Terminal state

A terminal state is a task state that ends an A2A task for good: TASK_STATE_COMPLETED, TASK_STATE_FAILED, TASK_STATE_CANCELED or TASK_STATE_REJECTED. The a2a.proto file marks exactly these four as terminal.

State	Meaning
`TASK_STATE_COMPLETED`	Finished successfully
`TASK_STATE_FAILED`	Finished with an error
`TASK_STATE_CANCELED`	Canceled before completion
`TASK_STATE_REJECTED`	The agent decided not to perform the task, at creation or later

What changes at a terminal state.

Send Message and Send Streaming Message to the task fail with UnsupportedOperationError (specification sections 3.1.1 and 3.1.2).
Cancel Task fails with TaskNotCancelableError (section 3.1.5).
Subscribe to Task fails with UnsupportedOperationError (section 3.1.6).
An open stream for the task must close, and a blocking Send Message call returns (sections 3.1.2 and 3.2.2).

Over JSON-RPC, UnsupportedOperationError maps to code -32004 (section 5.4). Illustrative response to a message sent to a finished task:

{
  "jsonrpc": "2.0",
  "id": 7,
  "error": {"code": -32004, "message": "Task is in a terminal state and cannot accept messages"}
}

Follow-up work. A task in a terminal state cannot restart, which gives clients a stable record of its inputs, outputs and artifacts. To refine a result or continue, the client sends a new message in the same contextId, optionally listing the earlier task in referenceTaskIds, and the agent creates a new task (Life of a Task guide).

States that are not terminal. TASK_STATE_INPUT_REQUIRED and TASK_STATE_AUTH_REQUIRED are interrupted states. The task is paused, still accepts messages, and can resume. TASK_STATE_SUBMITTED and TASK_STATE_WORKING mean the agent is still processing.

Sources

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