better-auth/docs/proxy.ts

import { NextRequest, NextResponse } from "next/server";
import { isMarkdownPreferred, rewritePath } from "fumadocs-core/negotiation";

const { rewrite: rewriteLLM } = rewritePath("/docs/*path", "/llms.txt/*path");

export function proxy(request: NextRequest) {
	if (isMarkdownPreferred(request)) {
		const result = rewriteLLM(request.nextUrl.pathname);

		if (result) {
			return NextResponse.rewrite(new URL(result, request.nextUrl));
		}
	}

	return NextResponse.next();
}

export const config = {
	matcher: "/docs/:path((?!.*\\.mdx$).*)*",
};