Are you using domain-based i18n or path-based?

Also, have you tried doing it in a return instead of beforeFIles?

You might also be able to do

rewrites: () => {
    return {
      beforeFiles: [
        {
          source: '/:locale/test',
          destination: `https://jsonplaceholder.typicode.com/posts/1`,
        },
      ],
    },
 }

@Marchy you can have a peak at the reproduction examples in the issue mentioned above https://github.com/vercel/next.js/issues/57616
it doesn't use beforeFiles and is just using the official nextjs example for i18n

Changing your source to /:locale/test should resolve this

@Marchy Can you reread that issue? We strictly do not want to use locale in this rewrite.
browse to /test should resolve to https://jsonplaceholder.typicode.com/posts/1
we explicitly do not want /:locale/test to do anything. These should 404.

Could you please do this in a reproduction repo instead of forking next.js? It's a huge download.

If your developing / supporting NextJS then the repo examples are usually the most sought after for reproduction repos. It's literally how the test cases are run. Nw, I think the issue is on their radar.

I don't believe this is an error within Next.js. You might just need to do something like

 rewrites: () => {
    return {
      beforeFiles: [
 {
          source: '/:locale/test',
          destination: `/test`,
        },
        {
          source: '/test',
          destination: `https://jsonplaceholder.typicode.com/posts/1`,
        },
      ],
    },
 }

@Marchy and what would be the result if then someone browsed to /en_US/test1?

https://nextjs-forum.com/post/1167473900519694386#message-1167650017385918474

locale: false fixes the default route but then breaks all locale routes to that rewrite.

What did you mean by this?

Thanks for trying to help @Marchy . I think you may only have read what I wrote in the above post in discord but have yet to read the explicit github issue that I also posted. It is very detailed and completely lays out what the problem is. https://github.com/vercel/next.js/issues/57616

Good luck 👍