[BUG] /background fails with "EUNKNOWN: unknown error, uv_spawn" — daemon autostart hard-codes the Windows PowerShell 5.1 absolute path, which enterprise Group Policy blocks

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Any entry point that needs the background supervisor daemon fails when no daemon is running:

> claude --bg "do something"
Starting background service…
Couldn't start the session — EUNKNOWN: unknown error, uv_spawn

Same for the /background (/bg) slash command. Regular interactive sessions, the Bash/PowerShell tools, and subagents all work fine.

Root cause (verified)

The Windows daemon autostart spawns the daemon via WMI (Win32_Process.Create) so it survives terminal close, and it invokes that through a hard-coded absolute path to Windows PowerShell 5.1:

%SYSTEMROOT%\System32\WindowsPowerShell\v1.0\powershell.exe -NoProfile -NonInteractive -EncodedCommand <base64>

On machines where Group Policy blocks that binary (a common enterprise hardening setup: WPSh 5.1 disabled in favor of PowerShell 7), CreateProcess fails with Win32 error 1260 (ERROR_ACCESS_DISABLED_BY_POLICY). libuv has no errno mapping for 1260, so the user sees the opaque EUNKNOWN: unknown error, uv_spawn.

Verification performed:

• A minimal CreateProcessW test against powershell.exe (5.1) fails with Win32 error 1260 under every flag combination (baseline, DETACHED_PROCESS, CREATE_NO_WINDOW, etc.).
• The same test against claude.exe succeeds with every combination, including DETACHED_PROCESS | CREATE_NEW_PROCESS_GROUP — so detached spawning itself is not restricted on this machine.
• The direct-spawn fallback after the WMI attempt does not appear to engage: after the failure, claude daemon status reports not running and daemon.log has no origin=transient start entry.

Workaround

Manually starting the supervisor first makes everything work, since the blocked autostart step is skipped:

claude daemon run     # in a separate terminal (or spawned hidden)
claude --bg "..."     # now succeeds; /background works too

Background sessions then spawn and run normally under the daemon.

Suggested fixes

1. Prefer pwsh.exe (PowerShell 7) for the WMI spawn helper when available, falling back to Windows PowerShell 5.1 — mirrors what the PowerShell tool already does.
2. When CreateProcess fails with 1260, surface a clear message ("blocked by Group Policy") and hint at the claude daemon run workaround instead of EUNKNOWN: unknown error, uv_spawn.
3. Investigate why the direct-spawn fallback after a failed WMI attempt doesn't leave a transient daemon running in this scenario.

What Should Happen?

claude --bg / /background should start the background supervisor daemon and detach the session, as it does on machines without this Group Policy restriction.

Ideally the daemon autostart should not depend on the Windows PowerShell 5.1 binary when pwsh.exe (PowerShell 7) is available — or at minimum, the error should clearly say the spawn was blocked by Group Policy instead of EUNKNOWN: unknown error, uv_spawn.

Error Messages/Logs

Steps to Reproduce

1. Use a Windows 11 machine where Group Policy blocks execution of Windows PowerShell 5.1 (C:\Windows\System32\WindowsPowerShell\v1.0\powershell.exe; CreateProcess fails with Win32 error 1260) while PowerShell 7 (pwsh.exe) is allowed.
2. Make sure no supervisor daemon is running: claude daemon stop --any (status should say not running).
3. Run claude --bg "test" from any project directory (or use /background inside an interactive session).
4. Observe:

``
Starting background service…
Couldn't start the session — EUNKNOWN: unknown error, uv_spawn
``

1. claude daemon status still reports not running, and daemon.log has no origin=transient entry.
2. Now run claude daemon run in a separate terminal and repeat step 3 — it succeeds.

Claude Model

None

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

2.1.209 (Claude Code)

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

Windows 11 Enterprise (10.0.26200), Claude Code native install. PowerShell 7 is present and allowed; only Windows PowerShell 5.1 is blocked by policy.

Possibly related but distinct: #21843 / #25558 (CLAUDE_CODE_SHELL ignored on Windows) cover shell selection for the Bash tool, not this daemon autostart path.