Fix 307 Temporary Redirect when use streamable_http

src/mcp/server/fastmcp/server.py

@@ -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 Mount, Router
 
         # Create session manager on first call (lazy initialization)
         if self._session_manager is None:
@@ -787,12 +787,24 @@ async def handle_streamable_http(
         ) -> None:
             await self.session_manager.handle_request(scope, receive, send)
 
-        # Create routes
-        routes: list[Route | Mount] = []
+        async def streamable_http_endpoint(request: Request):
+            return await handle_streamable_http(request.scope, request.receive, request._send)  # type: ignore[reportPrivateUsage]
+
+        # Normalize the main path (no trailing slash)
+        _main_path = self.settings.streamable_http_path.removesuffix("/")
+
+        streamable_router = Router(
+            routes=[
+                Route("/", endpoint=streamable_http_endpoint, methods=["GET", "POST"]),
+            ],
+            redirect_slashes=False,
+        )
+
+        routes: list[Route | Mount ] = []
         middleware: list[Middleware] = []
         required_scopes = []
 
-        # Add auth endpoints if auth provider is configured
+        # Auth endpoints if auth provider is configured
         if self._auth_server_provider:
             assert self.settings.auth
             from mcp.server.auth.routes import create_auth_routes
@@ -817,18 +829,21 @@ 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),
+                    _main_path,
+                    app=RequireAuthMiddleware(
+                        streamable_router, required_scopes
+                    ),
                 )
             )
         else:
             # Auth is disabled, no wrapper needed
             routes.append(
                 Mount(
-                    self.settings.streamable_http_path,
-                    app=handle_streamable_http,
+                    _main_path,
+                    app=streamable_router,
                 )
             )
 

tests/server/fastmcp/test_server.py

@@ -124,6 +124,20 @@ 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, Mount)]
+
+        # Verify routes exist
+        assert len(streamable_routes) == 1, "Should have two streamable routes"
+
+        # Verify path values
+        assert (
+            streamable_routes[0].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"""