[BUG] SendMessage broadcast (to: "*") rejects structured messages (shutdown_request)

Preflight Checklist

• [x] I have searched existing requests and this issue hasn't been reported yet
• [x] This is a single bug report (not multiple issues)

What's Wrong?

SendMessage with to: "*" (broadcast) rejects structured messages such as shutdown_request, returning:

Error: structured messages cannot be broadcast (to: "*")

When a team lead needs to shut down all teammates simultaneously (e.g., after a parallel scraping phase completes), broadcasting a shutdown_request is the natural approach. However, the framework explicitly blocks structured messages (JSON payloads with a type field) from being broadcast.

What Should Happen?

SendMessage with to: "*" should support structured messages — at minimum shutdown_request and shutdown_response, since "shut down all teammates" is the most common broadcast use case.

Broadcasting { "type": "shutdown_request", "reason": "cleanup" } to all teammates should work the same way as sending it to each teammate individually.

Error Messages/Logs

Error: structured messages cannot be broadcast (to: "*")

Steps to Reproduce

1. Create a team with 3+ teammates using TeamCreate
2. From the team lead, call SendMessage with:

``json
{
"to": "*",
"message": { "type": "shutdown_request", "reason": "cleanup" }
}
``

1. Observe error: structured messages cannot be broadcast

Workaround

Send N individual SendMessage calls, one per teammate:

{"to": "teammate-1", "message": {"type": "shutdown_request", "reason": "cleanup"}}
{"to": "teammate-2", "message": {"type": "shutdown_request", "reason": "cleanup"}}
// ... repeat for each teammate

This works but is verbose, wastes tokens proportional to team size, and breaks down when the number of teammates is dynamic.

Claude Model

claude-sonnet-4-20250514

Is this a regression?

Unknown — structured messages and broadcast may have always had this restriction.

Claude Code Version

2.1.32+

Platform

Anthropic API (1p)

Operating System

Linux (Ubuntu 22.04)

Terminal/Shell

bash

Additional Information

Related: #30140 (shared channel proposal) addresses the broader coordination problem, but this specific bug — broadcast rejecting structured messages — is an independent issue with the existing SendMessage primitive.

The restriction seems intentional but overly broad. Even if arbitrary structured messages shouldn't be broadcast, protocol messages like shutdown_request (which are designed for lifecycle management) should be exempt since shutting down all teammates is a core operation.