ExperimentalThe opaque session ID identifying the child in all ACP messages.
OptionaltitleThe parent's human-readable display title for this child. It need not be unique.
Omitted means unchanged; null clears it. If unset, the Client chooses
a fallback presentation.
OptionaldescriptionThe parent's human-readable description of the child's role or purpose.
Omitted means unchanged; null clears it. If unset, the Client chooses
a fallback presentation. This is current display metadata, not the
history of instructions sent to the child.
OptionalcapabilitiesClient-initiated session mutations permitted for this subagent session.
Omitted means unchanged; null clears the capability set and disables
child mutations. If never supplied, no session mutations are permitted.
Read-only operations retain their normal protocol semantics and capability
requirements. A concrete object replaces the whole capability set.
OptionalstateCurrent state snapshot for the child session.
Omitted means unchanged; null clears the current activity without
asserting idle or sending an unknown snapshot. A concrete state
replaces the previous state object wholesale. If never supplied, the
current activity is unset/unconfirmed.
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.
See protocol docs: Extensibility
Omitted means unchanged; null removes the metadata.
UNSTABLE
This capability is not part of the spec yet, and may be removed or changed at any point.
Notification that the enclosing parent session created and owns a child session.
Later updates modify the existing association's metadata, not its ownership.
Sent on the immediate parent session. The first update for an unknown [
SubagentUpdate::session_id] announces the child and MUST be sent before any live child traffic or live message naming the child as sender or recipient. Parents may message and reuse an announced child across multiple operations. Child events are delivered automatically on the same connection; no child load, resume, or subscription is needed.Only the subagent session ID is required. Omitted patch fields keep their previous values;
nullclears them. Clearing capabilities disables child mutations. Clearing state leaves current activity unset/unconfirmed: it does not imply idle, stop work, or create anunknownstate snapshot. A concrete state replaces the entire previous state object, not the session. The title and description provide the parent's display metadata for the child. They do not replace the content of individual messages or operations.