diff --git a/lang_agent/base.py b/lang_agent/base.py
index 68e7292..d862793 100644
--- a/lang_agent/base.py
+++ b/lang_agent/base.py
@@ -1,4 +1,4 @@
-from typing import List, Callable, Tuple
+from typing import List, Callable, Tuple, Dict
 from abc import ABC, abstractmethod
 from PIL import Image
 from io import BytesIO
@@ -6,6 +6,7 @@ import matplotlib.pyplot as plt
 from loguru import logger
 
 from langgraph.graph.state import CompiledStateGraph
+from langchain_core.messages import BaseMessage
 
 
 class LangToolBase(ABC):
@@ -54,4 +55,8 @@ class ToolNodeBase(GraphBase):
         returns 2 words, one for starting delayed yeilding, the other for ending delayed yielding,
         they should be of format ('[key1]', '[key2]'); key1 is starting, key2 is ending
         """
-        return None, None
\ No newline at end of file
+        return None, None
+    
+    @abstractmethod
+    def invoke(self, inp)->Dict[str, List[BaseMessage]]:
+        pass
\ No newline at end of file