Rewrites

withFairGardenIndicators adds a chain of rewrites to next.config.ts. They go in beforeFiles, the one phase where Next keeps applying rewrites after one has matched — each sees the path the previous ones produced. That is what makes this a chain of small steps rather than one rewrite per combination of values.

The chain

For locales: ['en', 'fr'], a theme preference and a tz flag:

1  /                    -> /en                         a path without a locale
   /:path((?!en|fr|_next|api|…).+) -> /en/:path        gets the default one
2  /:locale(en|fr)/:path*   -> /:locale/-/-/:path*      empty segments
3  /:locale/:prefs/:flags/:path*   cookie theme=light   -> /:locale/:prefs.theme~light/:flags/:path*
   /:locale/:prefs/:flags/:path*   cookie theme=dark    -> /:locale/:prefs.theme~dark/:flags/:path*
4  /:locale/-.:prefs/:flags/:path*  -> /:locale/:prefs/:flags/:path*   strip the marker
3  /:locale/:prefs/:flags/:path*   header x-vercel-ip-timezone America/(New_York|…) -> …/:flags.tz~EST/…
4  /:locale/:prefs/-.:flags/:path*  -> /:locale/:prefs/:flags/:path*

Step 3 is one rewrite per value. Keys are visited in sorted order, so the pairs come out sorted, and each rewrite's source refuses a segment that already has its key, so the first value whose pattern matches wins. The number of rewrites grows with the number of values, not combinations.

has matches the whole cookie or header value, as Next does — the pattern is anchored at both ends — so a listed value has to be exact and a pattern has to cover the whole string.

What is left alone

A path that starts with a locale gets no locale. A path that starts with _next, api or .well-known never does, nor does anything in the config's exclude. The plugin also looks at the app:

• every entry of public/, since a file there is served at its own name
• every directory at the top of app/ that is not a dynamic segment, such as app/api/ or a health check, since a route up there is outside the locale tree by construction
• metadata files at the top of app/ — sitemap.ts, robots.ts, manifest.ts, the icons — which Next serves under the same name with another extension

A route group or a parallel slot at the top of app/ could hold anything, so those are not inspected; name what they serve in exclude.

beforeFiles runs before the filesystem is checked, which is exactly why this matters: a rewrite that caught /favicon.ico would send it to /en/-/-/favicon.ico, and nothing is there.

Internal paths from outside

A request for /en/-/-/login — the internal form — is not served. The chain inserts the empty segments regardless of what follows the locale, so it becomes /en/-/-/-/-/login, which matches no route. The internal path is a cache key, not an address.

Redirects

With the default localePrefix: 'as-needed', /en and /en/… redirect for good to the path without the prefix, so the default locale has one URL per page. With 'always', an unprefixed path redirects, temporarily, to the default locale. The site root is never redirected by config in either mode; that is the proxy's call.

Order

The app's own beforeFiles rewrites come first and its redirects come first. A path the app's rewrites produce is localized like any other, so an app that rewrites /oidc/:path* to /api/oidc/:path* needs nothing more — api is excluded — though it should still exclude oidc so that its links to /oidc/… are not localized either.