Proxy your Next.JS sitemaps to Yoast sitemaps using the latest middleware!

middleware.ts

// middleware.ts
import { NextResponse } from "next/server";
import type { NextRequest } from "next/server";

// This function can be marked `async` if using `await` inside
export function middleware(request: NextRequest) {
  const url = request.nextUrl;
  url.pathname = "/api/sitemap";
  return NextResponse.rewrite(url);
}

export const config = {
  matcher: [
    /* Match all sitemap paths */
    "/([\\w\\d_-]*sitemap[\\w\\d_-]*.xml)/",
  ],
};

sitemap.ts

// pages/api/sitemap.ts
import type { NextApiRequest, NextApiResponse } from "next";

export default async function handler(
  req: NextApiRequest,
  res: NextApiResponse
) {
  const xmlRes = await fetch((process.env.NEXT_PUBLIC_WORDPRESS_URL ?? "") + req.url);
  let xml = await xmlRes.text();

  res.setHeader("Content-Type", "text/xml");
  res.write(xml);
  res.end();
}

Feel free to also reference https://gist.github.com/jordanmaslyn/7e28423ef43cdb6af068f5f192077746 for some configuration optimizations on the WordPress side.