ExperimentalA unique identifier for the message.
OptionalcontentComplete replacement content for this message.
Optional_The _meta property is reserved by ACP to allow clients and agents to attach additional
metadata to their interactions. Implementations MUST NOT make assumptions about values at
these keys. Omitted means no metadata update; null is an explicit clear signal.
See protocol docs: Extensibility
An agent message upsert.
Only [
AgentMessage::message_id] is required.contenthas patch semantics: an omitted field leaves existing message content unchanged,nullclears the value, and a concrete array replaces the previous value. For a newmessageId, omitted fields use client defaults.contentis replaced as a whole array; send[]ornullto clear it.Message updates and chunks are applied in the order they are received. When an
agent_messageupdate includescontent, that array replaces any content previously accumulated for the message, including content from earlier chunks. Later chunks with the samemessageIdappend to the current content.