SessionStart hook displays as 'error' despite successful execution (exit 0)

Description

SessionStart hooks that execute successfully (exit code 0) and output context to stdout display as "error" in the Claude Code startup UI, even though they function correctly.

Steps to Reproduce

1. Configure a SessionStart hook in ~/.claude/settings.json:

{
  "hooks": {
    "SessionStart": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "/path/to/hook-script.sh"
          }
        ]
      }
    ]
  }
}

1. Create a simple hook script that outputs context and exits 0:

#!/bin/bash
echo "# Session Context"
echo "Some useful context here"
exit 0

1. Start a new Claude Code session

Expected Behavior

The hook should display as successful (or not display an error state) since:

• The script exits with code 0
• The stdout content is correctly injected into Claude's context (verified - Claude receives the content)

Actual Behavior

The startup banner shows: SessionStart:startup hook error in red text, despite the hook functioning correctly.

Environment

• Claude Code version: v2.1.12
• Model: Opus 4.5
• OS: macOS (Darwin 24.2.0)

Notes

Per the hooks documentation, SessionStart hooks with exit code 0 should have their stdout "added directly as context" - this is working correctly. The issue is purely cosmetic/UI - the "error" label is misleading since the hook executed successfully and the context injection works.

🤖 Generated with Claude Code