Fix ASGI errors

fastapi_mcp/server.py

@@ -167,6 +167,6 @@ class FastApiMCP:
         # Route for MCP messages
         @router.post(f"{mount_path}/messages/")
         async def handle_post_message(request: Request):
-            await sse_transport.handle_fastapi_post_message(request)
+            return await sse_transport.handle_fastapi_post_message(request)
 
         logger.info(f"MCP server listening at {mount_path}")

fastapi_mcp/transport/sse.py

@@ -1,17 +1,20 @@
 from uuid import UUID
 from logging import getLogger
+from typing import Union
 
-from fastapi import Request, Response
+from anyio.streams.memory import MemoryObjectSendStream
+from fastapi import Request, Response, BackgroundTasks, HTTPException
+from fastapi.responses import JSONResponse
 from pydantic import ValidationError
 from mcp.server.sse import SseServerTransport
-from mcp.types import JSONRPCMessage
+from mcp.types import JSONRPCMessage, JSONRPCError, ErrorData
 
 
 logger = getLogger(__name__)
 
 
 class FastApiSseTransport(SseServerTransport):
-    async def handle_fastapi_post_message(self, request: Request) -> None:
+    async def handle_fastapi_post_message(self, request: Request) -> Response:
         """
         A reimplementation of the handle_post_message method of SseServerTransport
         that integrates better with FastAPI.
@@ -21,6 +24,8 @@ class FastApiSseTransport(SseServerTransport):
            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.
+        3. Use FastAPI's native response handling mechanisms and exception patterns to
+           avoid unexpected rabbit holes.
 
         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
@@ -28,30 +33,24 @@ class FastApiSseTransport(SseServerTransport):
         when using the original implementation.
         """
 
-        logger.debug("Handling POST message")
-        scope = request.scope
-        receive = request.receive
-        send = request._send
+        logger.debug("Handling POST message with FastAPI patterns")
 
         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)
+            raise HTTPException(status_code=400, detail="session_id is required")
 
         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)
+            raise HTTPException(status_code=400, detail="Invalid session ID")
 
         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)
+            raise HTTPException(status_code=404, detail="Could not find session")
 
         body = await request.body()
         logger.debug(f"Received JSON: {body.decode()}")
@@ -61,12 +60,49 @@ class FastApiSseTransport(SseServerTransport):
             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
+            # Create background task to send error
+            background_tasks = BackgroundTasks()
+            background_tasks.add_task(self._send_message_safely, writer, err)
+            response = JSONResponse(content={"error": "Could not parse message"}, status_code=400)
+            response.background = background_tasks
+            return response
+        except Exception as e:
+            logger.error(f"Error processing request body: {e}")
+            raise HTTPException(status_code=400, detail="Invalid request body")
 
-        logger.debug(f"Sending message to writer: {message}")
-        response = Response("Accepted", status_code=202)
-        await response(scope, receive, send)
-        await writer.send(message)
+        # Create background task to send message
+        background_tasks = BackgroundTasks()
+        background_tasks.add_task(self._send_message_safely, writer, message)
+        logger.debug("Accepting message, will send in background")
+
+        # Return response with background task
+        response = JSONResponse(content={"message": "Accepted"}, status_code=202)
+        response.background = background_tasks
+        return response
+
+    async def _send_message_safely(
+        self, writer: MemoryObjectSendStream[JSONRPCMessage], message: Union[JSONRPCMessage, ValidationError]
+    ):
+        """Send a message to the writer, avoiding ASGI race conditions"""
+
+        try:
+            logger.debug(f"Sending message to writer from background task: {message}")
+
+            if isinstance(message, ValidationError):
+                # Convert ValidationError to JSONRPCError
+                error_data = ErrorData(
+                    code=-32700,  # Parse error code in JSON-RPC
+                    message="Parse error",
+                    data={"validation_error": str(message)},
+                )
+                json_rpc_error = JSONRPCError(
+                    jsonrpc="2.0",
+                    id="unknown",  # We don't know the ID from the invalid request
+                    error=error_data,
+                )
+                error_message = JSONRPCMessage(root=json_rpc_error)
+                await writer.send(error_message)
+            else:
+                await writer.send(message)
+        except Exception as e:
+            logger.error(f"Error sending message to writer: {e}")