middleware.js

import { next, rewrite } from '@vercel/functions';

// Vercel Routing Middleware: serves the markdown twin of a docs page (built next to it as /page.md)
// to AI agents that send `Accept: text/markdown`. The home page has no twin, so it answers with llms.txt.
export const config = {
    // Pages (with or without .html) and .md twins only; skips assets and any other file
    matcher: '/((?!assets/)(?!.*\\.(?!(?:md|html)$)[^/.]+$).*)'
};

// Suffix of page URLs on this site: '' with cleanUrls, '.html' without
const PAGE_SUFFIX = '';

const quality = (accept, type) => {
    for (const part of accept.split(',')) {
        const [media, ...params] = part.split(';').map(value => value.trim().toLowerCase());
        if (media !== type) continue;
        const q = params.find(param => param.startsWith('q='));
        return q ? Number(q.slice(2)) || 0 : 1;
    }
    return 0;
};

const prefersMarkdown = accept => {
    const markdown = quality(accept, 'text/markdown');
    return markdown > 0 && markdown >= quality(accept, 'text/html');
};

export default async function middleware(request) {
    const url = new URL(request.url);

    if (url.pathname.endsWith('.md')) {
        return next({ headers: { Link: `<${url.origin}${url.pathname.slice(0, -3)}${PAGE_SUFFIX}>; rel="canonical"`, Vary: 'Accept' } });
    }
    if (!prefersMarkdown(request.headers.get('accept') || '')) {
        return next({ headers: { Vary: 'Accept' } });
    }

    const page = url.pathname.replace(/(\/index)?(\.html)?\/?$/, '');
    const markdown = new URL(page ? `${page}.md` : '/llms.txt', url.origin);
    // Not every page has a twin (archived versions, for one). Credentials are forwarded and redirects
    // not followed, so a protected preview's login redirect does not pass for an existing file.
    const headers = {};
    for (const name of ['cookie', 'x-vercel-protection-bypass']) {
        if (request.headers.has(name)) headers[name] = request.headers.get(name);
    }
    const twin = await fetch(markdown, { method: 'HEAD', headers, redirect: 'manual' }).catch(() => null);
    if (!twin?.ok) {
        return next({ headers: { Vary: 'Accept' } });
    }
    return rewrite(markdown, { headers: { Vary: 'Accept' } });
}