Status vs artifacts update for a2a executor task streaming

[supreme-gg-gg]

In adk-go, the A2A executor streams TaskArtifactUpdateEvent but in adk-python it streams TaskStatusUpdateEvent with the message. Is there a reason why they're different? I noticed that the a2a docs is not very strict with this but it does mention intermediate outputs should use TaskStatusUpdateEvent (meanwhile I think it's reasonable that using artifact update makes partial events easier to deal with). This discrepancy is causing some weird behaviour with a2a clients when switching between python and go ADK since their messages are streamed differently.

• adk-go: https://github.com/google/adk-go/blob/main/server/adka2a/processor.go#L107
• adk-python: https://github.com/google/adk-python/blob/main/src/google/adk/a2a/executor/a2a_agent_executor.py#L180

[yarolegovich]

Hi, I think it should be made more clear in Python code comments, but the implementation you referenced is the "legacy" one. It should and is kept for not breaking existing users on upgrade, but eventually it will be decomissioned.

Instructions on enabling the new (the behaviour as in Go) mode are here:
https://adk.dev/a2a/quickstart-exposing/#agent-executor-v2

A2A docs might unfortunately still be outdated, it's better to reference the spec:

Messages SHOULD NOT be used to deliver task outputs. Results SHOULD BE returned using Artifacts associated with a Task. This separation allows for a clear distinction between communication (Messages) and data output (Artifacts).

Messages in the protocol are a common source of confusion. They are supposed to carry short status updates ("Thinking", "Working", "Calling a tool"), which might not even be persisted in the history afterwards. The data model doesn't support streaming and is not strictly governed by the spec in terms of accumulation semantics:

Agents MAY choose to persist all Messages that contain important information in the Task history to ensure clients can retrieve it later. However, clients MUST NOT rely on this behavior unless negotiated out-of-band.

There were a lot of discussions related to this, for example a2aproject/A2A#1261