[BUG] Idle Claude Code instances consume 70-97% CPU in background

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 instances that are idle (no active conversation) consume excessive CPU (70-97%) continuously over extended periods. The problem worsens over time - instances running for 14+ hours show higher
CPU usage than recently started ones. Critically, the idle background instances use far more CPU than active instances with ongoing conversations.

• Main thread continuously reading /proc/self/statm (memory stats) via pread64()
• ~350 pread64() calls per second on this single file
• ~210 sched_yield() calls per second (yielding but immediately rescheduling)
• ~1,032 futex calls per second (thread synchronization spinning)

What Should Happen?

-

Error Messages/Logs

Steps to Reproduce

1. Start a Claude Code session in a zellij pane
2. Have a conversation or just start the session
3. Detach from the zellij session (leave it running in background)
4. Wait several hours (problem appears after ~6-8 hours)
5. Check CPU usage with top or ps aux

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.0.36

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Other

Additional Information

_No response_