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)`, with the messaging extension’s `…#StreamDeltaPayload` schema marker on the event’s own `metadata`
Non-partial event with content	`TaskStatusUpdateEvent(working, message=...)`
Non-partial event without content	Artifacts from `artifact_delta`, if any

A STREAM_DELTA sequence ends with the next durable message (TaskStatusUpdateEvent(working, message=...)) or with the terminal task status. Every STREAM_DELTA update carries content, and the next partial event after a durable message opens a new section with append=false.

Artifacts

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

Outbox

`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`

Terminal

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)`

Streaming Delta Semantics — append, lastChunk, and how a chunked reply ends
Streaming and Artifacts
Message Mapping

Python SDK

LangGraph

Google ADK

Client Events Reference

Streaming

Artifacts

Outbox

Terminal

​Streaming

​Artifacts

​Outbox

​Terminal

​Related Pages

Streaming

Artifacts

Outbox

Terminal

Related Pages