404 on dynamic routes without locale prefix after upgrading to Next.js 15.2.4

[irfanonk]

Hi,

After upgrading from Next.js 15.0.2 to 15.2.4, I'm getting 404s in deployment (Vercel) when accessing dynamic routes without a locale prefix — e.g. /course/:slug now returns 404. This happens when i first visit the dynamic link using next/link.
However when i refresh the page it works fine.
Additionally /tr/course/:courseId (for example) works fine too, and also everything works locally.

• App uses app/[language]/course/[slug]/page.tsx
• Rewrites are set up in next.config.ts to prepend default locale
• Dynamic route worked fine before 15.2.4

https://<deployed_branch_url>/course/:slug --> returns 404 in first visit but works on page refresh
https://<deployed_branch_url>/tr/course/:slug --> works normally

Additional information

rewrites in next.config.ts

  rewrites: async () => {
    const defaultLocale = DEFAULT_LOCALE_CODE;

    return [
      ...getAvailableLocales()
        .filter((locale) => locale.code !== defaultLocale)
        .map((locale) => ({
          source: `/${locale.code}/:path*`,
          destination: `/${locale.code}/:path*`,
        })),
      {
        source: '/:path((?!api).*)',
        destination: `/${defaultLocale}/:path*`,
      },
    ];
  },

middleware.ts

export function middleware(request: NextRequest) {
  return runMiddleware(request, [useAuthMiddleware, useI18nMiddleware]);
}

async function useI18nMiddleware(request: NextRequest, response: NextResponse) {
  if (
    !pathToRegexp('/((?!_next|api|_vercel|monitoring|.*\\..*).*)').test(
      request.nextUrl.pathname,
    )
  ) {
    return response;
  }

  const { pathname } = request.nextUrl;

  const pathnameHasLocale = !!getAvailableLocaleCodes().find(
    (locale) => pathname.includes(`/${locale}/`) || pathname === `/${locale}`,
  );

  if (pathnameHasLocale) {
    // If the pathname includes the default language, redirect to the path without the default language
    if (
      pathname.includes(`/${DEFAULT_LOCALE_CODE}/`) ||
      pathname === `/${DEFAULT_LOCALE_CODE}`
    ) {
      let newPathname = '';

      if (pathname.includes(`/${DEFAULT_LOCALE_CODE}/`))
        newPathname = pathname.replace(`/${DEFAULT_LOCALE_CODE}/`, '');
      else if (pathname === `/${DEFAULT_LOCALE_CODE}`) newPathname = '/';

      request.nextUrl.pathname = newPathname || '/';

      const response = NextResponse.redirect(request.nextUrl);

      response.cookies.set(LOCALE_COOKIE_KEY, pathname.split('/')[1]);

      return response;
    } else {
      response.cookies.set(LOCALE_COOKIE_KEY, pathname.split('/')[1]);

      return response;
    }
  }

  // If the pathname does not include any locale, add the user's preferred locale
  const locale = getUserLocaleFromRequest(request);

  // Only add the locale to the pathname if it's not the default language
  if (locale !== DEFAULT_LOCALE_CODE) {
    request.nextUrl.pathname = `/${locale}${pathname}`;

    const responseNew = NextResponse.redirect(request.nextUrl, {
      headers: response.headers,
    });

    responseNew.cookies.set(LOCALE_COOKIE_KEY, locale);

    return responseNew;
  } else {
    response.cookies.set(LOCALE_COOKIE_KEY, DEFAULT_LOCALE_CODE);

    return response;
  }
}

Example

No response