vercel__next.js/examples/cms-agilitycms/pages/api/preview.ts

import type { NextApiRequest, NextApiResponse } from "next";
import { validatePreview } from "../../lib/api";

export default async function handler(
  req: NextApiRequest,
  res: NextApiResponse,
) {
  // Check the secret and next parameters
  // This secret should only be known to this API route and the CMS

  //validate our preview key, also validate the requested page to preview exists
  const validationResp = await validatePreview({
    agilityPreviewKey: req.query.agilitypreviewkey,
    slug: req.query.slug,
    contentID: req.query.contentid,
  });

  if (validationResp.error) {
    return res.status(401).end(`${validationResp.message}`);
  }

  // Enable Draft Mode by setting the cookie
  res.setDraftMode({ enable: true });

  // Redirect to the slug
  if (!("slug" in validationResp)) {
    throw new Error("invariant missing slug in validation response");
  }
  res.writeHead(307, { Location: validationResp.slug });
  res.end();
}