Multiple CLI sessions corrupt ~/.claude.json on Windows (no file locking)

Problem

Running multiple claude CLI sessions simultaneously (e.g., in different VS Code terminal tabs for different projects) causes repeated corruption of ~/.claude.json on Windows.

The file gets detected as corrupted ("JSON Parse error: Unexpected EOF"), backed up, and replaced with a near-empty config — losing all project data, tool usage stats, tips history, etc.

Root cause

Multiple claude.exe processes read/write ~/.claude.json concurrently without any file locking. On Windows, there is no atomic rename() like on Unix, making partial reads during writes very likely.

Reproduction

1. Open 2-3 VS Code windows with different projects
2. Start claude in the integrated terminal of each
3. Use them concurrently
4. Within minutes, ~/.claude.json gets corrupted

Evidence

Over 5 days (Feb 21-26), we collected 43 "corrupted" backup files. 97% of them were valid JSON — the corruption is purely a timing issue where one process reads while another writes.

The file grew from 67KB to 80KB over time, then cascaded down to 77 bytes through repeated "detect corrupt → backup → replace with empty" cycles.

Impact

• Complete loss of project configurations (30 projects → 0)
• Loss of tool usage statistics, tips history, startup count
• Requires manual recovery from backup files
• Happens repeatedly, making multi-project workflows unreliable

Affected processes

All of these write to ~/.claude.json concurrently:

• Multiple CLI sessions (~/.local/bin/claude.exe)
• Claude Desktop App (Electron)
• VS Code Claude Code Extension (spawns its own claude.exe)

Suggested fix

• Use proper file locking (e.g., flock equivalent on Windows, or lock files)
• Or use atomic write pattern: write to temp file → MoveFileEx with MOVEFILE_REPLACE_EXISTING
• Or separate per-session state from shared config

Environment

• Windows 11 Pro, Claude Code CLI v2.1.59
• 3 concurrent CLI sessions in different VS Code terminals