Fix 307 Temporary Redirect when use streamable_http

src/mcp/server/fastmcp/server.py

@@ -790,6 +790,12 @@ async def handle_streamable_http(
         middleware: list[Middleware] = []
         required_scopes = []
 
+        # Always mount both /mcp and /mcp/ for full compatibility, regardless of default
+        # Verify that _main_path has root format -> /mcp
+        _main_path = self.settings.streamable_http_path.removesuffix("/")
+        # Format _alt_path so it ends with '/' -> /mcp/
+        _alt_path  = _main_path + "/"
+
         # Add auth endpoints if auth provider is configured
         if self._auth_server_provider:
             assert self.settings.auth
@@ -815,19 +821,27 @@ async def handle_streamable_http(
                     revocation_options=self.settings.auth.revocation_options,
                 )
             )
-            routes.append(
+            routes.extend([
                 Mount(
-                    self.settings.streamable_http_path,
+                    _main_path,
                     app=RequireAuthMiddleware(handle_streamable_http, required_scopes),
-                )
+                ),
+                Mount(
+                    _alt_path,
+                    app=RequireAuthMiddleware(handle_streamable_http, required_scopes),
+                )]
             )
         else:
             # Auth is disabled, no wrapper needed
-            routes.append(
+            routes.extend([
                 Mount(
-                    self.settings.streamable_http_path,
+                    _main_path,
                     app=handle_streamable_http,
-                )
+                ),
+                Mount(
+                    _alt_path,
+                    app=handle_streamable_http,
+                )]
             )
 
         routes.extend(self._custom_starlette_routes)

tests/server/fastmcp/test_server.py

@@ -124,6 +124,22 @@ 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)]
+        mount_routes = [r for r in app.routes if isinstance(r, Mount)]
+
+        # 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"""