diff --git a/metagpt/schema.py b/metagpt/schema.py
index 0119f5bbb..f45d1e36d 100644
--- a/metagpt/schema.py
+++ b/metagpt/schema.py
@@ -9,6 +9,7 @@
 from __future__ import annotations
 
 from dataclasses import dataclass, field
+from enum import StrEnum
 from typing import Type, TypedDict, Set, Optional
 
 from pydantic import BaseModel
@@ -16,6 +17,10 @@ from pydantic import BaseModel
 from metagpt.logs import logger
 
 
+class MessageTag(StrEnum):
+    Prerequisite = "prerequisite"
+
+
 class RawMessage(TypedDict):
     content: str
     role: str
@@ -61,6 +66,7 @@ class UserMessage(Message):
     """便于支持OpenAI的消息
        Facilitate support for OpenAI messages
     """
+
     def __init__(self, content: str):
         super().__init__(content, 'user')
 
@@ -70,6 +76,7 @@ class SystemMessage(Message):
     """便于支持OpenAI的消息
        Facilitate support for OpenAI messages
     """
+
     def __init__(self, content: str):
         super().__init__(content, 'system')
 
@@ -79,6 +86,7 @@ class AIMessage(Message):
     """便于支持OpenAI的消息
        Facilitate support for OpenAI messages
     """
+
     def __init__(self, content: str):
         super().__init__(content, 'assistant')