update: add the system prompt class in mcp server

examples/servers/system_prompt.py

@@ -0,0 +1,26 @@
+from mcp.server.fastmcp import FastMCP
+from mcp.server.fastmcp.prompts import base as mcp_messages
+
+mcp = FastMCP("weather")
+
+
+@mcp.prompt()
+def weather_system_prompt() -> mcp_messages.SystemMessage:
+    """
+    Creates a prompt asking an AI to weather
+    Args:
+        None: None
+    """
+
+    return mcp_messages.SystemMessage(
+        "You are a helpful weather agent. You answer questions clearly and simply. "
+        "If you don’t know something, say you don’t have that information."
+    )
+
+
+mcp_app = mcp.streamable_http_app()
+
+if __name__ == "__main__":
+    import uvicorn
+
+    uvicorn.run("system_prompt:mcp_app", host="0.0.0.0", port=8002, reload=True)

src/mcp/server/fastmcp/prompts/base.py

@@ -13,7 +13,7 @@
 class Message(BaseModel):
     """Base class for all prompt messages."""
 
-    role: Literal["user", "assistant"]
+    role: Literal["user", "assistant","system"]
     content: ContentBlock
 
     def __init__(self, content: str | ContentBlock, **kwargs: Any):
@@ -25,7 +25,7 @@ def __init__(self, content: str | ContentBlock, **kwargs: Any):
 class UserMessage(Message):
     """A message from the user."""
 
-    role: Literal["user", "assistant"] = "user"
+    role: Literal["user", "assistant","system"] = "user"
 
     def __init__(self, content: str | ContentBlock, **kwargs: Any):
         super().__init__(content=content, **kwargs)
@@ -34,13 +34,20 @@ def __init__(self, content: str | ContentBlock, **kwargs: Any):
 class AssistantMessage(Message):
     """A message from the assistant."""
 
-    role: Literal["user", "assistant"] = "assistant"
+    role: Literal["user", "assistant","system"] = "assistant"
 
     def __init__(self, content: str | ContentBlock, **kwargs: Any):
         super().__init__(content=content, **kwargs)
 
+class SystemMessage(Message):
+    """A message from the assistant."""
+
+    role: Literal["user", "assistant","system"] = "system"
+
+    def __init__(self, content: str | ContentBlock, **kwargs: Any):
+        super().__init__(content=content, **kwargs)
 
-message_validator = TypeAdapter[UserMessage | AssistantMessage](UserMessage | AssistantMessage)
+message_validator = TypeAdapter[UserMessage | AssistantMessage | SystemMessage](UserMessage | AssistantMessage | SystemMessage)
 
 SyncPromptResult = str | Message | dict[str, Any] | Sequence[str | Message | dict[str, Any]]
 PromptResult = SyncPromptResult | Awaitable[SyncPromptResult]

src/mcp/types.py

@@ -35,7 +35,7 @@
 
 ProgressToken = str | int
 Cursor = str
-Role = Literal["user", "assistant"]
+Role = Literal["user", "assistant","system"]
 RequestId = Annotated[int | str, Field(union_mode="left_to_right")]
 AnyFunction: TypeAlias = Callable[..., Any]