Route handler always returns X-Vercel-Cache: STALE

[joostmeijles]

Link to the code that reproduces this issue

https://github.com/joostmeijles/route-handler-stale/tree/main

Route handler example: https://github.com/joostmeijles/route-handler-stale/blob/main/app/items/%5Bslug%5D/route.ts

To Reproduce

1. Start the application in production mode
2. Visit http://localhost:3000/items/a
3. Inspect the response headers and verify that first visit has x-nextjs-cache: MISS
4. Visit http://localhost:3000/items/a
5. Inspect the response headers and verify that first visit has x-nextjs-cache: HIT
6. Deploy to Vercel
7. Visit https://routehandler-stale.vercel.app/items/a
8. Inspect the response headers and verify that it has always X-Vercel-Cache: STALE

Current vs. Expected behavior

Currently always X-Vercel-Cache: STALE is returned.

Expected is X-Vercel-Cache: MISS on first visit and X-Vercel-Cache: HIT for subsequent visits.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 22.2.0: Fri Nov 11 02:04:44 PST 2022; root:xnu-8792.61.2~4/RELEASE_ARM64_T8103
  Available memory (MB): 8192
  Available CPU cores: 8
Binaries:
  Node: 20.12.2
  npm: 10.5.0
  Yarn: N/A
  pnpm: 8.15.7
Relevant Packages:
  next: 14.3.0-canary.63 // Latest available version is detected (14.3.0-canary.63).
  eslint-config-next: N/A
  react: 19.0.0-beta-4508873393-20240430
  react-dom: 19.0.0-beta-4508873393-20240430
  typescript: 5.1.3
Next.js Config:
  output: N/A

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

App Router

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

Vercel (Deployed)

Additional context

No response