Partial Prefetching: generated routes return HTTP 500 on a cold custom-cache miss

Link to the code that reproduces this issue

https://github.com/krzkz94/nextjs-partial-prefetch-cache-miss-repro

To Reproduce

1. Clone the reproduction and install its pinned dependencies:

git clone https://github.com/krzkz94/nextjs-partial-prefetch-cache-miss-repro.git
cd nextjs-partial-prefetch-cache-miss-repro
npm ci
npm test

2. The script builds the app successfully, starts a fresh production server, and requests /en and /fr. Both paths are returned by generateStaticParams().

3. Observe HTTP 500 for both generated routes. The test exits with status 1 because it expects HTTP 200.

Build logs, server logs, and JSON results are saved under artifacts/<next-version>/<mode>-<bundler>/.

To reproduce with Turbopack:

npm run test:turbopack

Run these controls sequentially:

npm run test:filesystem
npm run test:prefetch-off

Both controls return HTTP 200.

Current vs. Expected behavior

Current:

The production build succeeds and generates /en and /fr, but the first requests to those routes return HTTP 500 when all of the following are configured:

• cacheComponents: true
• partialPrefetching: true
• A custom incremental cache handler that initially returns null
• cacheMaxMemorySize: 0

The server trace shows the concrete route missing, followed by a generic fallback-shell miss:

[cache.get] {"key":"/en","kind":"APP_PAGE","isFallback":false,"hit":false}
[cache.get] {"key":"/[locale]","kind":"APP_PAGE","isFallback":true,"hit":false}
Error: Route "/[locale]": Next.js encountered uncached or runtime data during prerendering.
code: 'NEXT_STATIC_GEN_BAILOUT'

Expected:

/en should return HTTP 200 with Locale: en, and /fr should return HTTP 200 with Locale: fr. A cache miss should allow these generated routes to render successfully.

The page only awaits params and renders the locale. It performs no data fetching.

Provide environment information

Operating System:
    Platform: darwin
    Arch: arm64
    Version: Darwin Kernel Version 27.0.0: Tue Aug 11 21:05:48 PDT 2026; root:xnu-13432.1.9~1/RELEASE_ARM64_T6041
    Available memory (MB): 65536
    Available CPU cores: 16
  Binaries:
    Node: 24.11.1
    npm: 11.6.2
    Yarn: N/A
    pnpm: 8.15.9
  Relevant Packages:
    next: 16.4.0-canary.33
    eslint-config-next: N/A
    react: 19.2.0
    react-dom: 19.2.0
    typescript: N/A
  Next.js Config:
    output: N/A

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

cacheComponents, Dynamic Routes, Partial Prerendering (PPR), Runtime

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

next start (local)

Additional context

Verified results:

• Next.js 16.3.5 with webpack: build succeeds; both generated routes return 500.
• Next.js 16.4.0-canary.33 with webpack and Turbopack: build succeeds; both generated routes return 500.
• Using Next's default filesystem cache, while retaining partial prefetching and cacheMaxMemorySize: 0: both routes return 200.
• Keeping the custom cache but disabling partial prefetching: both routes return 200.

The reproduction uses only Next.js, React, and React DOM. The custom cache is a small in-memory Map. There is no Redis, external service, third-party cache library, authentication, or dependency patch.

The trace suggests investigating the handling of the generic /[locale] fallback shell during runtime regeneration versus build-time shell validation. The repository demonstrates the behavior without bundling or prescribing a particular fix.

Clone the linked reproduction, run npm ci and npm test, then compare the failing partial-prefetching run with npm run test:filesystem and npm run test:prefetch-off. Start from the cache.get trace for /en and /[locale] and the runtime handling of the generic fallback shell; done means generated /en and /fr requests return HTTP 200 with their locale responses.