better-auth/demo/nextjs/middleware.ts

import { client } from "@/lib/auth-client";
import { NextRequest, NextResponse } from "next/server";

export default async function authMiddleware(request: NextRequest) {
	const { data: session } = await client.getSession({
		fetchOptions: {
			headers: {
				//get the cookie from the request
				cookie: request.headers.get("cookie") || "",
			},
		},
	});

	if (!session) {
		return NextResponse.redirect(new URL("/", request.url));
	}
	return NextResponse.next();
}

export const config = {
	matcher: ["/dashboard"],
};