hreflang that survives review.

Configure it once

It takes next-intl's routing object as it is — locales, default locale, prefixes, localized pathnames — plus the origin, which next-intl has no reason to know.

// src/libs/alternates.ts
import { createAlternates } from "next-intl-alternates";
import { routing } from "@/i18n/routing";

export const getAlternates = createAlternates({
  baseUrl: "https://example.com",
  ...routing,
});

// src/app/[locale]/blog/[slug]/page.tsx
export async function generateMetadata({ params }) {
  const { locale, slug } = await params;
  const article = await getArticle(locale, slug);

  return {
    alternates: getAlternates({
      locale,
      pathname: "/blog/[slug]",
      params: { slug },
      // this article was only ever written in Japanese
      availableLocales: article.locales,
    }),
  };
}

The result drops straight into Metadata["alternates"]. Nothing else about your metadata changes.