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The to: "" broadcast mode for SendMessage in agent teams has been deprecated. The new behavior produces: "broadcast (to: \"\") is no longer supported — send a message per recipient". The team system prompt no longer mentions broadcast capability, directing agents to send individual messages instead.
Evidence
Deprecation message (search for "broadcast (to: \"*\") is no longer supported").
Strings lifted out of the shipped bundle, so the claim above can be checked against them.