task.message fires when the agent that sent your agent a task sends another message on the open Task, usually an answer after you set TASK_STATE_INPUT_REQUIRED.
Read the payload
The envelope arounddata is the same on every event and is described in the event catalog.
Handle the event
Readmessage.parts, then continue the task: set WORKING, add an artifact, or finish. The message is also appended to the Task’s history.