Intermittent "Not Found 404" returned on first visit after idle time (Next 15 With App Router)

[llorenspujol]

Description of the bug

On the first visit to a page after some idle time (usually 1–2 hours is enough), I sometimes receive the default Next.js 404 page, even though the page definitely exists. If I refresh the page, it loads correctly 100% of the time.

The affected path is /[lang]/events/[slug] (others also, but focus on this only to keep it simple). The strange part is that neither the not-found.tsx from this route nor the global app/global-not-found.tsx are used when this happens; it just falls back to the built-in Next.js 404... How can this really be possible?

Here is a log from one of these failures:

All inner requests return 200, but the overall response is a 404. As mentioned, this only happens after the app has been idle for 1–2 hours, and it’s easy to reproduce by visiting a direct link.

Example of a link that may return 404 on first time:
https://athletiks.io/ca/events/run-music-con-camilo-10-dagost-del-2025-3de5b3

It doesn’t happen every single time, but it happens often enough to affect real users. I initially suspected a firewall issue, but logs show that isn’t the cause... (apparently)

While not critical (since refreshing always fixes it), this significantly worsens user experience, especially because many of our clients share direct links in WhatsApp groups.

Expected behavior

It should never return 404 on a page that 100% exists and all inner requests have been successful. And in case of failure, it should use the custom not-found for that path.

Environment

• Next.js 15.3 (With App Router)
• Hosted on Vercel
• next-intl for i18n, locales: ca, es, en
• Middleware handles locale routing

Steps to reproduce

Go to this link in production, it may fail as described. If it does not fail, try another browser agent. But should definetly fail on first visit on many users:

https://athletiks.io/ca/events/run-music-con-camilo-10-dagost-del-2025-3de5b3

Additional information

No response

Example

https://athletiks.io/ca/events/run-music-con-camilo-10-dagost-del-2025-3de5b3

[llorenspujol]

And it keeps happening over and over… here are today’s logs. All these pages exist, every one of them. Yet each one returned a 404 Not Found, and after a refresh, it just returned 200 OK.

Our current user experience is heavily affected, and I hope someone can shed some light on this!

Thanks in advance 🙏

[asilvafx]

Did you find any solution ?cheers

[llorenspujol]

Yep, i added a comment with the solution that worked for me!

[llorenspujol]

Finally I found the problem.

It was in the middleware.ts, and it was caused because Supabase auth overwrote the headers from next-intl, exactly what’s explained here: #70582 (comment)

In my case, I decided to simply not recreate a new response after Supabase sets the cookies. That solved the issue immediately 👇