[BUG] Complete UI freeze during Bash tool execution - Thread deadlock

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 completely freezes during Bash tool execution. The timer stops updating, Esc
doesn't interrupt, and no keyboard input is possible. The process must be killed with
kill -9 (SIGTERM is ignored).

This issue started occurring a few days ago and happens irregularly - sometimes every
few minutes, sometimes every few hours. Any Bash command can trigger it.

What Should Happen?

Timer should continue counting

• Esc should interrupt the request at any time
• UI should remain responsive during Bash execution

Error Messages/Logs

No error messages - the UI freezes completely.                                          
                                                                                          
  Thread state analysis via /proc/<pid>/task/*/wchan:                                     
  - Main thread: ep_poll (waiting for epoll events)                                       
  - Worker threads: 18x futex_wait_queue (blocked on mutex)                               
  - I/O thread: wait_woken                                                                
                                                                                          
  Process shows State: R (running) but event loop never executes.                         
  Process ignores SIGTERM, only responds to SIGKILL.

Steps to Reproduce

1. Start Claude Code
2. Execute any Bash tool command
3. The UI may freeze completely at random
4. The status line stops updating (timer stuck)
5. Pressing Esc does nothing
6. No keyboard input possible
7. Process ignores SIGTERM - must be killed via kill -9 <pid>

Specific trigger: Serial port access reliably triggers the freeze:
stty -F /dev/ttyUSB0 115200 raw -echo && timeout 8 cat /dev/ttyUSB0 2>&1 || true

However, the freeze also occurs with non-blocking commands and background workarounds.
Any Bash tool execution can trigger it.

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.19 (Claude Code)

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Other

Additional Information

## Technical Analysis

This is a thread deadlock, NOT GC thrashing:

• Main thread stuck in ep_poll, waiting for events
• UI thread blocked on futex_wait_queue - waiting for mutex
• One thread holds a lock and is waiting for I/O
• UI thread waits for the same lock → deadlock

### Debug Log Analysis

With --debug flag enabled, the log shows:

• No errors or warnings before the freeze
• Last entries are repeated [ToolSearch:optimistic] lines (20+ identical at same

timestamp)

• Then logging stops completely
• The deadlock occurs synchronously - even the logging thread is blocked

### Key Difference from #19344 (GC Death Spiral)

| Metric | #19344 | This Issue |
|--------|--------|------------|
| CPU | 150-180% | Low |
| Thread state | GC functions | futex_wait_queue |
| Memory | Growing | Stable |

### Additional Evidence

After killing the frozen process:

• Terminal showed buffered keystrokes typed during freeze
• Confirms UI thread was blocked, process alive, input buffered but never read

### Related Issues

• #19344 - different root cause (GC vs deadlock)
• #20572 - similar symptoms, different trigger