[BUG] Regression in 2.1.27: Infinite CPU loop on project startup

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Claude Code 2.1.27 enters an infinite CPU loop (99%+ CPU) when starting any session in a project
with significant session history. Downgrading to 2.1.25 immediately fixes the issue. This occurs after submitting first prompt, or running /resume

What Should Happen?

Claude should not hang and use 100% of cpu core.

Error Messages/Logs

- Multiple Claude processes spawn at 99% CPU on startup
  - Affects both new sessions and /resume
  - Affects both terminal CLI and VSCode extension
  - No error message - silent infinite loop
  - Only fix is kill -9 (or downgrade)

Steps to Reproduce

1. Have a project with substantial session history (100+ sessions)
2. Use Claude Code 2.1.27
3. Try to start any session in that project
4. Observe infinite CPU loop

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

2.1.25

Claude Code Version

2.1.27

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux - Pop!_OS 24.04

Terminal/Shell

Other

Additional Information

• Killing Claude process can leave ~/.claude/history.jsonl.lock as a directory instead of a file
• This stale lock may contribute to the issue but isn't the root cause