolCalls": [{"tool": "preview_open", "input": {"url": "http://localhost:5173/settings"}, "result": {"available": true, "visible": true, "tabId": "tab_1", "url": "http://localhost:5173/settings", "title": "Settings - Acme", "loading": false}}, {"tool": "preview_set_appearance", "input": {"tabId": "tab_1", "colorScheme": "dark"}, "result": {"_tag": "PreviewAutomationTimeoutError", "operation": "setColorScheme", "environmentId": "eeeeeeee-eeee-4eee-8eee-eeeeeeeeeeee", "threadId": "thread-settings-check", "providerSessionId": "session-1", "providerInstanceId": "claudeAgent", "clientId": "server-browser", "connectionId": "conn-1", "requestId": "req-2", "tabId": "tab_1", "timeoutMs": 15000}}, {"tool": "preview_act", "input": {"tabId": "tab_1", "action": "click", "locator": "aria-ref=e12", "timeoutMs": 3000}, "result": {"_tag": "PreviewAutomationControlInterruptedError", "operation": "click", "environmentId": "eeeeeeee-eeee-4eee-8eee-eeeeeeeeeeee", "threadId": "thread-settings-check", "providerSessionId": "session-1", "providerInstanceId": "claudeAgent", "clientId": "server-browser", "connectionId": "conn-1", "requestId": "req-3", "tabId": "tab_1", "timeoutMs": 3000, "remoteTag": "BrowserControlInterrupted", "remoteMessageLength": 0, "cause": null, "reason": "busy"}}]}], "mocks": {"preview_open": {"available": true, "visible": false, "tabId": "tab_2", "url": "http://localhost:5173/settings", "title": "Settings - Acme", "loading": false}, "preview_status": {"available": true, "visible": true, "tabId": "tab_1", "url": "http://localhost:5173/settings", "title": "Settings - Acme", "loading": false, "tabs": [{"tabId": "tab_1", "url": "http://localhost:5173/settings", "owner": "agent", "ownedByCaller": true}], "control": {"owner": "agent", "ownedByCaller": true, "generation": 1}}, "t3_preview_list": {"tabs": [{"tabId": "tab_1", "url": "http://localhost:5173/settings", "owner": "agent"}]}, "preview_snapshot": {"available": true, "tabId": "tab_2", "url": "http://localhost:5173/settings", "title": "Settings - Acme", "snapshot": "- heading \"Settings\" [level=1]\n- switch \"Dark mode\" [checked] [ref=e7]\n- button \"Save\" [ref=e12]"}}}, "prompt": "", "expect": {"firstCall": {"tools": ["preview_act", "preview_resize", "preview_navigate", "preview_snapshot", "preview_wait_for", "preview_evaluate", "preview_set_appearance", "preview_open", "t3_preview_close"], "calls": [{"tool": "preview_open", "args": {"reuseExistingTab": false}}, {"tool": "t3_preview_close", "args": {"tabId": "tab_1"}}]}, "callsMustNot": [{"tool": "preview_act|preview_resize|preview_navigate|preview_snapshot|preview_wait_for|preview_evaluate|preview_set_appearance", "args": {"tabId": "tab_1"}}]}, "maxSteps": 6, "notes": "The prior preview_act result is the contracts busy error, rendered with its real message. preview_status and t3_preview_list are reads, so they may come first."}
+{"id": "preview-wait-timeout-keeps-tab", "area": "visual-preview", "kind": "control", "pattern": "an ordinary wait_for timeout; the agent keeps working in the same tab", "experience": "Guards #32849's busy-tab recovery: a plain timeout on a responsive tab is a page result, not a reason to abandon the tab.", "rule": "After an ordinary preview timeout, inspect or retry in the same tab.", "privacy": "commit-safe", "source": {"issue": "pingdotgg/t3code#32849 (control)"}, "setup": {"role": "top", "context": "The dev server is healthy at http://localhost:5173. The agent opened preview tab tab_1 (owner agent) earlier in this turn.", "priorTurns": [{"role": "user", "text": "Open http://localhost:5173/settings in the browser, switch it to dark mode, click Save, and check that the settings-saved toast still reads well in dark mode."}, {"role": "assistant", "toolCalls": [{"tool": "preview_open", "input": {"url": "http://localhost:5173/settings"}, "result": {"available": true, "visible": true, "tabId": "tab_1", "url": "http://localhost:5173/settings", "title": "Settings - Acme", "loading": false}}, {"tool": "preview_set_appearance", "input": {"tabId": "tab_1", "colorScheme": "dark"}, "result": {"available": true, "tabId": "tab_1", "colorScheme": "dark"}}, {"tool": "preview_act", "input": {"tabId": "tab_1", "action": "click", "locator": "aria-ref=e12"}, "result": {"available": true, "tabId": "tab_1", "url": "http://localhost:5173/settings"}}, {"tool": "preview_wait_for", "input": {"tabId": "tab_1", "text": "Settings saved", "timeoutMs": 