diff --git a/metagpt/__init__.py b/metagpt/__init__.py
index 16359ca19..2980109dd 100644
--- a/metagpt/__init__.py
+++ b/metagpt/__init__.py
@@ -1,9 +1,11 @@
 #!/usr/bin/env python 
 # -*- coding: utf-8 -*-
-# @Time    : 2023/4/24 22:26
-# @Author  : alexanderwu
-# @File    : __init__.py
-# @Desc    : mashenquan, 2023/8/22. Add `Message` for importing by external projects.
+"""
+@Time    : 2023/4/24 22:26
+@Author  : alexanderwu
+@File    : __init__.py
+@Desc    : mashenquan, 2023/8/22. Add `Message` for importing by external projects.
+"""
 
 from metagpt.schema import Message
 
diff --git a/metagpt/schema.py b/metagpt/schema.py
index 4e6cba4ca..749e0fd56 100644
--- a/metagpt/schema.py
+++ b/metagpt/schema.py
@@ -4,6 +4,7 @@
 @Time    : 2023/5/8 22:12
 @Author  : alexanderwu
 @File    : schema.py
+@Desc    : mashenquan, 2023/8/22. Add tags to enable custom message classification.
 """
 from __future__ import annotations
 
@@ -29,7 +30,7 @@ class Message:
     cause_by: Type["Action"] = field(default="")
     sent_from: str = field(default="")
     send_to: str = field(default="")
-    tags: Set = field(default_factory=Set)
+    tags: Set = field(default_factory=set())
 
     def __str__(self):
         # prefix = '-'.join([self.role, str(self.cause_by)])