Link to the code that reproduces this issue

https://github.com/christiancenti/reproduction-sitemap-bug

To Reproduce

1. Use MetadataRoute.Sitemap to generate a sitemap with language alternates in Next.js.
2. Inspect the resulting sitemap XML.
3. Submit the sitemap to Google Search Console.
4. Observe the "incorrect namespace" error.

Current vs. Expected behavior

Current:
The sitemap XML generated by the Next.js Metadata system uses https in the namespace declarations:

This causes Google Search Console to report an "incorrect namespace" error.

Expected:
The generated sitemap XML should use the official http namespaces as per spec:

• xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
• xmlns:xhtml="http://www.w3.org/1999/xhtml"

Google Search Console should accept the sitemap without namespace errors.

Provide environment information

Relevant Packages:
next: 15.4.0-canary.129

Additional context

• File reference: packages/next/src/build/webpack/loaders/metadata/resolve-route-data.ts
• See Google community discussion: https://support.google.com/webmasters/thread/357720871/sitemap-error-incorrect-namespace-in-google-search-console

Thank you for your help!