Handle SSE Disconnects Properly (#612)

tests/shared/test_sse.py

@@ -10,6 +10,7 @@ import uvicorn
 from pydantic import AnyUrl
 from starlette.applications import Starlette
 from starlette.requests import Request
+from starlette.responses import Response
 from starlette.routing import Mount, Route
 
 from mcp.client.session import ClientSession
@@ -83,13 +84,14 @@ def make_server_app() -> Starlette:
     sse = SseServerTransport("/messages/")
     server = ServerTest()
 
-    async def handle_sse(request: Request) -> None:
+    async def handle_sse(request: Request) -> Response:
         async with sse.connect_sse(
             request.scope, request.receive, request._send
         ) as streams:
             await server.run(
                 streams[0], streams[1], server.create_initialization_options()
             )
+        return Response()
 
     app = Starlette(
         routes=[