[BUG] ~/claude.json race condition with multiple Claude CLI sessions active in different directories

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?

I have been using multiple Claude CLI windows in different projects for months. Suddenly today I'm experiencing this issue repeatedly.
Claude Code v2.1.59
Opus 4.6 · Claude Max
Claude CLI Error:
Claude configuration file at C:\Users\joe\.claude.json is corrupted: JSON Parse error: Unexpected EOF
The corrupted file has been backed up to: C:\Users\joe\.claude\backups\.claude.json.corrupted.1772119104739
A backup file exists at: C:\Users\joe\.claude\backups\.claude.json.backup.1772119081716
You can manually restore it by running: cp "C:\Users\joe\.claude\backups\.claude.json.backup.1772119081716"
"C:\Users\joe\.claude.json" --- I keep getting this. my disk has 197GB free. I am running 2 claude CLI sessions
on the machine in different directories and on different projects - could they be corrupting each other?

● Yes, that's almost certainly the cause. Both CLI sessions share the same ~/.claude.json file, and when they write to
it concurrently there's a race condition — one session truncates the file while the other is mid-write, resulting in
the corrupted/truncated JSON (hence "Unexpected EOF").

This is a known limitation. The Claude CLI doesn't use file locking on that config file.

Workaround options:

1. Run only one session at a time — simplest fix
2. Restore the backup when it happens — the CLI already tells you the command:

cp "C:\Users\joe\.claude\backups\.claude.json.backup.1772119081716" "C:\Users\joe\.claude.json"

There's no fix you can apply on your end to prevent the race condition — it would need to be addressed in the CLI
itself. If it's happening frequently, I'd suggest filing an issue at https://github.com/anthropics/claude-code/issues.

What Should Happen?

You should do basic multi-process locking on file access as I would expect from a junior programmer. Or, create a separate .json file for each CLI using well-known techniques, or separate into global/local .json files. This is an absurd bug to encounter.

Error Messages/Logs

Claude configuration file at C:\Users\joe\.claude.json is corrupted: JSON Parse error: Unexpected EOF
  The corrupted file has been backed up to: C:\Users\joe\.claude\backups\.claude.json.corrupted.1772119104739
  A backup file exists at: C:\Users\joe\.claude\backups\.claude.json.backup.1772119081716
  You can manually restore it by running: cp "C:\Users\joe\.claude\backups\.claude.json.backup.1772119081716"
  "C:\Users\joe\.claude.json

Steps to Reproduce

1. open multiple claude cli interfaces on your computer.
2. attempt to use them as the claude developers suggest and work on multiple projects or aspects simultaneously
3. fun error ensues.

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

not sure, but this error appeared recently

Claude Code Version

2.1.59 (Claude Code)

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

_No response_