[BUG] Background task/agent notifications interrupt in-progress prompt typing

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 a background task or subagent (e.g. one launched via the Agent/Task tool, or a "broker" completion notification) finishes while I'm in the middle of typing a prompt in the input box, the notification message is inserted/sent immediately, splitting my in-progress text. This causes my prompt to be submitted incomplete (cut off mid-sentence), interrupting my typing flow.

What Should Happen?

Background task/agent completion notifications should be delivered automatically (as they are today) without touching or submitting the current input buffer. The user's in-progress prompt should remain intact in the input box, unaffected by the incoming notification, so they can finish typing and submit it themselves.

Error Messages/Logs

Steps to Reproduce

1. Start typing a multi-line or longer prompt in the Claude Code CLI input.
2. While still typing (before pressing Enter), have a background agent/task complete and emit its notification ("broker" message).
3. Observe that the notification is delivered into the conversation immediately, and the text currently being typed is submitted/interrupted partway through instead of remaining in the input box untouched.

Claude Model

None

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

2.1.228

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_