`"use cache"` doesn't work for route handlers

[6TELOIV]

Link to the code that reproduces this issue

https://github.com/6TELOIV/use-cache-route-handler-repro

To Reproduce

1. Start dev (npm run dev)
2. Visit http://localhost:3000/string
   • Observe the response
3. Visit http://localhost:3000/response
   • Observe the error

Current vs. Expected behavior

With the current static/dynamic routes implementation, A dev can mark a route handler's GET method "static".

With the new "use cache" directive, this doesn't seem to work at all. Additionally, the error message seems opaque/confusing, refering to client and server components even though the method does nothing react-ish:

[Error: Only plain objects, and a few built-ins, can be passed to Client Components from Server Components. Classes or null prototypes are not supported.
  {}] {
  environmentName: 'Cache',
  digest: ''
}

Provide environment information

Operating System:
  Platform: win32
  Arch: x64
  Version: Windows 10 Enterprise
  Available memory (MB): 32401
  Available CPU cores: 8
Binaries:
  Node: 22.11.0
  npm: 10.9.0
  Yarn: 1.22.22
  pnpm: 10.5.2
Relevant Packages:
  next: 15.2.1-canary.1 // Latest available version is detected (15.2.1-canary.1).
  eslint-config-next: 15.2.1-canary.1
  react: 19.0.0
  react-dom: 19.0.0
  typescript: 5.7.3
Next.js Config:
  output: N/A

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

dynamicIO, Use Cache

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

next dev (local), next build (local), next start (local), Vercel (Deployed), Other (Deployed)

Additional context

Failing in 15.2.1-canary.1.