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Why Next.js 15 randomly fails on Cloudflare Workers (and how to fix it)

#nextjs #cloudflare #typescript #webdev

If you've tried deploying Next.js 15 App Router to
Cloudflare Workers, you've probably hit builds that
randomly fail with cryptic errors.

I've been running a production app on CF Workers for
months. Here are the 4 bugs I found — none of them
are documented in one place.

Bug 1: Race condition on pages-manifest.json

Next.js 15.5+ uses worker threads during build. This
causes intermittent ENOENT errors:

Error: ENOENT: no such file or directory,
open '.next/server/pages-manifest.json'
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Fix: Disable worker threads in your build script:

process.env.NEXT_PRIVATE_WORKER_THREADS = 'false'
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Bug 2: pages-manifest.json is never generated

App Router-only projects don't generate
pages-manifest.json — but opennextjs-cloudflare
requires it at deploy time.

Fix: Pre-create it after build:

const manifest = {
  '/_app': 'pages/_app.js',
  '/_document': 'pages/_document.js',
  '/_error': 'pages/_error.js',
}
fs.writeFileSync(manifestPath, JSON.stringify(manifest))
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Bug 3: Turbopack stubs break esbuild

Next.js generates Pages Router stubs that reference
[turbopack]_runtime.js. This causes esbuild to
fail silently during the Cloudflare bundle step.

Fix: Delete the stubs after Next.js build completes:

const stubFiles = ['_app.js', '_document.js', '_error.js']
for (const f of stubFiles) {
  const p = path.join(pagesDir, f)
  if (fs.existsSync(p)) fs.unlinkSync(p)
}
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Bug 4: Supabase auth cookies break on edge

The standard Supabase client stores cookies in a way
that doesn't work on Cloudflare Workers (no
document.cookie access in SSR context).

Fix: Use @supabase/ssr with explicit cookie handlers:

import { createServerClient } from '@supabase/ssr'

export function createClient() {
  const cookieStore = cookies()
  return createServerClient(
    process.env.NEXT_PUBLIC_SUPABASE_URL!,
    process.env.NEXT_PUBLIC_SUPABASE_ANON_KEY!,
    {
      cookies: {
        getAll() { return cookieStore.getAll() },
        setAll(cookiesToSet) {
          cookiesToSet.forEach(({ name, value, options }) =>
            cookieStore.set(name, value, options)
          )
        },
      },
    }
  )
}
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Putting it all together

All 4 fixes live in a single scripts/build.mjs that
wraps next build. Run it instead of next build
directly.

I packaged this build script along with all the other
patterns (Tailwind CSS 4, i18n, PWA, security headers,
WCAG 2.2 AA) into a starter template:

👉 Edge Starter on Lemon Squeezy — $39

The template builds consistently and is tested in
production. Happy to answer questions about any of
the fixes in the comments.
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