SessionStart hook with background process silently blocks claude-code after v2.1.87

Description

After updating to Claude Code 2.1.87, the Code mode in Claude Desktop became completely unresponsive when a SessionStart hook contained a background process (caffeinate -s &).

Root Cause

The background process (caffeinate -s &) inherits the parent process's stdin/stdout file descriptors. Since claude-code communicates with the Desktop App via stream-json through these pipes, the background process holding them open caused claude-code to wait indefinitely.

This behavior was tolerated in previous versions but became fatal after 2.1.87 tightened the subprocess communication mechanism.

Symptoms

• Code mode produces no response after message is submitted
• Chat and Cowork modes work normally
• Log shows: Session timed out after 649s (hadFirstResponse=false, reason=no_response)
• Subprocess starts but seconds_since_stderr=never

Workaround

Use nohup with full file descriptor cutoff:

nohup caffeinate -s </dev/null >/dev/null 2>&1 & echo $! > ~/.claude/caffeinate.pid

Suggested Improvements

1. When a hook causes claude-code to hang on first response, surface an error message instead of silently timing out
2. Add documentation warning: hook commands must not inherit parent's stdin/stdout — recommend nohup ... </dev/null >/dev/null 2>&1 & for background processes
3. Include a changelog note when hook execution environment changes between versions

Environment

• Claude Code: 2.1.87
• OS: macOS (Apple Silicon)