Next.js 16 beta: `dynamicParams = false` breaks parallel route with top-level dynamic segment

[amannn]

Link to the code that reproduces this issue

amannn/nextjs-bug-repro-dynamicfalse@ce4aec8

To Reproduce

1. Run pnpm i && pnpm dev
2. Visit /en
3. Click on "Story" at the top

Current vs. Expected behavior

I expect the page to render correctly, but it fails with a 404.

Provide environment information

Operating System:
  Platform: darwin
  Arch: x64
  Version: Darwin Kernel Version 24.6.0: Mon Jul 14 11:28:17 PDT 2025; root:xnu-11417.140.69~1/RELEASE_X86_64
  Available memory (MB): 16384
  Available CPU cores: 12
Binaries:
  Node: 24.4.1
  npm: 11.4.2
  Yarn: 1.22.22
  pnpm: 10.17.1
Relevant Packages:
  next: 16.0.0-canary.0 // Latest available version is detected (16.0.0-canary.0).
  eslint-config-next: 16.0.0-canary.0
  react: 19.1.0
  react-dom: 19.1.0
  typescript: 5.9.3
Next.js Config:
  output: N/A

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

Internationalization (i18n), Linking and Navigating, Not Found, Parallel & Intercepting Routes

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

next dev (local), next build (local)

Additional context

It seems like the behavior is caused by this:

export const dynamicParams = false;

After removing this line, the app works fine.

Note that this works fine in Next.js 15.x.

[wyattjoh]

So I investigated this case, and think that this behaviour might be intentional. In Next.js 15.5.4, when we build, we get the following output:

Route (app)
┌ ○ /_not-found
├ ● /[locale]
├   └ /en
└ ƒ /[locale]/stories/[slug]

Which is technically wrong, as the route with the [slug] is under the [locale], which defines dynamicParams = false which applies to the entire route, not just the [locale] segment.

Next.js 15.6.0-canary.47 produces the following output:

Route (app)
┌ ○ /_not-found
├ ● /[locale]
├   └ /en
└ ● /[locale]/stories/[slug]
    └ /en/stories/[slug]

Which I'd argue is actually more accurate, but in this case, still has the same rules of dynamicParams getting applied, and as no slug was generated, any unknown pages should trigger a 404.

[amannn]

Thanks for investigating this!

So you're saying that a dynamicParams = false located in a layout.tsx for a top-level dynamic segment like [locale] is inherited throughout all pages within this segment? (which might practically be all routes for an app with such an i18n setup)

I can get my reproduction working by adding a layout for the story detail page with:

// src/app/[locale]/stories/[slug]/layout.tsx

export function generateStaticParams() {
  return [];
}

export default function StoriesSlugLayout({
  children
}: LayoutProps<'/[locale]/stories/[slug]'>) {
  return children;
}

Is this the intended solution for this case? If you want to keep a fully dynamic behavior for this page, that doesn't seem to be supported, is that correct? Is there a way to avoid inheriting dynamicParams = false, or at least flip it back on for certain segments? (this is more a general question for my understanding, maybe it could be an addition to the docs on dynamicParams)

It also seems a bit odd to me that this error is only encountered once you add a parallel route like in the example, is this intended?

Thank you for your help @wyattjoh!