Bumping the project from next.js 15 to 16 causes useLocale() returning default locale majority of the time

[RaenonX]

Sorry if the question is vague in advance and thanks for maintaining this lib!

My project largely follows what's in https://github.com/amannn/next-intl/blob/main/examples/example-app-router/src/app/%5Blocale%5D/layout.tsx at next.js 15, then when I bumped it to 16, without changing anything, useLocale() would return the default locale majority of the time.

What could be the issue here and how do I fix it? Side note, this only happens in PROD build (after running next build) but not the DEV build.

Tried checking the commit history of https://github.com/amannn/next-intl/blob/main/examples/example-app-router as well, but I only see middleware.ts renaming, which I've done it already.

I do want to get a minimal reproduction repo, but given the current scale of my project, I think it's worth for a few guesses before trimming down my repo.

[amannn]

This could be relevant: My page content isn’t localized despite the pathname containing a locale prefix.

But yeah, comparing it with example-app-router might be a good otherwise if you experience an issue! Generally the update to Next.js 16 was straightforward for that example.

[RaenonX]

Thanks!

In the end it seems like the proxy isn't picked up in the PROD build for whatever reason, and doesn't seem to be an issue related to next-intl. Having the file named middleware.ts works fine, but renaming it to proxy.ts seems to be causing the issue.

I tried putting it either at the project root as proxy.ts or src/proxy.ts, but no luck. What's even weird is that it only breaks after building the app. The dev build works completely fine. Still investigating it...

[RaenonX]

Seems to be related to vercel/next.js#85243

[amannn]

Seems to be related to vercel/next.js#85243

Oh wow, thanks for sharing this. I'll follow along in that issue …