Custom 404 in pages router not work as expected when using both pages router and app router #82229
Description
Link to the code that reproduces this issue
https://github.com/yunsii/nextjs-custom-errors-reproduction
To Reproduce
- next dev
- return custom 404 as expected when opening http://localhost:3000/404
- return default 404 when opening http://localhost:3000/404-in-page-router is not what we expected
When I rename app dir to others name and restart server, it will return custom 404 when opening http://localhost:3000/404-in-page-router
Also I tried to use /app/not-found.page.tsx, and opening http://localhost:3000/404-in-page-router will encounter error:
> [email protected] dev /home/yuns/work/hello-next
> next dev
▲ Next.js 15.4.5
- Local: http://localhost:3000
- Network: http://10.255.255.254:3000
✓ Starting...
✓ Ready in 1371ms
○ Compiling /404-in-page-router ...
✓ Compiled /404-in-page-router in 606ms (281 modules)
○ Compiling /_not-found ...
✓ Compiled /_not-found in 1029ms (858 modules)
⨯ [Error [InvariantError]: Invariant: Expected clientReferenceManifest to be defined. This is a bug in Next.js.]
⨯ [Error [InvariantError]: Invariant: Expected clientReferenceManifest to be defined. This is a bug in Next.js.]
✓ Compiled /_error in 383ms (931 modules)
GET /404-in-page-router 500 in 3578ms
Current vs. Expected behavior
Return consistent 404 page in pages router.
Provide environment information
Operating System:
Platform: linux
Arch: x64
Version: #1 SMP PREEMPT_DYNAMIC Thu Jun 5 18:30:46 UTC 2025
Available memory (MB): 58254
Available CPU cores: 20
Binaries:
Node: 22.15.0
npm: 10.9.2
Yarn: N/A
pnpm: 10.12.4
Relevant Packages:
next: 15.4.5 // Latest available version is detected (15.4.5).
eslint-config-next: 15.4.5
react: 19.1.0
react-dom: 19.1.0
typescript: 5.8.3
Next.js Config:
output: N/AWhich area(s) are affected? (Select all that apply)
Error Handling, Pages Router
Which stage(s) are affected? (Select all that apply)
next dev (local)
Additional context
No response