Windows: .claude.json repeatedly corrupted by concurrent non-atomic writes

Description

On Windows, ~/.claude.json gets corrupted on nearly every restart. Multiple claude.exe processes write to the same config file concurrently without file locking or atomic writes, causing a race condition that truncates/empties the file.

Steps to Reproduce

1. Install Claude Code on Windows 11 (native install, version 2.1.59)
2. Run claude and use it normally for a session
3. Exit the session (/exit or close terminal)
4. Relaunch claude
5. Error appears on startup:

``
Claude configuration file at C:\Users\<user>\.claude.json is corrupted: JSON Parse error: Unrecognized token ''
The corrupted file has already been backed up.
``

1. Repeat steps 2-5 — the error occurs on nearly every restart.

Evidence

8 concurrent claude.exe processes observed via tasklist during a single session, all targeting the same ~/.claude.json.

47 corrupted snapshots accumulated in ~/.claude/backups/ in a single day, many within the same millisecond:

.claude.json.corrupted.1772128941435   77 bytes
.claude.json.corrupted.1772128941449   77 bytes
.claude.json.corrupted.1772128941450   77 bytes
.claude.json.corrupted.1772128941463   77 bytes

Corruption patterns

Truncated mid-write (trailing comma, no closing brace — 80 bytes):

{
  "clientDataCache": {
    "data": {},
    "timestamp": 1772128933289
  },
   

Partial write (only first key flushed — 77 bytes):

{
  "clientDataCache": {
    "data": {},
    "timestamp": 1772129541144
  }
}

The full valid config file is ~5KB with 38 top-level keys.

Suggested Fix

Use atomic writes: write JSON to a temporary file in the same directory, then fs.rename() over the target. On Windows, rename on the same volume is near-atomic and eliminates the truncate-then-write race. Alternatively, implement file locking via fs.open with exclusive flags or a lockfile to serialize concurrent writes.

Environment

• OS: Windows 11 Pro 10.0.22631
• Shell: Git Bash (bash.exe)
• Claude Code version: 2.1.59
• Install method: native