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Activity is structured progress that is not conversation content — a search running, a checklist filling in, a step a UI renders as its own widget. It is materialised as a message so it keeps its place in the sequence, but its content is an object, not text.

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Events

Activity binds the snapshot–delta pattern, scoped per activity message and matched by messageId.

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ACTIVITY_SNAPSHOT

Creates or replaces one activity message.
  • activityType is an open string: the set is the producer’s, not the protocol’s. A consumer MUST tolerate types it does not recognise.
  • A snapshot for a messageId the consumer has not seen creates the activity message, in sequence position at the point of arrival.
  • A snapshot for an existing activity replaces its content and its activityType. replace is OPTIONAL; absent means it does, and that meaning is normative. An explicit replace: false asks the consumer to leave the existing message as it stands — content and activityType; the snapshot’s own values apply only when it creates the message. It is not a merge.
  • Replacement replaces content, not the metadata accumulated so far — metadata keeps merging under the metadata rules. Attribution is the exception: a replacing snapshot re-mints the activity, so the message’s subagentRunId becomes the snapshot’s own, including becoming absent.
  • A replace: false snapshot for an existing activity does not re-mint ownership: the established owner stands, and the snapshot’s own attribution MUST agree with it. The producer duty is the same one the subagent rules put on streamed continuations; the consumer obligation is deliberately weaker — the established owner stands and a consumer is not required to reject the disagreement, where a streamed continuation’s mismatch must be rejected.

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ACTIVITY_DELTA

Amends one activity message’s content with an RFC 6902 patch against it. The delta’s activityType replaces the message’s — a delta MAY retype the activity it amends, and a delta that does not intend to MUST repeat the current type, since the field is required.
  • A producer MUST NOT send a delta for an activity message it has not created with a snapshot — the snapshot is the baseline the pattern requires.
  • A delta naming a message that does not exist, or one that is not an activity message, is skipped; the consumer SHOULD surface a warning, and MUST NOT fail the run.
  • The patched result MUST still be an object: content is one by schema, and a producer MUST NOT send a patch — a root replacement, say — whose application would make it anything else. A consumer is not required to detect the violation.
  • Patch failure handling is the pattern’s, with resynchronisation by a fresh snapshot of the same messageId.

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Activity messages in the conversation

An activity message appears in history with the fixed role activity. It is rendering material for the consumer, not conversation the agent resumes from: a consumer MUST strip activity messages from the messages it sends in run input. How activity messages survive a MESSAGES_SNAPSHOT is specified with that event.

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Message Flow

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Data Types

ActivitySnapshotEvent, ActivityDeltaEvent and ActivityMessage are defined by the schema reference. ActivityMessage stands alone rather than composing the base message, because its content is an object rather than a string.

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Error Handling

Malformed activity events are fatal as malformed known values. The two tolerated failures — a delta against a missing or non-activity message, and a well-formed patch that does not apply — warn and skip, as above; both leave the producer and consumer possibly disagreed about that activity, which the producer’s next snapshot repairs.