> ## Documentation Index
> Fetch the complete documentation index at: https://agentclientprotocol.com/llms.txt
> Use this file to discover all available pages before exploring further.

# 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](/rfds/message-id) has moved to Completed and optional Agent-generated `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](/protocol/v1/prompt-turn#message-ids).