[BUG] Critical regression: .claude.json corruption from concurrent sessions - unacceptable for production use

Environment

• Claude Code version: 2.1.59
• OS: Windows 10/11 (MSYS_NT-10.0-26100)
• Concurrent sessions: 11 (normal for methodology-driven multi-agent workflows)

Problem

.claude.json is being corrupted repeatedly due to non-atomic writes from concurrent sessions. This is a critical regression that makes Claude Code unreliable for any serious multi-session workflow.

Claude configuration file at C:\Users\RBatt\.claude.json is corrupted: JSON Parse error: Unexpected EOF
The corrupted file has been backed up to: C:\Users\RBatt\.claude.json.corrupted.1772075903627

This happened twice within minutes during normal operation.

Root Cause

All Claude sessions share a single global config file:

• ~/.claude.json contains toolUsage, clientDataCache, projects, etc.
• These fields are written frequently (every tool call updates toolUsage)
• With N concurrent sessions, you have N processes doing read-modify-write without file locking
• Result: truncated JSON, corrupted config, interrupted workflows

Evidence

$ tasklist | grep claude
claude.exe   23592   730 MB
claude.exe   10004   359 MB
claude.exe   38728   218 MB
... (11 total processes)

All 11 fighting over one file. This is Computer Science 101 - concurrent writes need locking.

Impact

• Workflow interruption: Session stops, user must manually restore backup
• Data loss risk: If backup is also corrupted, project-specific settings lost
• Trust erosion: Users running production agent workflows cannot rely on Claude Code

Expected Behavior

Concurrent Claude Code sessions should:

1. Use atomic writes (write to temp file, rename)
2. Use file locking (flock/LockFileEx)
3. Or separate per-session state from shared config

This Is a Regression

This did not happen in earlier versions. Users have been running 10+ concurrent sessions without issue. Something in recent versions (2.1.x) broke the concurrency model.

Duplicates

I see #28824, #28813, #28809 filed today for the same issue. The volume of reports should indicate severity. This needs a hotfix, not a backlog ticket.

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Respectfully: shipping a CLI tool that corrupts its own config file when users run multiple instances is not acceptable. Multi-session usage is a core workflow for agent-based development. Please prioritize this fix.