diff --git a/src/a2a/server/request_handlers/default_request_handler.py b/src/a2a/server/request_handlers/default_request_handler.py
index a1b8d565..d311691f 100644
--- a/src/a2a/server/request_handlers/default_request_handler.py
+++ b/src/a2a/server/request_handlers/default_request_handler.py
@@ -200,6 +200,12 @@ async def _setup_message_execution(
                 )
 
             task = task_manager.update_with_message(params.message, task)
+        elif params.message.taskId:
+            raise ServerError(
+                error=TaskNotFoundError(
+                    message=f'Task {params.message.taskId} was specified but does not exist'
+                )
+            )
 
         # Build request context
         request_context = await self._request_context_builder.build(
diff --git a/tests/server/request_handlers/test_default_request_handler.py b/tests/server/request_handlers/test_default_request_handler.py
index 06243d9d..2d9e9960 100644
--- a/tests/server/request_handlers/test_default_request_handler.py
+++ b/tests/server/request_handlers/test_default_request_handler.py
@@ -1779,3 +1779,85 @@ async def test_on_resubscribe_to_task_in_terminal_state(terminal_state):
         in exc_info.value.error.message
     )
     mock_task_store.get.assert_awaited_once_with(task_id)
+
+
+@pytest.mark.asyncio
+async def test_on_message_send_task_id_provided_but_task_not_found():
+    """Test on_message_send when taskId is provided but task doesn't exist."""
+    task_id = 'nonexistent_task'
+    mock_task_store = AsyncMock(spec=TaskStore)
+
+    request_handler = DefaultRequestHandler(
+        agent_executor=DummyAgentExecutor(), task_store=mock_task_store
+    )
+
+    params = MessageSendParams(
+        message=Message(
+            role=Role.user,
+            messageId='msg_nonexistent',
+            parts=[Part(root=TextPart(text='Hello'))],
+            taskId=task_id,
+            contextId='ctx1',
+        )
+    )
+
+    from a2a.utils.errors import ServerError
+
+    # Mock TaskManager.get_task to return None (task not found)
+    with patch(
+        'a2a.server.request_handlers.default_request_handler.TaskManager.get_task',
+        return_value=None,
+    ):
+        with pytest.raises(ServerError) as exc_info:
+            await request_handler.on_message_send(
+                params, create_server_call_context()
+            )
+
+    assert isinstance(exc_info.value.error, TaskNotFoundError)
+    assert exc_info.value.error.message
+    assert (
+        f'Task {task_id} was specified but does not exist'
+        in exc_info.value.error.message
+    )
+
+
+@pytest.mark.asyncio
+async def test_on_message_send_stream_task_id_provided_but_task_not_found():
+    """Test on_message_send_stream when taskId is provided but task doesn't exist."""
+    task_id = 'nonexistent_stream_task'
+    mock_task_store = AsyncMock(spec=TaskStore)
+
+    request_handler = DefaultRequestHandler(
+        agent_executor=DummyAgentExecutor(), task_store=mock_task_store
+    )
+
+    params = MessageSendParams(
+        message=Message(
+            role=Role.user,
+            messageId='msg_nonexistent_stream',
+            parts=[Part(root=TextPart(text='Hello'))],
+            taskId=task_id,
+            contextId='ctx1',
+        )
+    )
+
+    from a2a.utils.errors import ServerError
+
+    # Mock TaskManager.get_task to return None (task not found)
+    with patch(
+        'a2a.server.request_handlers.default_request_handler.TaskManager.get_task',
+        return_value=None,
+    ):
+        with pytest.raises(ServerError) as exc_info:
+            # Need to consume the async generator to trigger the error
+            async for _ in request_handler.on_message_send_stream(
+                params, create_server_call_context()
+            ):
+                pass
+
+    assert isinstance(exc_info.value.error, TaskNotFoundError)
+    assert exc_info.value.error.message
+    assert (
+        f'Task {task_id} was specified but does not exist'
+        in exc_info.value.error.message
+    )