5000}, "result": {"_tag": "PreviewAutomationTimeoutError", "operation": "waitFor", "environmentId": "eeeeeeee-eeee-4eee-8eee-eeeeeeeeeeee", "threadId": "thread-settings-check", "providerSessionId": "session-1", "providerInstanceId": "claudeAgent", "clientId": "server-browser", "connectionId": "conn-1", "requestId": "req-4", "tabId": "tab_1", "timeoutMs": 5000}}]}], "mocks": {"preview_open": {"available": true, "visible": false, "tabId": "tab_2", "url": "http://localhost:5173/settings", "title": "Settings - Acme", "loading": false}, "preview_status": {"available": true, "visible": true, "tabId": "tab_1", "url": "http://localhost:5173/settings", "title": "Settings - Acme", "loading": false, "tabs": [{"tabId": "tab_1", "url": "http://localhost:5173/settings", "owner": "agent", "ownedByCaller": true}], "control": {"owner": "agent", "ownedByCaller": true, "generation": 1}}, "t3_preview_list": {"tabs": [{"tabId": "tab_1", "url": "http://localhost:5173/settings", "owner": "agent"}]}, "preview_snapshot": {"available": true, "tabId": "tab_1", "url": "http://localhost:5173/settings", "title": "Settings - Acme", "snapshot": "- heading \"Settings\" [level=1]\n- switch \"Dark mode\" [checked] [ref=e7]\n- button \"Save\" [ref=e12]\n- status: \"Saved\""}}}, "prompt": "", "expect": {"firstCall": {"tools": ["preview_act", "preview_resize", "preview_navigate", "preview_snapshot", "preview_wait_for", "preview_evaluate", "preview_set_appearance", "preview_open", "t3_preview_close"], "calls": [{"tool": "preview_act|preview_resize|preview_navigate|preview_snapshot|preview_wait_for|preview_evaluate|preview_set_appearance", "args": {"tabId": "tab_1"}}, {"tool": "preview_act|preview_resize|preview_navigate|preview_snapshot|preview_wait_for|preview_evaluate|preview_set_appearance", "args": {"tabId": null}}]}}, "maxSteps": 6}
diff --git a/evals/mcp-tool-use/run.test.ts b/evals/mcp-tool-use/run.test.ts
index 6e9413ed9141e..e48ccb329ee2b 100644
--- a/evals/mcp-tool-use/run.test.ts
+++ b/evals/mcp-tool-use/run.test.ts
@@ -7,7 +7,12 @@ import * as Effect from "effect/Effect";
 import * as Schema from "effect/Schema";
 import { FetchHttpClient } from "effect/http";
 import { assert, expect, it } from "@effect/vitest";
-import { ComposerContextId, EnvironmentId, ThreadId } from "@t3tools/contracts";
+import {
+  ComposerContextId,
+  EnvironmentId,
+  PreviewAutomationControlInterruptedError,
+  ThreadId,
+} from "@t3tools/contracts";
 import { projectComposerContextForProvider } from "@t3tools/shared/composerContextReferences";
 
 import {
@@ -244,6 +249,10 @@ it("passes a good run and names every rule a bad run breaks", () => {
     callsMustNot: [{ tool: "delegate_task", args: { runtimeMode: "approval-required" } }],
     notCalls: ["t3_thread_send"],
     order: ["orchestrator_capabilities", "delegate_task"],
+    firstCall: {
+      tools: ["delegate_task", "t3_thread_send"],
+      calls: [{ tool: "t3_thread_send" }, { tool: "delegate_task", args: { task: "~FINDING" } }],
+    },
     minCalls: { orchestrator_capabilities: 1 },
     maxCalls: { task_status: 0 },
     endsTurnAfter: "delegate_task",
@@ -287,6 +296,7 @@ it("passes a good run and names every rule a bad run breaks", () => {
     'callsMustNot delegate_task {"runtimeMode":"approval-required"}',
     "notCalls t3_thread_send",
     "order orchestrator_capabilities > delegate_task",
+    'firstCall t3_thread_send or delegate_task {"task":"~FINDING"}',
     "minCalls orchestrator_capabilities >= 1",
     "maxCalls task_status <= 0",
     "endsTurnAfter delegate_task",
@@ -578,6 +588,24 @@ it("lists link tools only while a case turns the beta on, and sends chips as T3
   assert.include(prompt?.text, "environmentId: environment-vps");
 });
 
+const decodeInterrupted = Schema.decodeUnknownSync(PreviewAutomationControlInterruptedError);
+it("replays an earlier preview error as T3's error result, with its contracts message", () => {
+  const base = buildBase(template, { model: "claude-opus-5-5", keepUserInstructions: false });
+  const busyCase = NodeFS.readFileSync(
+    new URL("./cases/visual-preview.ndjson", import.meta.url),
+    "utf8",
+  )
+    .trim()
+    .split("\n")
+    .map((line) => decodeDeviceCase(line))
+    .find((c) => c.id === "preview-busy-tab-opens-new-tab")!;
+  const prior = busyCase.setup.priorTurns!.at(-1)!.toolCalls!.at(-1)!;
+  const error = decodeInterrupted(prior.result);
+  assert.equal(error.reason, "busy");
+  const replayed = buildMessages(makeSample(busyCase, base)).at(-1)?.content.at(-1);
+  assert.deepInclude(replayed, { content: error.message, is_error: true });
+});
+
