Related: #5687 (dependency upgrade that introduced the regression).

Summary

Requests to marktext.me returned HTTP 500 / Cloudflare 1101 after #5687 upgraded Next.js to 16.4.0. Production Worker logs and a local reproduction both showed Unexpected loadManifest(/.next/server/preview-props.json) call!: Next.js loads this new manifest during server initialization, but OpenNext 1.20.10 does not inline it. Builds and deployments still succeeded.

Pin Next.js to 15.5.27, the latest 15.x patch and a version accepted by the current OpenNext adapter. Keep the other dependency upgrades. Use Next 15's required jsx: preserve setting and add the CSS import declaration needed by TypeScript 6.

Add a local Workers HTTP smoke test before production and preview uploads in GitHub Actions. It checks the homepage, documentation, robots.txt, sitemap.xml, and the search index, including response status, content type, and expected content. This catches the original runtime failure that lint, type checking, and builds missed.

Type of change

• Bug fix (non-breaking, fixes an issue)
• Documentation update

Test plan

• New runtime smoke test: fails with HTTP 500 on the original Next.js 16.4.0 bundle; passes on the fixed bundle.
• Manually tested on macOS with local workerd and the production Cloudflare Worker.
• Website lint, type checking, OpenNext build, and cf:smoke passed.
• Root pnpm run lint passed (existing warnings).
• Root pnpm run typecheck: blocked during Muya declaration generation by missing local dependencies (marked, marked-highlight, fuse.js, and others); only the website dependency tree was installed for this incident. Website type checking passes independently.
• Production checks after the emergency deployment: homepage, developer documentation, robots.txt, and sitemap.xml return HTTP 200; www redirects to the apex with HTTP 301.

Notes for reviewers

The fixed Worker was already deployed directly to restore service (version adc71ba3-0809-4b03-b5ba-c49c6924aff6). Merge this PR to persist the fix in the GitHub Actions deployment source. The smoke test validates local runtime behavior; it does not check production DNS, credentials, or domain routing. Revisit the version pin when OpenNext supports the new manifest.

Before:

GET https://marktext.me/ -> 500, error code: 1101
Worker exception: Unexpected loadManifest(/.next/server/preview-props.json) call!

After:

GET https://marktext.me/ -> 200
GET https://marktext.me/docs/dev/architecture -> 200
GET https://www.marktext.me/?smoke=1 -> 301
Location: https://marktext.me/?smoke=1

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.