user_message_uuids

The uuids of every message you sent that Claude Code answered in this turn. When you send several messages close together, Claude Code can merge them into one turn, and user_message_uuid then names only the last of them. To match the reply to any of the merged messages, look for that message's uuid anywhere in this list. Requires Agent SDK v0.3.259 or later.

Claude Code sets the list together with user_message_uuid on each reply frame that carries that field and on the result. For the full set of frames that carry user_message_uuid, and the version each requires, see user_message_uuid. The list always contains user_message_uuid and holds at most 64 entries.

When Claude Code picks up a regular message you sent while a turn was running, it adds that message's uuid to the result's list.

When a first reply or result carries user_message_uuid without the list, it came from an earlier Claude Code version, so fall back to the single field.