Web UI does not open after ExitPlanMode on Windows + Claude Code (0.19.15) — server runs, plan captured

[NotMyself]

Description

On Windows with Claude Code, the PermissionRequest:ExitPlanMode hook fires correctly and the Plannotator server starts on a random port — but the browser tab never auto-opens, so the user sees no UI and the plan-mode permission appears to be granted without review.

Structurally this looks like the same class of bug as #551 (closed; OpenCode + Linux + xdg-open), but with cmd.exe /c start ${url} failing silently in the Claude Code hook subprocess context on Windows.

Environment

OS	Windows 11 Pro 10.0.26200
Agent	Claude Code 2.1.140
Plannotator	0.19.15 (plugin); bun 1.3.11 (binary)
Default browser	Microsoft Edge
Install	/plugin install plannotator@plannotator (user scope)

Steps to reproduce

1. Enable the plannotator plugin in Claude Code.
2. Enter plan mode (Shift+Tab) and ask Claude to make a plan.
3. Wait for the model to call ExitPlanMode.

Expected behavior

Browser tab auto-opens at http://localhost:<random> showing the plan UI for annotation/approval.

Actual behavior

• ExitPlanMode permission is granted effectively instantly with no UI prompt.
• No browser tab opens.
• The plannotator server is running. Confirmed by:
  • ~/.plannotator/sessions/<pid>.json is created with a fresh port + URL.
  • The plan is captured in ~/.plannotator/history/<project>/<title>-<date>/NNN.md.
  • curl -sI http://localhost:<port>/ returns HTTP 200 with the expected ~17 MB HTML payload — the UI is reachable, just never opened.

So the hook works end-to-end except for openBrowser().

Root cause hypothesis

In packages/server/browser.ts:

if (platform === "win32" || wsl) {
  await $`cmd.exe /c start ${url}`.quiet();
}

Wrapped in try/catch with .quiet(). When this fails in the Claude Code hook subprocess context — likely the same "child process can't reach the interactive desktop / env not propagated" mode that #551 hit on Linux/OpenCode — the failure is swallowed and only the VS Code IPC fallback is attempted. On a non-VS-Code Claude Code install there's no IPC registry to fall back to, so the user sees nothing.

(Bun's shell template-tag $ interpolation may also be a factor — possible URL escaping/splitting that confuses cmd.exe start — but I haven't verified that vs. the subprocess-context theory.)

Related

• Web UI does not open automatically when Plannotator Code Review is triggered #551 — Web UI does not open automatically (closed via fix: non-blocking browser launch in Pi extension (Linux) #292, but later commenters reported the issue still occurring on Mac M4 and PLANNOTATOR_BROWSER didn't help; this Windows case looks like the same shape).
• Annotate feedback can be lost when submit auto-closes localhost session after CLI timeout on Windows/Codex #678 — Different failure mode (submit-side timeout on Windows/Codex, not open-side).

Workaround (interactive)

The UI is reachable; you can open it manually:

Get-Content ~/.plannotator/sessions/*.json |
  ConvertFrom-Json |
  Sort-Object startedAt |
  Select-Object -Last 1 -ExpandProperty url

Open the printed URL in a browser. Plan / annotation flow works normally from there.

Notes

• Worked in 0.8.0 (which registered the hook with an absolute exe path and presumably a different open mechanism). 0.19.x regression.
• Have not yet tested PLANNOTATOR_BROWSER=<full path to chrome.exe> on Windows — the analogous workaround was reported as not-fixing-it in Web UI does not open automatically when Plannotator Code Review is triggered #551.
• Happy to test patches against this setup.

[backnotprop]

Thanks for detailed summary @NotMyself

What terminal environment are you using. powershell?

Are you using claude agents?

[NotMyself]

@backnotprop I use Windows Terminal hosting PowerShell 7 and the native claude code binary. I was using claude agents when the issue happened.

[backnotprop]

There might be some limitations with how they launch sessions with claude agents, I'm exploring solution paths