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

An outbox payload is not announced as an event of its own. The server merges it into the task it is already keeping, and the terminal Task that closes the stream is what carries it — the same object a unary SendMessage returns and a later GetTask answers with.

`a2a_outbox` value	Client Receives
`Message`	Nothing on the wire at that moment; the message is in the terminal task’s `history`
`Task` patch	Nothing on the wire at that moment; its history, artifacts and metadata are on the terminal task

Look for the reply in the closing task, not among the status updates. A turn that used the outbox says nothing through the thread, so an empty stream of status events is what an outbox reply looks like from outside.

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`	`TaskStatusUpdateEvent(completed)`, with the payload merged into the closing task
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