[Bug] High CPU usage (50-126%) in idle state - main thread busy-wait

Bug Report

Claude Code Version

2.1.29 (installed via brew on macOS)

Description

The Claude Code main process consumes 50-126% CPU continuously even when idle. The main thread is in state R (Running) instead of S (Sleeping), indicating a busy-wait loop in the core event loop.

System Info

• macOS 26.2 Tahoe (Darwin 25.2.0)
• MacBookPro16,1 (Intel Core i9-9880H, 16GB RAM)
• Shell: zsh 5.9

Evidence

Process state:

PID    COMMAND          %CPU TIME     STAT
83726  claude           50.7  30:38   R   ← Main thread RUNNING instead of SLEEPING

Thread breakdown:

• Main thread: STAT=R, 50.7% CPU, 25min CPU time
• Other threads: STAT=S, 0-2% CPU, sleeping normally

Subprocess comparison:

83726  claude              31:00.99  ← The problem
83734  npm exec mcp-server   0:02.14  ← Normal
83735  npm exec chrome...    0:02.06  ← Normal
83744  pyright-langserver    1:02.02  ← Normal
83745  clangd                0:00.08  ← Normal

MCP servers and LSPs consume minimal CPU. The issue is in Claude Code's main event loop.

Impact

• Reduced battery life on laptops
• Fan noise
• System slowdown during long idle sessions
• 30+ minutes of CPU time accumulated in ~1 hour

Expected Behavior

When idle, the main thread should be in state S (Sleeping) with near 0% CPU, waiting for events via kqueue/epoll instead of busy-waiting.

Steps to Reproduce

1. Start Claude Code: claude
2. Let it idle (no commands, no active operations)
3. Run: top -o cpu or ps -p <claude_pid> -o pid,stat,%cpu,time
4. Observe main thread at 50-126% CPU with STAT=R

Diagnosis

The main event loop appears to be using busy-wait polling instead of proper event notification. This is a classic performance bug where the loop doesn't block/sleep when no events are pending.

Suggested Fix

Ensure the main event loop blocks on kqueue/selector when idle, only waking when there's actual work to do (input, IPC events, timer events, etc.).