Protocol Details

Transport

All A2A communication uses JSON-RPC 2.0 over HTTP POST to a single endpoint (the server root /). The AgentCard is served at GET /agentCard.

Request format

{
  "jsonrpc": "2.0",
  "id": "abc-123",
  "method": "tasks/send",
  "params": {
    "message": {
      "messageId": "msg-1",
      "role": "user",
      "parts": [{ "text": "hello", "kind": "text" }]
    }
  }
}

Response format

{
  "jsonrpc": "2.0",
  "id": "abc-123",
  "result": {
    "id": "task-uuid",
    "contextId": "ctx-uuid",
    "status": { "state": "completed", "timestamp": "2026-01-01T00:00:00Z" },
    "artifacts": [ ... ]
  }
}

Methods

Method	Description
tasks/send	Send a message, execute synchronously, return completed task
tasks/sendSubscribe	Send a message, stream events via SSE
tasks/get	Retrieve a task by ID
tasks/list	List all tasks
tasks/cancel	Cancel a non-terminal task
tasks/pushNotification/set	Register a push notification endpoint
tasks/pushNotification/get	Retrieve push notification config
tasks/pushNotification/delete	Remove push notification config
tasks/pushNotification/list	List push notification configs

Error codes

Code	Constant	Description
-32700	PARSE_ERROR	Invalid JSON
-32600	INVALID_REQUEST	Not a valid JSON-RPC 2.0 object
-32601	METHOD_NOT_FOUND	Unknown method
-32602	INVALID_PARAMS	Missing or invalid parameters
-32603	INTERNAL_ERROR	Unhandled server error
-32001	TASK_NOT_FOUND	No task with the given ID
-32002	TASK_NOT_CANCELABLE	Task is already in a terminal state
-32003	PUSH_NOT_SUPPORTED	Agent doesn't support push notifications
-32004	UNSUPPORTED_OPERATION	Operation not supported (e.g. SSE on non-streaming agent)
-32005	CONTENT_TYPE_NOT_SUPPORTED
-32006	INVALID_AGENT_RESPONSE
-32007	EXTENSION_REQUIRED
-32008	VERSION_NOT_SUPPORTED	Unsupported A2A-Version header value

Version negotiation

Clients may include an A2A-Version header. The server accepts 1.0 and 0.3. Any other value returns a VERSION_NOT_SUPPORTED error.

POST / HTTP/1.1
A2A-Version: 1.0
Content-Type: application/json

Task lifecycle

submitted
    │
    ▼
working
    │
    ├──→ completed   (terminal)
    ├──→ failed      (terminal)
    ├──→ canceled    (terminal)
    ├──→ rejected    (terminal)
    ├──→ input_required  (interrupted — waiting for user)
    └──→ auth_required   (interrupted — waiting for credentials)

Terminal tasks cannot be canceled (TASK_NOT_CANCELABLE).
Interrupted tasks can be resumed by sending a new message.

Streaming (SSE)

tasks/sendSubscribe keeps the HTTP connection open and streams text/event-stream events:

data: {"jsonrpc":"2.0","result":{"type":"TaskStatusUpdateEvent","taskId":"…","status":{"state":"working"},"final":false}}

data: {"jsonrpc":"2.0","result":{"type":"TaskArtifactUpdateEvent","taskId":"…","artifact":{…},"final":false}}

data: {"jsonrpc":"2.0","result":{"type":"TaskStatusUpdateEvent","taskId":"…","status":{"state":"completed"},"final":true}}

Events with "final": true signal that the stream is complete.