EBUSY: concurrent instances race on .claude.json (Windows)

Summary

Running multiple Claude Code instances simultaneously on Windows causes EBUSY: resource busy or locked errors when writing to ~/.claude.json. The file is 72KB and written with writeFileSync without file locking, so concurrent instances race on it.

Reproduction

1. Open two Claude Code sessions (e.g., terminal + desktop app, or two terminals)
2. Use both actively
3. One will eventually hit:

ERROR EBUSY: resource busy or locked, open 'C:\Users\<user>\.claude.json'

  at writeFileSync (unknown)
  VZ8 (B:/~BUN/root/src/entrypoints/cli.js:135:1271)
  NcK (B:/~BUN/root/src/entrypoints/cli.js:442:23227)
  p6 (B:/~BUN/root/src/entrypoints/cli.js:442:21289)
  <anonymous> (B:/~BUN/root/src/entrypoints/cli.js:8305:475)
  Au (B:/~BUN/root/src/entrypoints/cli.js:492:63164)
  zm8 (B:/~BUN/root/src/entrypoints/cli.js:492:76228)
  dR (B:/~BUN/root/src/entrypoints/cli.js:492:76109)
  zm8 (B:/~BUN/root/src/entrypoints/cli.js:492:77018)
  dR (B:/~BUN/root/src/entrypoints/cli.js:492:76109)

Root cause

Windows file locks are mandatory (unlike Unix advisory locks), so writeFileSync from one process blocks/fails when another process has the file open. A write-to-temp-then-rename pattern or proper file locking would fix this.

Environment

• Windows 11 Pro 10.0.26200
• Claude Code (latest as of 2026-04-10)

---
Filed by Jeff Yaw, Yaw Labs (jeff@yaw.sh)