A2A Request Lifecycle

( What is A2A?)

• Client sends GET /.well-known/agent-card to retrieve the remote agent’s Agent Card
• Client evaluates skills and capabilities listed in the Agent Card to confirm the agent matches task requirements

• Client parses securitySchemes from the Agent Card
• If OpenID Connect: client requests a JWT from the Auth Server
• Auth Server returns a signed JWT; client attaches it to subsequent requests via HTTP headers

• Client parses the endpoint URL from the Agent Card
• Client sends POST /sendMessage with JWT
• Server processes the request, creates a Task, and returns a Task Response
• For long-running operations, the client periodically polls the server for task status updates

• Client sends POST /sendMessageStream with JWT
• Server opens a Server-Sent Events (SSE) connection and streams multiple events:
  1. Task Submitted
  2. TaskStatusUpdateEvent (progress)
  3. TaskArtifactUpdateEvent (incremental artifact delivery)
  4. TaskStatusUpdateEvent: Completed
• Client can subscribe to receive these updates in real time without polling

( What is A2A?)

• Request/Response (Polling) — synchronous request; client polls server for updates on long-running tasks
• Server-Sent Events (SSE) — client opens a stream to receive real-time incremental results over a persistent HTTP connection
• Push Notifications — server actively sends asynchronous webhook notifications to the client for extended or disconnected scenarios

( Agentic Web 2507.21206)

1. Client agent receives user query and creates a new Task with a unique ID
2. Client retrieves Agent Cards to identify remote agents matching task requirements
3. Client and remote agents exchange Messages under the A2A protocol
4. Task state is continuously updated as execution progresses
5. Once complete, output is encapsulated and delivered as an Artifact