SessionStart hooks not firing on Windows (startup or /new)

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report
• [x] I am using the latest version of Claude Code

What's Wrong?

SessionStart hooks configured in ~/.claude/settings.json do not fire on Windows, neither on initial claude launch nor on /new.

The hook script works perfectly when executed manually from the shell. Other hooks (PreToolUse, PostToolUse, SubagentStop) fire correctly — only SessionStart is affected.

What Should Happen?

The SessionStart hook should fire when:

1. Claude Code is launched (claude in terminal)
2. A new conversation is started with /new

Evidence

Hook counter file (axon_hook_counters.json) after a full session shows:

{"counts": {"post_tool_use": 6, "pre_tool_use": 1, "subagent_stop": 1}, "alerted": false}

Note: session_start is completely absent — the hook never ran.

Manual execution of the same script works:

echo '{"cwd":"E:/Proyectos/jarkoterminal"}' | python ~/.claude/hooks/axon/axon_session_start.py
# Returns valid JSON with additionalContext — exit code 0

Hook Configuration

{
  "hooks": {
    "SessionStart": [
      {
        "matcher": "",
        "hooks": [
          {
            "type": "command",
            "command": "PYTHONUTF8=1 AXON_URL=https://example.com AXON_API_KEY=xxx python \"C:\Users\user\.claude\hooks\axon\axon_session_start.py\""
          }
        ]
      }
    ]
  }
}

Environment

• Claude Code version: 2.1.81
• Platform: Windows 11 Pro (10.0.26200)
• Shell: Git Bash
• Python: 3.x (works fine for all other hooks)

Related Issues

• #34431 — SessionStart:resume missing cwd on Windows
• #34072 — SessionStart not firing on /clear