According to A2A spec section 8.2, it should return -32002 (TaskNotCancelableError)

Author: maeste

src/a2a/server/request_handlers/default_request_handler.py

@@ -36,6 +36,7 @@
     MessageSendParams,
     Task,
     TaskIdParams,
+    TaskNotCancelableError,
     TaskNotFoundError,
     TaskPushNotificationConfig,
     TaskQueryParams,
@@ -124,6 +125,14 @@ async def on_cancel_task(
         if not task:
             raise ServerError(error=TaskNotFoundError())
 
+        # Check if task is in a non-cancelable state (completed, canceled, failed, rejected)
+        if task.status.state in TERMINAL_TASK_STATES:
+            raise ServerError(
+                error=TaskNotCancelableError(
+                    message=f'Task cannot be canceled - current state: {task.status.state}'
+                )
+            )
+
         task_manager = TaskManager(
             task_id=task.id,
             context_id=task.context_id,