modelcontextprotocol · praboud-ant · May 2, 2025 · May 2, 2025 · May 2, 2025 · May 2, 2025
diff --git a/examples/servers/simple-prompt/mcp_simple_prompt/server.py b/examples/servers/simple-prompt/mcp_simple_prompt/server.py
@@ -90,6 +90,7 @@ async def get_prompt(
    if transport == "sse":
        from mcp.server.sse import SseServerTransport
        from starlette.applications import Starlette
+        from starlette.responses import Response
        from starlette.routing import Mount, Route

        sse = SseServerTransport("/messages/")
@@ -101,6 +102,7 @@ async def handle_sse(request):
                await app.run(
                    streams[0], streams[1], app.create_initialization_options()
                )
+            return Response()

        starlette_app = Starlette(
            debug=True,

diff --git a/examples/servers/simple-resource/mcp_simple_resource/server.py b/examples/servers/simple-resource/mcp_simple_resource/server.py
@@ -46,6 +46,7 @@ async def read_resource(uri: FileUrl) -> str | bytes:
    if transport == "sse":
        from mcp.server.sse import SseServerTransport
        from starlette.applications import Starlette
+        from starlette.responses import Response
        from starlette.routing import Mount, Route

        sse = SseServerTransport("/messages/")
@@ -57,11 +58,12 @@ async def handle_sse(request):
                await app.run(
                    streams[0], streams[1], app.create_initialization_options()
                )
+            return Response()

        starlette_app = Starlette(
            debug=True,
            routes=[
-                Route("/sse", endpoint=handle_sse),
+                Route("/sse", endpoint=handle_sse, methods=["GET"]),
                Mount("/messages/", app=sse.handle_post_message),
            ],
        )

diff --git a/examples/servers/simple-tool/mcp_simple_tool/server.py b/examples/servers/simple-tool/mcp_simple_tool/server.py
@@ -60,6 +60,7 @@ async def list_tools() -> list[types.Tool]:
    if transport == "sse":
        from mcp.server.sse import SseServerTransport
        from starlette.applications import Starlette
+        from starlette.responses import Response
        from starlette.routing import Mount, Route

        sse = SseServerTransport("/messages/")
@@ -71,11 +72,12 @@ async def handle_sse(request):
                await app.run(
                    streams[0], streams[1], app.create_initialization_options()
                )
+            return Response()

        starlette_app = Starlette(
            debug=True,
            routes=[
-                Route("/sse", endpoint=handle_sse),
+                Route("/sse", endpoint=handle_sse, methods=["GET"]),
                Mount("/messages/", app=sse.handle_post_message),
            ],
        )

diff --git a/src/mcp/server/fastmcp/server.py b/src/mcp/server/fastmcp/server.py
@@ -589,6 +589,7 @@ async def handle_sse(scope: Scope, receive: Receive, send: Send):
                    streams[1],
                    self._mcp_server.create_initialization_options(),
                )
+            return Response()

        # Create routes
        routes: list[Route | Mount] = []

diff --git a/src/mcp/server/session.py b/src/mcp/server/session.py
@@ -97,9 +97,6 @@ def __init__(
        self._exit_stack.push_async_callback(
            lambda: self._incoming_message_stream_reader.aclose()
        )
-        self._exit_stack.push_async_callback(
-            lambda: self._incoming_message_stream_writer.aclose()
-        )

    @property
    def client_params(self) -> types.InitializeRequestParams | None:
@@ -137,6 +134,10 @@ def check_client_capability(self, capability: types.ClientCapabilities) -> bool:

        return True

+    async def _receive_loop(self) -> None:
+        async with self._incoming_message_stream_writer:
+            await super()._receive_loop()
+
    async def _received_request(
        self, responder: RequestResponder[types.ClientRequest, types.ServerResult]
    ):

diff --git a/src/mcp/server/sse.py b/src/mcp/server/sse.py
@@ -10,7 +10,7 @@

    # Create Starlette routes for SSE and message handling
    routes = [
-        Route("/sse", endpoint=handle_sse),
+        Route("/sse", endpoint=handle_sse, methods=["GET"]),
        Mount("/messages/", app=sse.handle_post_message),
    ]

@@ -22,12 +22,18 @@ async def handle_sse(request):
            await app.run(
                streams[0], streams[1], app.create_initialization_options()
            )
+        # Return empty response to avoid NoneType error
+        return Response()

    # Create and run Starlette app
    starlette_app = Starlette(routes=routes)
    uvicorn.run(starlette_app, host="0.0.0.0", port=port)
 ```

+Note: The handle_sse function must return a Response to avoid a "TypeError: 'NoneType'
+object is not callable" error when client disconnects. The example above returns
+an empty Response() after the SSE connection ends to fix this.
+
 See SseServerTransport class documentation for more details.
 """

@@ -121,11 +127,22 @@ async def sse_writer():
                    )

        async with anyio.create_task_group() as tg:
-            response = EventSourceResponse(
-                content=sse_stream_reader, data_sender_callable=sse_writer
-            )
+
+            async def response_wrapper(scope: Scope, receive: Receive, send: Send):
+                """
+                The EventSourceResponse returning signals a client close / disconnect.
+                In this case we close our side of the streams to signal the client that
+                the connection has been closed.
+                """
+                await EventSourceResponse(
+                    content=sse_stream_reader, data_sender_callable=sse_writer
+                )(scope, receive, send)
+                await read_stream_writer.aclose()
+                await write_stream_reader.aclose()
+                logging.debug(f"Client session disconnected {session_id}")
+
            logger.debug("Starting SSE response task")
-            tg.start_soon(response, scope, receive, send)
+            tg.start_soon(response_wrapper, scope, receive, send)

            logger.debug("Yielding read and write streams")
            yield (read_stream, write_stream)

diff --git a/tests/shared/test_sse.py b/tests/shared/test_sse.py
@@ -10,6 +10,7 @@
 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=[