Task lifecycle (A2A)

The task lifecycle is the sequence of states an A2A task moves through, from the moment an agent creates it in response to a message until it reaches one of four terminal states, after which it can never change again.

Message or task. Not every exchange becomes a task. An agent can answer a message with a single Message and keep no state, or it can create a Task with a server-generated id and track the work. Once an agent has created a task, later responses about that work are Task objects.

The stages.

1. Created: TASK_STATE_SUBMITTED means the agent has acknowledged the task, and TASK_STATE_WORKING means it is processing.
2. Interrupted: TASK_STATE_INPUT_REQUIRED waits for more input from the client. TASK_STATE_AUTH_REQUIRED waits for authorization, often delivered out of band. The client continues by sending a message with the same taskId.
3. Terminal: TASK_STATE_COMPLETED, TASK_STATE_FAILED, TASK_STATE_CANCELED or TASK_STATE_REJECTED. An agent can reject a task when it is created or later, once it decides it cannot or will not proceed.

Rules that follow.

• A blocking SendMessage, the default, returns once the task is terminal or interrupted. With returnImmediately set, it returns straight after creating the task.
• Messages sent to a terminal task fail with UnsupportedOperationError, and CancelTask on one fails with TaskNotCancelableError.
• A refinement or follow-up starts a new task in the same contextId, and can point back with referenceTaskIds. The finished task stays immutable, which keeps every artifact traceable to one unit of work.
• Clients follow progress by polling GetTask, by streaming, or through push notifications.

An interrupted task, illustrative:

{
  "id": "5f1c2d9e-8b7a-4e3c-9d21-6a0b4c7e2f13",
  "contextId": "ctx-7",
  "status": {
    "state": "TASK_STATE_INPUT_REQUIRED",
    "message": {
      "messageId": "a9e4b1c2-7d3f-4e8a-b6c5-0f2d1e3a4b5c",
      "role": "ROLE_AGENT",
      "parts": [{"text": "Which delivery address should I use?"}]
    },
    "timestamp": "2026-09-26T14:03:11.482Z"
  }
}

Neighbouring terms. Task state is the enum value at each step, and a terminal state is one of the four end points.