[BUG] Plan mode: first file write always fails, succeeds on retry

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?

• Every time plan mode writes/updates the plan file, the first attempt fails with "Error writing file"
• Immediately retries and succeeds
• 100% reproducible, not intermittent
• Platform: linux (WSL2)
• Version: 2.1.37
• WSL2 kernel 6.6.87.2
• Ubuntu 24.04.3 LTS
• 8 CPUs, 7.6 GB RAM
• Platform: Linux (WSL2 6.6.87.2, Ubuntu 24.04.3)

What Should Happen?

the message "● Updated plan
⎿  Error writing file" should not be displayed in the console.

Error Messages/Logs

● Updated plan
  ⎿  Error writing file

Steps to Reproduce

1. Start a conversation with a non-trivial task (e.g. "add a new page to the admin dashboard")
2. Claude enters plan mode and writes the plan file
3. I see the error message inline in the console.
4. Claude automatically retries
5. Second attempt succeeds
6. It seems to happen all plan requests

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

v2.1.29

Claude Code Version

2.1.37 (Claude Code)

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

WSL (Windows Subsystem for Linux)

Additional Information

_No response_