[BUG] Claude Code 2.1.261 crashes

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) 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 message (including /status) in the message box cause a crash, not at the app level, but the message doesn't get processed.
I am using the desktop version of Claude on Enterprise Windows.

### What Should Happen?

This should work normally.

### Error Messages/Logs

```shell
Claude Code returned an error result: Sandbox required but unavailable: sandbox is enabled but the Windows sandbox is not active on this session (feature gate off). Set sandbox.failIfUnavailable=false to allow unsandboxed execution.

Error: sandbox required but unavailable: sandbox is enabled but the Windows sandbox is not active on this session (feature gate off)
sandbox.failIfUnavailable is set — refusing to start without a working sandbox.
```

### Steps to Reproduce

Just open a chat in claude code and type anything

### Claude Model

Sonnet (default)

### Is this a regression?

Yes, this worked in a previous version

### Last Working Version

2.1.261

### Claude Code Version

2.1.261

### Platform

Anthropic API

### Operating System

Windows

### Terminal/Shell

Other

### Additional Information

_No response_

Start by reproducing Claude Code 2.1.261 on Windows with sandbox enabled and sandbox.failIfUnavailable set, then send any prompt such as /status. Trace where the sandbox availability check raises "sandbox required but unavailable" and how Windows sandbox feature gates are detected. Done means messages no longer fail unexpectedly in this setup, or the error path is intentionally handled or documented.

Written by the indexing model from the issue text.