A2A Streaming (SSE)

Before writing code

Fetch live docs:

1. Fetch https://a2a-protocol.org/latest/specification/ for the streaming section
2. Web-search site:github.com a2aproject A2A streaming SSE message/stream for streaming protocol details
3. Web-search site:github.com a2aproject a2a-samples streaming for streaming implementation examples
4. Fetch SDK docs for streaming server/client classes and event types

Conceptual Architecture

What A2A Streaming Is

A2A streaming uses Server-Sent Events (SSE) to deliver real-time task updates from server to client. Instead of waiting for the entire task to complete (message/send), the client receives incremental updates as the agent works (message/stream).

When to Use Streaming

• Long-running tasks — Keep the client informed of progress
• Incremental results — Deliver partial artifacts as they're generated
• Interactive UX — Show real-time agent activity to end users
• Token streaming — Stream LLM output tokens as they're generated

How It Works

1. Client sends message/stream (same payload as message/send)
2. Server responds with Content-Type: text/event-stream
3. Server sends SSE events as the task progresses
4. Final event contains a terminal task state
5. Connection closes after the terminal event

SSE Event Types

Event Format

SSE events follow the standard format:

event: <event-type>
data: <JSON payload>

Each event's data field contains a JSON object with:

• taskId — Which task this event belongs to
• type — Event type discriminator
• Type-specific fields (status update, artifact data, message content)

Re-subscription

If the SSE connection drops, the client can re-subscribe:

• Call tasks/resubscribe with the taskId
• Server resumes sending events from the current state
• Events that were sent before the reconnection are NOT replayed (unless the server implements replay)

Server-Side Implementation

The server must:

1. Declare streaming: true in the Agent Card capabilities
2. Handle message/stream method
3. Keep the HTTP connection open
4. Send SSE events as the agent processes
5. Send a final event with a terminal state
6. Close the connection cleanly

Client-Side Implementation

The client must:

1. Check Agent Card for streaming capability
2. Send message/stream instead of message/send
3. Parse SSE events (use an SSE client library)
4. Handle each event type appropriately
5. Handle connection drops and re-subscribe if needed

Backpressure and Buffering

• Servers should buffer events if the client can't consume fast enough
• Set reasonable connection timeouts
• Send periodic keepalive comments (:keepalive\n\n) to prevent proxy timeouts
• Clients should process events asynchronously to avoid blocking the stream

Best Practices

• Always send a terminal event before closing the stream
• Include the full task status in TaskStatusUpdateEvent (not just the state name)
• Use TaskArtifactUpdateEvent for incremental delivery of large results
• Implement heartbeat/keepalive to prevent connection timeouts
• Handle client disconnection gracefully on the server side
• Test with slow clients to verify backpressure handling
• Set appropriate timeouts on both client and server

Fetch the specification for exact SSE event schemas, event naming conventions, and re-subscription protocol before implementing.