What probably matters to youSection of the release
What
SDK assistant messages can now carry user_message_uuid, the id you passed as options.uuid when you sent the prompt. It is stamped on the turn's first reply frame only, so a consumer can bind a reply to the send it answers without waiting for the result message.
Details
Sits at the wrapper level next to type and session_id, never inside message.content.
Also attached to error results (error_max_turns, error_during_execution, error_max_budget_usd, error_max_structured_output_retries) and to success results even when is_error is set.
Absent on subagent frames, on synthetic or scheduled turns, and on turns where you sent no client uuid.
Evidence
Client uuid of the user message that triggered this turn (submitMessage options.uuid), stamped on the turn's FIRST reply frame only, user_message_uuid
Strings lifted out of the shipped bundle, so the claim above can be checked against them.
