Streaming Responses

A2A defines two transport modes: unary (message/send — one request, one response) and streaming (message/stream — Server-Sent Events with incremental updates). A2X supports both and picks the right one automatically based on the client's request.

When to enable streaming

Enable SSE when any of these apply:

• Your agent's responses are long enough that users would feel a "typing" pause.
• You want to show intermediate tool-call events ("thinking…", "searching…").
• You're orchestrating multi-agent pipelines and want to surface progress.

Skip it when responses are short and latency-insensitive — the unary path is simpler to reason about.

Server side: already on by default

If you're using toA2x(), streaming is already wired up. The short version:

toA2x(agent, { port: 4000, defaultUrl: 'http://localhost:4000/a2a' });

The AgentCard emitted at /.well-known/agent.json advertises capabilities.streaming: true, and the POST /a2a endpoint serves SSE for message/stream requests.

If you're wiring manually (see Framework Integration), the key piece is:

import { AgentExecutor, StreamingMode } from '@a2x/sdk';
 
const executor = new AgentExecutor({
  runner,
  runConfig: { streamingMode: StreamingMode.SSE },
});

StreamingMode.SSE lets the runner emit incremental events. The rest is automatic.

What the client actually receives

For a message/stream call, the client gets an SSE stream of typed events. Each event is one of:

See Consuming Streams for the client-side iteration pattern.

Disabling streaming

If you want a deliberately non-streaming agent (e.g. for backpressure reasons), drop streamingMode:

const executor = new AgentExecutor({ runner, runConfig: {} });

The server will still accept message/stream requests, but responses come as single-chunk streams. Most clients handle this correctly.

Debugging SSE

SSE is plain text over HTTP. curl works:

curl -N \
  -H 'Accept: text/event-stream' \
  -H 'Content-Type: application/json' \
  -X POST http://localhost:4000/a2a \
  -d '{
    "jsonrpc": "2.0",
    "id": "1",
    "method": "message/stream",
    "params": { "message": { "role": "user", "parts": [{ "text": "Hi" }] } }
  }'

The -N flag disables curl's output buffering so you see events as they arrive.