claude-code - 💡(How to fix) Fix additionalContext in UserPromptSubmit hook output is silently dropped [1 comments, 2 participants]

TL;DR

The issue can be worked around by using plain stdout instead of returning JSON from the UserPromptSubmit hook.

Guidance

• The likely cause is that the assistant is not designed to handle JSON output from hooks, which is why plain stdout works as a workaround.
• To verify, try registering a new hook with plain stdout output and check if the assistant can see the context.
• The current workaround is to use plain stdout, such as echo "user-message-submitted: $TS", which surfaces correctly as a system-reminder.
• Consider checking the hook documentation for any updates or changes to how JSON output is handled.

Example

{
  "hooks": [{
    "type": "command",
    "command": "echo 'user-message-submitted: 2026-04-24T13:43:01Z'",
    "timeout": 3
  }]
}

Notes

This workaround may not be ideal for all use cases, especially if the hook needs to return structured data. It's unclear if this is a bug or an intended behavior, and further investigation may be needed to determine the root cause.

Recommendation

Apply workaround: using plain stdout is the most straightforward solution given the current information, as it allows the assistant to see the context as intended.