better-auth/examples/tanstack-example/app/client.tsx

import { StartClient } from "@tanstack/start";
import { hydrateRoot } from "react-dom/client";
import { createRouter } from "./router";

const router = createRouter();

const root = document.getElementById("root");
if (!root) {
	throw new Error("Root element not found");
}

hydrateRoot(root, <StartClient router={router} />);