Streaming and Async

Streaming and Asynchronous Operations for Long-Running Tasks

Many agent operations are long-running, multi-step, or require human intervention. A2A provides mechanisms to keep clients informed—whether they stay connected or not.

Streaming with Server-Sent Events (SSE)

SSE is ideal when the client can keep an HTTP connection open and wants incremental results or real-time status updates.

Key points:

• Server capability: the AgentCard declares capabilities.streaming: true.
• Initiate streaming: clients call message/stream.
• Event types (examples):
  • Task (current state snapshot)
  • TaskStatusUpdateEvent (state changes and intermediate messages)
  • TaskArtifactUpdateEvent (artifact chunks via append/lastChunk)
• Termination: the server ends a cycle by emitting a final status update (commonly with final: true) and closing the stream.
• Resubscription: clients can reconnect to an active task stream using tasks/resubscribe if the connection drops.

Push notifications (disconnected scenarios)

For tasks that take minutes, hours, or days—or when clients can’t keep a connection open—A2A supports push notifications to a client webhook.

Key points:

• Server capability: the AgentCard declares capabilities.pushNotifications: true.
• Configuration: the client provides a PushNotificationConfig (webhook URL, optional token, optional auth details), either in the initial request or via a task method such as tasks/pushNotificationConfig/set.
• Client action: after receiving a notification, the client typically calls tasks/get to fetch the full updated Task.

Security considerations (push notifications)

Push notifications are outbound, server-initiated HTTP requests, so both sides must take care:

• Server-side: validate webhook URLs (prevent SSRF/DDoS amplification), enforce allowlists/verification, and authenticate to the webhook.
• Client-side: verify the caller (signatures/JWT/HMAC/API keys), prevent replay (timestamps/nonces), and rotate keys safely.