Dynamic `sitemap.ts` under a dynamic segment with `generateStaticParams` is collapsed to `/-/sitemap.xml` — per-param sitemaps no longer emitted (regression 16.2.3 → 16.3)

Link to the code that reproduces this issue

https://github.com/Debbl/next-dynamic-sitemap-repro

To Reproduce

1. Clone the reproduction repository and check out the plain repro commit — note that main HEAD additionally contains a pnpm patch (patches/next@16.3.4.patch) demonstrating the proposed fix, so building main as-is does NOT reproduce the bug:

   git clone https://github.com/Debbl/next-dynamic-sitemap-repro
   cd next-dynamic-sitemap-repro
   git checkout d2e42032449c7f92113ba0e591b2979f4fd1de55  # plain repro, no patch
   

2. pnpm install
3. pnpm build (runs next build with output: 'export')
4. find out -name 'sitemap*'

The app has a dynamic [lang] segment with a code-based sitemap:

app/
  layout.js
  [lang]/
    page.js       # generateStaticParams -> en, zh
    sitemap.js    # generateStaticParams -> en, zh; export const dynamic = 'force-static'

Current vs. Expected behavior

Current behavior (next 16.3.4)

The sitemap route is collapsed to a single --placeholder pathname. generateStaticParams of sitemap.js is never invoked:

└   /-/sitemap.xml
  └ ● /-/sitemap.xml

Only out/-/sitemap.xml is emitted. out/en/sitemap.xml and out/zh/sitemap.xml do not exist, so any robots.txt pointing at /{lang}/sitemap.xml (the URLs previous versions produced, which search consoles have indexed) now 404s.

The same happens without output: 'export': prerender-manifest.json routes contains only /-/sitemap.xml, no /en/sitemap.xml / /zh/sitemap.xml entries.

Expected behavior (next 16.2.3)

└ ● /[lang]/sitemap.xml
  ├ /en/sitemap.xml
  └ /zh/sitemap.xml

out/en/sitemap.xml and out/zh/sitemap.xml are emitted (verify with pnpm add next@16.2.3 && rm -rf .next out && pnpm build).

Root cause analysis

This looks like fallout from #93873 ("Prerender static metadata under dynamic segments to canonical pathname"). That optimization is meant for static metadata files (e.g. /[id]/apple-icon.png), whose bytes don't depend on params. But the check matches by pathname shape only:

• In isPageStatic (packages/next/src/build/utils.ts), isStaticMetadataFile(pathname) matches /[lang]/sitemap.xml and takes the buildStaticMetadataStaticPaths shortcut, skipping buildAppStaticPaths entirely — so the userland generateStaticParams exported by sitemap.js (which the metadata route loader re-exports via createReExportsCode) is never called.
• One-liner showing the collapse:

  node -e "const{getStaticMetadataPrerenderPathname:f}=require('next/dist/lib/metadata/get-metadata-route.js');console.log(f('/[lang]/sitemap.xml'))"
  # → /-/sitemap.xml
  

A guard on the shortcut fixes it while keeping the #93873 optimization for true static files (static metadata files never export generateStaticParams — getStaticAssetRouteCode emits none):

if (
  routeModule.definition.kind === RouteKind.APP_ROUTE &&
  isStaticMetadataFile(pathname) &&
  !segments.some((segment) => typeof segment.generateStaticParams === 'function')
) {

Verified locally against the repro: per-param sitemaps are restored, and a true static file (app/[lang]/icon.png) still collapses to a single out/-/icon.png.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin 25.5.0
Binaries:
  Node: 24.13.0
  npm: 11.6.2
  Yarn: N/A
  pnpm: 10.26.0
Relevant Packages:
  next: 16.3.4
  eslint-config-next: N/A
  react: 19.2.8
  react-dom: 19.2.8
  typescript: N/A
Next.js Config:
  output: export

Which area(s) are affected? (Select all that apply)

Metadata, Dynamic Routes, Output

Which stage(s) are affected? (Select all that apply)

next build (local)

Additional context

• Regression window: 16.2.3 works, 16.3.4 does not (introduced by #93873, first shipped in the 16.3 line).
• Reproduces locally with both next build + output: 'export' and a plain server build (the export just makes the missing files obvious; in a server build the collapsed route lands in prerenderManifest.routes as /-/sitemap.xml the same way).
• Real-world impact: localized sitemaps under app/[lang]/sitemap.ts disappear from the build output, breaking robots.txt Sitemap: entries and search-console registrations that point at /{lang}/sitemap.xml.
• Repro repo layout: commit d2e4203 is the plain reproduction; main HEAD (831e964) additionally applies the proposed fix as a pnpm patch (patches/next@16.3.4.patch) — building main shows the expected per-param output (out/en/sitemap.xml, out/zh/sitemap.xml) while a true static file (app/[lang]/icon.png) still collapses to a single out/-/icon.png, i.e. the #93873 optimization is preserved.
• The repro README contains the 16.2.3 comparison steps and the root-cause notes; happy to open a PR with the guard described above.

Start with packages/next/src/build/utils.ts, the isPageStatic path, and getStaticMetadataPrerenderPathname; use the plain repro commit d2e4203 and run pnpm install, pnpm build, and find out -name 'sitemap*'. Done means per-param sitemap files appear for en and zh, while the static icon case remains collapsed; compare with the existing 16.2.3 output.