fastapi-native routes

examples/full_schema_description_example.py

@@ -10,7 +10,6 @@ from fastapi_mcp import FastApiMCP
 # Add MCP server to the FastAPI app
 mcp = FastApiMCP(
     items.app,
-    mount_path="/mcp",
     name="Item API MCP",
     description="MCP server for the Item API",
     base_url="http://localhost:8000",

examples/simple_example.py

@@ -10,7 +10,6 @@ from fastapi_mcp import FastApiMCP
 # Add MCP server to the FastAPI app
 mcp = FastApiMCP(
     items.app,
-    mount_path="/mcp",
     name="Item API MCP",
     description="MCP server for the Item API",
     base_url="http://localhost:8000",

fastapi_mcp/server.py

@@ -1,21 +1,25 @@
 from contextlib import asynccontextmanager
 from typing import Dict, Optional, Any, List, Union, AsyncIterator
 
-from fastapi import FastAPI, Request
+from fastapi import FastAPI, Request, APIRouter
 from fastapi.openapi.utils import get_openapi
 from mcp.server.lowlevel.server import Server
-from mcp.server.sse import SseServerTransport
 import mcp.types as types
 
 from fastapi_mcp.openapi.convert import convert_openapi_to_mcp_tools
 from fastapi_mcp.execute import execute_api_tool
+from fastapi_mcp.transport.sse import FastApiSseTransport
+
+from logging import getLogger
+
+
+logger = getLogger(__name__)
 
 
 class FastApiMCP:
     def __init__(
         self,
         fastapi: FastAPI,
-        mount_path: str = "/mcp",
         name: Optional[str] = None,
         description: Optional[str] = None,
         base_url: Optional[str] = None,
@@ -29,7 +33,6 @@ class FastApiMCP:
         self.name = name
         self.description = description
 
-        self._mount_path = mount_path
         self._base_url = base_url
         self._describe_all_responses = describe_all_responses
         self._describe_full_response_schema = describe_full_response_schema
@@ -41,10 +44,12 @@ class FastApiMCP:
         Create an MCP server from the FastAPI app.
 
         Args:
-            app: The FastAPI application
+            fastapi: The FastAPI application
             name: Name for the MCP server (defaults to app.title)
             description: Description for the MCP server (defaults to app.description)
-            base_url: Base URL for API requests (defaults to http://localhost:$PORT)
+            base_url: Base URL for API requests. If not provided, the base URL will be determined from the
+                FastAPI app's root path. Although optional, it is highly recommended to provide a base URL,
+                as the root path would be different when the app is deployed.
             describe_all_responses: Whether to include all possible response schemas in tool descriptions
             describe_full_response_schema: Whether to include full json schema for responses in tool descriptions
 
@@ -126,37 +131,42 @@ class FastApiMCP:
 
         return mcp_server
 
-    def mount(self) -> None:
+    def mount(self, router: Optional[FastAPI | APIRouter] = None, mount_path: str = "/mcp") -> None:
         """
         Mount the MCP server to the FastAPI app.
 
         Args:
-            app: The FastAPI application
+            router: The FastAPI app or APIRouter to mount the MCP server to. If not provided, the MCP
-            mcp_server: The MCP server to mount
+                    server will be mounted to the FastAPI app.
-            operation_map: A mapping of operation IDs to operation details
             mount_path: Path where the MCP server will be mounted
-            base_url: Base URL for API requests
         """
         # Normalize mount path
-        if not self._mount_path.startswith("/"):
+        if not mount_path.startswith("/"):
-            self._mount_path = f"/{self._mount_path}"
+            mount_path = f"/{mount_path}"
-        if self._mount_path.endswith("/"):
+        if mount_path.endswith("/"):
-            self._mount_path = self._mount_path[:-1]
+            mount_path = mount_path[:-1]
+
+        if not router:
+            router = self.fastapi
 
         # Create SSE transport for MCP messages
-        sse_transport = SseServerTransport(f"{self._mount_path}/messages/")
+        sse_transport = FastApiSseTransport(f"{mount_path}/messages/")
 
-        # Define MCP connection handler
+        # Route for MCP connection
+        @router.get(mount_path)
         async def handle_mcp_connection(request: Request):
-            async with sse_transport.connect_sse(request.scope, request.receive, request._send) as streams:
+            async with sse_transport.connect_sse(request.scope, request.receive, request._send) as (reader, writer):
                 await self.mcp_server.run(
-                    streams[0],
+                    reader,
-                    streams[1],
+                    writer,
                     self.mcp_server.create_initialization_options(
                         notification_options=None, experimental_capabilities={}
                     ),
                 )
 
-        # Mount the MCP connection handler
+        # Route for MCP messages
-        self.fastapi.get(self._mount_path)(handle_mcp_connection)
+        @router.post(f"{mount_path}/messages/")
-        self.fastapi.mount(f"{self._mount_path}/messages/", app=sse_transport.handle_post_message)
+        async def handle_post_message(request: Request):
+            await sse_transport.handle_fastapi_post_message(request)
+
+        logger.info(f"MCP server listening at {mount_path}")

fastapi_mcp/transport/sse.py

@@ -0,0 +1,72 @@
+from uuid import UUID
+from logging import getLogger
+
+from fastapi import Request, Response
+from pydantic import ValidationError
+from mcp.server.sse import SseServerTransport
+from mcp.types import JSONRPCMessage
+
+
+logger = getLogger(__name__)
+
+
+class FastApiSseTransport(SseServerTransport):
+    async def handle_fastapi_post_message(self, request: Request) -> None:
+        """
+        A reimplementation of the handle_post_message method of SseServerTransport
+        that integrates better with FastAPI.
+
+        A few good reasons for doing this:
+        1. Avoid mounting a whole Starlette app and instead use a more FastAPI-native
+           approach. Mounting has some known issues and limitations.
+        2. Avoid re-constructing the scope, receive, and send from the request, as done
+           in the original implementation.
+
+        The combination of mounting a whole Starlette app and reconstructing the scope
+        and send from the request proved to be especially error-prone for us when using
+        tracing tools like Sentry, which had destructive effects on the request object
+        when using the original implementation.
+        """
+
+        logger.debug("Handling POST message")
+        scope = request.scope
+        receive = request.receive
+        send = request._send
+
+        session_id_param = request.query_params.get("session_id")
+        if session_id_param is None:
+            logger.warning("Received request without session_id")
+            response = Response("session_id is required", status_code=400)
+            return await response(scope, receive, send)
+
+        try:
+            session_id = UUID(hex=session_id_param)
+            logger.debug(f"Parsed session ID: {session_id}")
+        except ValueError:
+            logger.warning(f"Received invalid session ID: {session_id_param}")
+            response = Response("Invalid session ID", status_code=400)
+            return await response(scope, receive, send)
+
+        writer = self._read_stream_writers.get(session_id)
+        if not writer:
+            logger.warning(f"Could not find session for ID: {session_id}")
+            response = Response("Could not find session", status_code=404)
+            return await response(scope, receive, send)
+
+        body = await request.body()
+        logger.debug(f"Received JSON: {body.decode()}")
+
+        try:
+            message = JSONRPCMessage.model_validate_json(body)
+            logger.debug(f"Validated client message: {message}")
+        except ValidationError as err:
+            logger.error(f"Failed to parse message: {err}")
+            response = Response("Could not parse message", status_code=400)
+            await response(scope, receive, send)
+            await writer.send(err)
+            return
+
+        logger.debug(f"Sending message to writer: {message}")
+        response = Response("Accepted", status_code=202)
+        await response(scope, receive, send)
+        await writer.send(message)