[BUG] Claude Code Freezes on Continuous/Streaming Output (Serial Ports, Long-Running Processes)

[BUG] Claude Code Freezes on Continuous/Streaming Output (Serial Ports, Long-Running Processes)

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 immediately freezes when rendering continuous/unbuffered stream output, specifically from:

• Serial ports (cat /dev/tty.usbmodem*)
• Long-running processes with continuous output
• Any unbuffered stream without discrete termination

Steps to Reproduce

Minimal reproduction:

1. Start fresh Claude Code session
2. Run: cat /dev/tty.usbmodem21401 (or any serial port)
3. Immediate freeze - terminal completely unresponsive
4. Must force-kill Claude Code

Test script provided:

# Clone the repo and run:
cd ~/claude-code
./test_stream_freeze.sh continuous  # Will freeze immediately

See test_stream_freeze.sh and FREEZE_BUG_REPRODUCTION.md in the repo for full test suite and analysis.

What Should Happen?

Expected behavior:

• Stream output should render in real-time
• Should have timeout mechanism (e.g., 5 seconds)
• Should not deadlock on unbuffered streams
• Should remain responsive even with continuous output

Actual behavior:

• Complete freeze after starting stream
• No timeout mechanism
• Terminal becomes unresponsive
• Must force-kill Claude Code process

Confirmed Facts

✅ Happens in fresh sessions (not memory accumulation)
✅ Immediate freeze (< 1 second after starting stream)
✅ Reproducible with serial ports
✅ Workaround exists: Redirect to file first (timeout 5 cat /dev/tty.* > file && cat file)

Root Cause Analysis

Hypothesis:

1. Claude Code's stream parser tries to buffer continuous output
2. Parser waits for stream termination or line boundaries
3. With continuous streams, neither occurs
4. Parser/renderer enters deadlock waiting for completion
5. UI thread becomes unresponsive

Why file redirection works:

timeout 5 cat /dev/tty.usbmodem* > /tmp/out.txt
cat /tmp/out.txt  # Works - discrete, bounded output

File redirection provides discrete, complete output rather than continuous stream.

Claude Model

Sonnet (default)

Is this a regression?

Unknown - may have existed since serial port support was added

Last Working Version

_N/A - appears to have never worked for continuous streams_

Claude Code Version

1.0.113 (VSCode Extension)

Platform

Anthropic API

Operating System

macOS (Darwin 25.1.0)

Terminal/Shell

zsh + VS Code integrated terminal