ADK Client Events Reference

This page lists all A2A events that Aion Server emits to clients during ADK agent execution, organized by event category.

Streaming

STREAM_DELTA is a transitory artifact — it is not persisted to the Task’s durable state. The client uses it for live display only.

ADK Event	Client Receives
Partial event with content	`TaskArtifactUpdateEvent(STREAM_DELTA, append=..., last_chunk=false)`
Non-partial event with content	`TaskArtifactUpdateEvent(STREAM_DELTA, last_chunk=true)` + `TaskStatusUpdateEvent(working, message=...)`
Non-partial event without content	`TaskArtifactUpdateEvent(STREAM_DELTA, last_chunk=true)`

STREAM_DELTA close events are only emitted if at least one partial event was sent beforehand.

ADK Event	Client Receives
`artifact_delta` entry in event actions	`TaskArtifactUpdateEvent` per artifact

`a2a_outbox` value	Client Receives
`Message`	`TaskStatusUpdateEvent(working, message=...)`
`Task` patch with metadata	`TaskStatusUpdateEvent(working)` (for metadata)
`Task` patch with history messages	N × `TaskStatusUpdateEvent(working, message=...)`
`Task` patch with artifacts	M × `TaskArtifactUpdateEvent`

Every execution ends with exactly one terminal event.

Outcome	Client Receives
Agent finishes, no outbox, delta text present	`TaskStatusUpdateEvent(working, delta text)` + `TaskStatusUpdateEvent(completed)`
Agent finishes, no outbox, no delta text	`TaskStatusUpdateEvent(completed)`
Agent finishes with `a2a_outbox`	(outbox events) + `TaskStatusUpdateEvent(completed)`
Unhandled exception	`TaskStatusUpdateEvent(failed)`