diff --git a/src/a2a/server/request_handlers/default_request_handler.py b/src/a2a/server/request_handlers/default_request_handler.py
index fd378cf47..6a38933f6 100644
--- a/src/a2a/server/request_handlers/default_request_handler.py
+++ b/src/a2a/server/request_handlers/default_request_handler.py
@@ -180,14 +180,21 @@ async def on_cancel_task(
 
         consumer = EventConsumer(queue)
         result = await result_aggregator.consume_all(consumer)
-        if isinstance(result, Task):
-            return result
+        if not isinstance(result, Task):
+            raise ServerError(
+                error=InternalError(
+                    message='Agent did not return valid response for cancel'
+                )
+            )
 
-        raise ServerError(
-            error=InternalError(
-                message='Agent did not return valid response for cancel'
+        if result.status.state != TaskState.canceled:
+            raise ServerError(
+                error=TaskNotCancelableError(
+                    message=f'Task cannot be canceled - current state: {result.status.state}'
+                )
             )
-        )
+
+        return result
 
     async def _run_event_stream(
         self, request: RequestContext, queue: EventQueue
diff --git a/tests/server/request_handlers/test_default_request_handler.py b/tests/server/request_handlers/test_default_request_handler.py
index e8906554a..88fb7d3e5 100644
--- a/tests/server/request_handlers/test_default_request_handler.py
+++ b/tests/server/request_handlers/test_default_request_handler.py
@@ -263,6 +263,56 @@ async def test_on_cancel_task_cancels_running_agent():
     mock_agent_executor.cancel.assert_awaited_once()
 
 
+@pytest.mark.asyncio
+async def test_on_cancel_task_completes_during_cancellation():
+    """Test on_cancel_task fails to cancel a task due to concurrent task completion."""
+    task_id = 'running_agent_task_to_cancel'
+    sample_task = create_sample_task(task_id=task_id)
+    mock_task_store = AsyncMock(spec=TaskStore)
+    mock_task_store.get.return_value = sample_task
+
+    mock_queue_manager = AsyncMock(spec=QueueManager)
+    mock_event_queue = AsyncMock(spec=EventQueue)
+    mock_queue_manager.tap.return_value = mock_event_queue
+
+    mock_agent_executor = AsyncMock(spec=AgentExecutor)
+
+    # Mock ResultAggregator
+    mock_result_aggregator_instance = AsyncMock(spec=ResultAggregator)
+    mock_result_aggregator_instance.consume_all.return_value = (
+        create_sample_task(task_id=task_id, status_state=TaskState.completed)
+    )
+
+    request_handler = DefaultRequestHandler(
+        agent_executor=mock_agent_executor,
+        task_store=mock_task_store,
+        queue_manager=mock_queue_manager,
+    )
+
+    # Simulate a running agent task
+    mock_producer_task = AsyncMock(spec=asyncio.Task)
+    request_handler._running_agents[task_id] = mock_producer_task
+
+    from a2a.utils.errors import (
+        ServerError,  # Local import
+        TaskNotCancelableError,  # Local import
+    )
+
+    with patch(
+        'a2a.server.request_handlers.default_request_handler.ResultAggregator',
+        return_value=mock_result_aggregator_instance,
+    ):
+        params = TaskIdParams(id=task_id)
+        with pytest.raises(ServerError) as exc_info:
+            await request_handler.on_cancel_task(
+                params, create_server_call_context()
+            )
+
+    mock_producer_task.cancel.assert_called_once()
+    mock_agent_executor.cancel.assert_awaited_once()
+    assert isinstance(exc_info.value.error, TaskNotCancelableError)
+
+
 @pytest.mark.asyncio
 async def test_on_cancel_task_invalid_result_type():
     """Test on_cancel_task when result_aggregator returns a Message instead of a Task."""
diff --git a/tests/server/request_handlers/test_jsonrpc_handler.py b/tests/server/request_handlers/test_jsonrpc_handler.py
index 19cf8be06..1d1b3c5d1 100644
--- a/tests/server/request_handlers/test_jsonrpc_handler.py
+++ b/tests/server/request_handlers/test_jsonrpc_handler.py
@@ -149,6 +149,7 @@ async def test_on_cancel_task_success(self) -> None:
         call_context = ServerCallContext(state={'foo': 'bar'})
 
         async def streaming_coro():
+            mock_task.status.state = TaskState.canceled
             yield mock_task
 
         with patch(
@@ -160,6 +161,7 @@ async def streaming_coro():
             assert mock_agent_executor.cancel.call_count == 1
             self.assertIsInstance(response.root, CancelTaskSuccessResponse)
             assert response.root.result == mock_task  # type: ignore
+            assert response.root.result.status.state == TaskState.canceled
             mock_agent_executor.cancel.assert_called_once()
 
     async def test_on_cancel_task_not_supported(self) -> None: