[Bug]: preview_snapshot can fail or time out on loaded pages and leave tab automation unusable

[gregbartell]

Before submitting

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

Area

apps/desktop

Steps to reproduce

1. Start T3 Code Desktop.

2. Open a Codex-backed thread with the T3 preview MCP tools available.

3. Serve any local web page:

   python -m http.server 8000 --directory build/site

4. Open the page in a fresh preview tab:

   {
     "show": true,
     "reuseExistingTab": false,
     "url": "http://localhost:8000/index.html"
   }

5. Confirm the page is loaded with preview_status.

6. Call preview_snapshot.

7. Retry preview_snapshot if the first call fails.

8. Call a simple preview_evaluate afterward:

   ({ href: location.href, ready: document.readyState })

Expected behavior

On a loaded page, preview_snapshot should return page state or a clear recoverable error.

A snapshot failure should not poison the tab. Follow-up preview_evaluate should continue to work, and preview_open(show: true) should recover the tab if the preview needs to be reattached.

Actual behavior

On a loaded page, the first snapshot returned a generic execution error:

{
  "error": {
    "_tag": "PreviewAutomationExecutionError",
    "operation": "snapshot",
    "failureCount": 1
  }
}

A retry timed out after 15 seconds:

{
  "error": {
    "_tag": "PreviewAutomationTimeoutError",
    "operation": "snapshot",
    "failureCount": 1
  }
}

A simple preview_evaluate immediately afterward also timed out:

PreviewAutomationTimeoutError: Preview automation evaluate timed out after 15000ms.

Reopening the same tab with preview_open(show: true) did not restore evaluation in this run. Creating a fresh tab did restore evaluation.

In another run, snapshot failed with an UnknownVizError in the action timeline, while evaluation stayed responsive. This suggests there may be more than one snapshot failure mode.

Impact

Major degradation or frequent failure

Version or commit

T3 Code desktop 0.0.28-1 via t3code-bin AUR package.

Environment

Arch Linux x86_64, kernel 7.0.13-arch1-1, T3 Code desktop AppImage package via t3code-bin, Codex provider.

Logs or stack traces

Representative timeout:

PreviewAutomationTimeoutError: Preview automation evaluate timed out after 15000ms.

Representative snapshot error:

{
  "error": {
    "_tag": "PreviewAutomationExecutionError",
    "operation": "snapshot",
    "failureCount": 1
  }
}

Screenshots, recordings, or supporting files

No screenshot included. The page was loaded and inspectable by status before snapshot; the useful evidence is the snapshot/evaluate error sequence.

Workaround

Use preview_evaluate and preview_status for inspection when a full snapshot is not required. If snapshot or a follow-up evaluation starts timing out, abandon that tab and open a fresh one.

[daemon-james]

Confirmed on T3 Code 0.0.31 on Linux with the Codex provider.

During a dashboard inspection session:

• preview_snapshot timed out while preview_status still reported the page as loaded.
• Follow-up preview_evaluate and preview_navigate calls also timed out, and preview action entries remained in a running state.
• The t3 serve process sustained roughly 100–146% CPU and about 2.1 GiB RSS during the degraded period.
• The host itself remained 77–95% idle with no I/O wait and about 13 GiB memory available; the local API and frontend were still responsive.
• When the automation backlog eventually cleared, t3 serve CPU fell to about 3% and preview calls recovered.

This adds a server-side resource symptom to the existing tab-poisoning report. It suggests timed-out preview automation may leave queued or stalled work inside T3, rather than the target application being responsible. I avoided further retries once this was isolated, since repeated calls appeared to make the degraded period longer.