diff --git a/metagpt/actions/design_api_an.py b/metagpt/actions/design_api_an.py
index a7a6fa57e..cffb09c76 100644
--- a/metagpt/actions/design_api_an.py
+++ b/metagpt/actions/design_api_an.py
@@ -45,6 +45,7 @@ REFINED_FILE_LIST = ActionNode(
     example=["main.py", "game.py", "new_feature.py"],
 )
 
+#optional,because low success reproduction of class diagram in non py project.
 DATA_STRUCTURES_AND_INTERFACES = ActionNode(
     key="Data structures and interfaces",
     expected_type=Optional[str],