diff --git a/metagpt/schema.py b/metagpt/schema.py
index 647ea03f6..08b4e3c92 100644
--- a/metagpt/schema.py
+++ b/metagpt/schema.py
@@ -204,6 +204,7 @@ class Message(BaseModel):
     cause_by: str = Field(default="", validate_default=True)
     sent_from: str = Field(default="", validate_default=True)
     send_to: set[str] = Field(default={MESSAGE_ROUTE_TO_ALL}, validate_default=True)
+    metadata: Dict[str, str] = Field(default_factory=dict)  # metadata for `content` and `instruct_content`
 
     @field_validator("id", mode="before")
     @classmethod