Fix 307 Temporary Redirect when use streamable_http

src/mcp/server/fastmcp/server.py

@@ -618,7 +618,7 @@ async def run_streamable_http_async(self) -> None:
         import uvicorn
 
         starlette_app = self.streamable_http_app()
-
+        starlette_app.router.redirect_slashes = False
         config = uvicorn.Config(
             starlette_app,
             host=self.settings.host,
@@ -770,7 +770,7 @@ async def sse_endpoint(request: Request) -> Response:
     def streamable_http_app(self) -> Starlette:
         """Return an instance of the StreamableHTTP server app."""
         from starlette.middleware import Middleware
-        from starlette.routing import Mount
+        from starlette.routing import Route
 
         # Create session manager on first call (lazy initialization)
         if self._session_manager is None:
@@ -782,16 +782,21 @@ def streamable_http_app(self) -> Starlette:
             )
 
         # Create the ASGI handler
-        async def handle_streamable_http(
-            scope: Scope, receive: Receive, send: Send
-        ) -> None:
-            await self.session_manager.handle_request(scope, receive, send)
+        async def handle_streamable_http(request: Request) -> Response:
+            await self.session_manager.handle_request(
+                request.scope, request.receive, request._send
+            )
+            return Response()
 
         # Create routes
-        routes: list[Route | Mount] = []
+        routes: list[Route] = []
         middleware: list[Middleware] = []
         required_scopes = []
 
+        # Always register both /mcp and /mcp/ for full compatibility
+        _main_path = self.settings.streamable_http_path.removesuffix("/")
+        _alt_path = _main_path + "/"
+
         # Add auth endpoints if auth provider is configured
         if self._auth_server_provider:
             assert self.settings.auth
@@ -817,19 +822,39 @@ async def handle_streamable_http(
                     revocation_options=self.settings.auth.revocation_options,
                 )
             )
-            routes.append(
-                Mount(
-                    self.settings.streamable_http_path,
-                    app=RequireAuthMiddleware(handle_streamable_http, required_scopes),
-                )
+            routes.extend(
+                [
+                    Route(
+                        _main_path,
+                        endpoint=RequireAuthMiddleware(
+                            handle_streamable_http, required_scopes
+                        ),
+                        methods=["GET", "POST", "OPTIONS"],
+                    ),
+                    Route(
+                        _alt_path,
+                        endpoint=RequireAuthMiddleware(
+                            handle_streamable_http, required_scopes
+                        ),
+                        methods=["GET", "POST", "OPTIONS"],
+                    ),
+                ]
             )
         else:
             # Auth is disabled, no wrapper needed
-            routes.append(
-                Mount(
-                    self.settings.streamable_http_path,
-                    app=handle_streamable_http,
-                )
+            routes.extend(
+                [
+                    Route(
+                        _main_path,
+                        endpoint=handle_streamable_http,
+                        methods=["GET", "POST", "OPTIONS"],
+                    ),
+                    Route(
+                        _alt_path,
+                        endpoint=handle_streamable_http,
+                        methods=["GET", "POST", "OPTIONS"],
+                    ),
+                ]
             )
 
         routes.extend(self._custom_starlette_routes)

tests/server/fastmcp/test_server.py

@@ -124,6 +124,23 @@ async def test_starlette_routes_with_mount_path(self):
             mount_routes[0].path == "/messages"
         ), "Mount route path should be /messages"
 
+        mcp = FastMCP()
+        app = mcp.streamable_http_app()
+
+        # Find routes by type
+        streamable_routes = [r for r in app.routes if isinstance(r, Route)]
+
+        # Verify routes exist
+        assert len(streamable_routes) == 2, "Should have two streamable routes"
+
+        # Verify path values
+        assert (
+            streamable_routes[0].path == "/mcp"
+        ), "Streamable route path should be /mcp"
+        assert (
+            streamable_routes[1].path == "/mcp/"
+        ), "Streamable route path should be /mcp/"
+
     @pytest.mark.anyio
     async def test_non_ascii_description(self):
         """Test that FastMCP handles non-ASCII characters in descriptions correctly"""