[BUG] UserPromptSubmit type: command sometimes skipped

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?

When I ask a question, I want to force Claude to start a discussion, not to start working. So I've created a UserPromptSubmit hook that detects question marks in user prompts, but it does not appear to fire. It worked in a previous session, but it doesn't anymore. The hook command works correctly when tested manually in the shell, but the systemMessage it returns is not displayed in the UI when a user prompt containing ? is submitted.

What Should Happen?

When a user prompt containing ? is submitted, the hook should fire and display its systemMessage ("Question detected — Claude must discuss, not act.") in the UI. The assistant should receive the additionalContext instructing it not to call code-modifying tools.

Error Messages/Logs

No error messages visible. The hook fails silently — no systemMessage appears and the assistant proceeds as if the hook didn't fire.

Steps to Reproduce

Hook configuration in .claude/settings.json:

  {
    "hooks": {
      "UserPromptSubmit": [
        {
          "hooks": [
            {
              "type": "command",
              "command": "jq -r '.user_prompt // \"\"' | grep -q '?' && echo '{\"systemMessage\":\"Question detected — Claude must discuss, not
  act.\",\"hookSpecificOutput\":{\"hookEventName\":\"UserPromptSubmit\",\"additionalContext\":\"STOP. The user asked a question...\"}}' || true"
            }
          ]
        }
      ]
    }
  }

Manual test confirms the command works:

echo '{"user_prompt":"can we stick with JAX-RS standards?"}' \
    | jq -r '.user_prompt // ""' \
    | grep -q '?' && echo "MATCHED" || echo "NO MATCH"
  # Output: MATCHED

The prompt that should have triggered it: can we stick with JAX-RS standards?

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.87

Platform

Google Vertex AI

Operating System

macOS

Terminal/Shell

iTerm2

Additional Information

_No response_