messageId fields are stabilized in ACP v1.
Agents can attach opaque, unique IDs to streamed message chunks. Chunks with the same ID belong to the same message, while a changed ID marks a new message, giving Clients a reliable boundary beyond changes in chunk type.
For the protocol documentation, see Message IDs.Announcements
Message IDs are stabilized
Announcement that optional Agent-generated message IDs are now part of the stable ACP v1 protocol.
Published: June 5, 2026
The Message ID RFD has moved to Completed and optional Agent-generated