Discussion: Should Message events terminate the event stream?

[MipaChan]

Message Events Should Not Terminate Event Streams

Current Behavior

The event stream terminates when receiving any Message event, effectively limiting agents to sending a single message per task execution.

# Python: EventConsumer.consume_all()
is_final_event = (... or isinstance(event, Message) ...)

# TypeScript: ExecutionEventQueue.events()
if (event.kind === 'message' || ...) { break; }

Use Cases We Need

Many real-world scenarios require streaming multiple messages and artifacts:

message: <Chunk1>
message: <Chunk2>
message: <Chunk3>
artifact: <Chunk1>
artifact: <Chunk2>
artifact: <Chunk3>

Examples:

• Streaming LLM responses: Long-form content generation with real-time updates
• Multi-step workflows: Progress updates during complex operations
• Tool-calling patterns: Back-and-forth communication for clarifications
• Mixed content streams: Alternating between messages and artifacts

Why This Design is Problematic

1. Inconsistent with Artifact behavior: Artifacts support streaming with firstChunk/lastChunk, but Messages don't
2. Forces workarounds: Developers must use artifacts for simple text streaming, which is semantically incorrect
3. Limits agent capabilities: Cannot implement common patterns like progress reporting or interactive workflows
4. Poor developer experience: Counter-intuitive that sending a message terminates the entire stream

Proposal

1. Add Streaming Support to Messages

Similar to Artifacts, add firstChunk and lastChunk fields to Message:

interface Message {
  text: string;
  firstChunk?: boolean;
  lastChunk?: boolean;
}

2. Change Termination Logic

Only terminate the stream when:

• TaskStatusUpdateEvent.final = true is received
• Or when a Message/Artifact with lastChunk = true is received

3. Why Now is the Right Time

The A2A protocol currently has a very small audience and is still in early stages. Making this breaking change now will:

• Prevent future migration issues
• Establish better patterns from the start
• Avoid fragmenting the ecosystem with workarounds

Questions for Discussion

1. Is there a specific reason why Messages were designed to be terminal events?
2. Would adding firstChunk/lastChunk to Messages align with the protocol's vision?