[Bug]: preview_snapshot always times out for tabs in delegated child threads and for open:false tabs (macOS 0.1.1) #23936

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

This is the remaining part of #16567, which was closed with the note that background-tab snapshots can still time out because the underlying screenshot capture remains unbounded. The Windows reports (#23139, #17483) describe the same mechanism; this is the macOS / delegated-child case.

Area

apps/desktop

Steps to reproduce

Same tiny static page, all within ~3 minutes:

1. preview_open({url:"http://127.0.0.1:8765/tiny.html", reuseExistingTab:false, open:false})
2. preview_snapshot({tabId}) fails after about 10 s.
3. preview_open({tabId, open:true}), then preview_snapshot({tabId}) succeeds.
4. Have a delegate_task child run steps 1–3. Step 3 still fails, because the child thread is never on screen.

Second repro on a public site: I gave one Claude child (Sonnet 5.5) and one Codex child (GPT-6.1 Sol) the same QA task on https://todomvc.com/examples/react/dist/ without naming any tool. Both chose T3's preview tools first; preview_open and preview_evaluate worked, but preview_snapshot timed out every time (Claude 3 of 3, Codex 2 of 2), and both had to fall back to an external headless browser to finish.

Expected behavior

Snapshots work for background tabs and for tabs opened by delegated child threads. The preview_open description says open=false is for "background-only automation". If that can't be supported, fail immediately with a clear error (like the existing capture-unavailable "present the target tab" message) and document the limitation.

Actual behavior

preview_snapshot succeeds only when the tab is shown in the inline preview of the thread on screen. Otherwise it waits ~10 s and fails with Preview snapshot failed: Preview automation snapshot timed out after 15000ms.

In those same tabs, preview_open, preview_navigate, preview_evaluate, preview_click and t3_preview_close all work, and requestAnimationFrame still fires. Only the snapshot (screenshot capture) hangs.

This breaks the coordinator pattern: a parent agent delegates browser checks to children, and none of them can take a snapshot.

Secondary: the error message is misleading. The server trace shows each failure at ~10,000 ms. The host throws tooBusy() ("The page was too busy to capture a snapshot in time…") from SNAPSHOT_TIMEOUT_MS = 1e4, then classifyResponseError rebuilds it from the broker context, which uses timeoutMs: 15e3. Agents see the wrong duration and lose the real reason.

Impact

Major degradation or frequent failure

Version or commit

T3 Code (Beta) 0.1.1 (also checked: 0.1.2-nightly.20261022.3526 does not include PR #22743)

Environment

macOS 27.0.1, Apple Silicon. Agents: Claude (Opus 5.5 parent, Sonnet 5.5 child via delegate_task), Codex (GPT-6.1 Sol child).

Logs or stack traces

PreviewAutomationBroker.awaitResponse 10003ms Failure PreviewAutomationTimeoutError: Preview automation snapshot timed out after 15000ms.
    at classifyResponseError (.../binCli-DF59J6P3.mjs:310107:48)
PreviewAutomationBroker.respond   # late response arrives ~10 s after the failure

Workaround

The on-screen thread can snapshot with open:true. Child threads can use preview_evaluate for DOM checks, and an external headless browser (e.g. Playwright CLI) for screenshots.

Related

• preview_snapshot on a background tab never returns and blocks every later action on that tab #16567: original report; this is the part noted as still open
• [Bug]: Background viewer capture can hold captureLock and block agent calls past the 10s snapshot deadline (nightly 3457) #23022: background viewer capture holds captureLock past the 10 s snapshot deadline (same timeout path)
• [Bug]: preview_recording_start on a tab that draws no frames never returns and blocks every later action on that tab #23514: preview_recording_start on a tab that draws no frames never returns
• [Bug]: Windows preview snapshots and remote viewing stall while T3 is completely covered #23139, [Bug]: Agent browser stalls / drops connection on Windows when its panel is off-screen (switching threads) #17483: Windows variants (covered window, switching threads)
• PR fix(desktop): agent preview tabs keep working while T3 is hidden #22743: proposed fix, "agent preview tabs keep working while T3 is hidden" (draft)