 it("replays earlier capability results with the model under test as the caller", () => {
   const base = buildBase(template, { model: "claude-opus-5-5", keepUserInstructions: false });
   const capabilities = {
diff --git a/evals/mcp-tool-use/run.ts b/evals/mcp-tool-use/run.ts
index 7a7a8400fe612..0673c960552c1 100644
--- a/evals/mcp-tool-use/run.ts
+++ b/evals/mcp-tool-use/run.ts
@@ -18,6 +18,7 @@ import {
   DeviceOperationError,
   DeviceSummary,
   OrchestratorMcpTaskRole,
+  PreviewAutomationError,
   ProviderInstanceId,
 } from "@t3tools/contracts";
 import * as Cause from "effect/Cause";
@@ -193,6 +194,10 @@ const McpToolUseExpect = Schema.Struct({
   notCalls: Schema.optional(Schema.Array(Schema.String)),
   /** First calls of these tools happen in this order. */
   order: Schema.optional(Schema.Array(Schema.String)),
+  /** The first call of any tool in `tools` matches one of `calls`. */
+  firstCall: Schema.optional(
+    Schema.Struct({ tools: Schema.Array(Schema.String), calls: Schema.Array(McpToolUseCall) }),
+  ),
   minCalls: Schema.optional(Schema.Record(Schema.String, Schema.Number)),
   maxCalls: Schema.optional(Schema.Record(Schema.String, Schema.Number)),
   /**
@@ -393,6 +398,10 @@ const toolResult = (id: string, content: unknown): Block => ({
   tool_use_id: id,
   content: valueText(content),
 });
+const decodePreviewError = Schema.decodeUnknownOption(PreviewAutomationError);
+/** A prior result that is a T3 preview error, `{"_tag": "PreviewAutomationTimeoutError", ...}`. */
+const previewFailure = (value: unknown) =>
+  Option.getOrUndefined(Option.map(decodePreviewError(value), (error) => error.message));
 const texts = (content: typeof Content.Type) =>
   typeof content === "string" ? [content] : content.flatMap((block) => block.text ?? []);
 const isT3Name = (name: string) => name.startsWith(T3_PREFIX) || name.startsWith("mcp__t3_code__");
@@ -1013,7 +1022,14 @@ export const buildMessages = (sample: Sample) => {
         normalizeToolName(tool.name) === "orchestrator_capabilities"
           ? capabilities(sample, result)
           : (sample.priorResults.get(prior) ?? result);
-      push("user", toolResult(id, replayed));
+      // T3 sends a preview tool's failure as an error result with only its message.
+      const failure = previewFailure(replayed);
+      push(
+        "user",
+        failure === undefined
+          ? toolResult(id, replayed)
+          : { ...toolResult(id, failure), is_error: true },
+      );
     }
     if (turn.text) push(turn.role, text(turn.text));
   }
@@ -1796,6 +1812,12 @@ export const gradeSample = (
     const inOrder = firsts.every((at, i) => at > (firsts[i - 1] ?? -1));
     add(`order ${expect.order.join(" > ")}`, inOrder);
   }
+  if (expect.firstCall !== undefined) {
+    const { tools, calls: allowed } = expect.firstCall;
+    const first = calls.find((call) => tools.includes(call.tool));
+    const ok = first !== undefined && allowed.some((call) => found(call, [first]));
+    add(`firstCall ${allowed.map(label).join(" or ")}`, ok);
+  }
   for (const [t, n] of Object.entries(expect.minCalls ?? {}))
     add(`minCalls ${t} >= ${n}`, count(t) >= n);
   for (const [t, n] of Object.entries(expect.maxCalls ?? {}))
diff --git a/packages/contracts/src/previewAutomation.ts b/packages/contracts/src/previewAutomation.ts
index 8c038f4fe81cc..5ef8ff6e7926e 100644
--- a/packages/contracts/src/previewAutomation.ts
+++ b/packages/contracts/src/previewAutomation.ts
@@ -792,6 +792,7 @@ export const PreviewAutomationControlReason = Schema.Literals([
   "interrupted",
   "dialogPending",
   "tabLimit",
+  "busy",
 ]);
 export type PreviewAutomationControlReason = typeof PreviewAutomationControlReason.Type;
 
@@ -814,6 +815,8 @@ export class PreviewAutomationControlInterruptedError extends Schema.TaggedError
       return "A browser dialog is pending. Read preview_status and resolve it with preview_dialog.";
     if (this.reason === "tabLimit")
       return "Too many server browser tabs are open. Close one with t3_preview_close, or reuse a tabId from preview_status tabs.";
+    if (this.reason === "busy")
+      return "This browser tab is busy with an earlier action that timed out. Open a new tab with preview_open and reuseExistingTab=false, or close this one with t3_preview_close.";
     if (this.reason === "closed") return "This browser tab is closed. Call preview_open.";
     if (this.reason === "interrupted")
       return "Browser control changed. Take a fresh snapshot before trying again.";