Parallel sessions crash simultaneously on Windows (native install)

Description

Running two parallel Claude Code sessions (separate terminals, separate worktrees/branches) on Windows causes both sessions to terminate simultaneously without any logged error. This has happened twice since switching to the native Windows installation (v2.1.50).

Environment

• OS: Windows 11 Pro 10.0.26200
• Claude Code version: 2.1.50 (native installation, not npm)
• Installation: Native Windows installer
• Shell: bash (Git Bash)

Steps to Reproduce

1. Open two separate terminal windows
2. Launch claude in each terminal, working on different branches/worktrees
3. Both sessions actively working (using subagents, API calls, tool use)
4. After some time, both sessions terminate simultaneously

Expected Behavior

Each session should be independent — one crashing should not affect the other.

Actual Behavior

Both sessions die at the same time. No error message is displayed. Debug logs (~/.claude/debug/) show no fatal error — the process simply stops, suggesting an external kill.

Possible Root Cause

Debug logs show both sessions perform atomic writes to the same ~/.claude.json file using temp files + rename:

Renaming C:\Users\<user>\.claude.json.tmp.16576.1771764251179 to C:\Users\<user>\.claude.json

If both sessions attempt this simultaneously, there could be a race condition on Windows (where file rename semantics differ from POSIX). One session's rename could fail or corrupt the shared state, causing both to exit.

Additional Context

• Sessions use different git worktrees and branches (no shared git resources)
• Both sessions use subagents (Haiku + Opus), generating concurrent API connections
• The crash is not reproducible on demand — it seems to depend on timing of concurrent writes
• This did not happen (or was not noticed) before switching from npm to native installation

Suggested Fix

• Use per-session state files instead of a shared ~/.claude.json, or
• Implement proper file locking (e.g., lockfile / flock equivalent on Windows) for shared state writes