Next should error if both app and src/app exist

[RJWadley]

currently, if a project uses src/app and app is created unintentionally (for example, through a script, command, or some other misconfiguration on the part of the user), next will 404 on every page until the offending app folder is deleted.

this is especially frustrating for less experienced users because empty folders are invisible to git! so even if you were to revert the offending script/config/command, every single route will continue to 404 without any feedback whatsoever.

a user might, for example:
> have a fully working next app using src/app
> attempt to port some configuration from a project that uses app
> unintentionally create an empty app folder
> notice all pages are 404ing
> revert changes
> all pages still 404ing. no other console errors.
> check out main branch
> all pages still 404ing. weird. must be the cache.
> delete node_modules, .next, and reinstall
> all pages still 404ing.
> waste lots of precious time

maybe there’s a reason this is the behavior, but it seems like erroring (or at least warning) would make sense here

[sunnypatell]

having both app/ and src/app/ in a Next.js project silently breaking all routes with 404 errors is frustrating, especially without a clear error message.

Why This Happens

• Next.js prioritizes app/ over src/app/, so if an empty app/ folder exists, it overrides the actual src/app/ directory.
• Since an empty app/ folder has no pages, every route results in a 404 without any warnings.
• Git doesn’t track empty folders, making debugging even harder.

Potential Fixes

✅ Improve Developer Feedback
Next.js should log a warning or error when both app/ and src/app/ exist.

Example:

[Next.js] Conflict detected: Both `/app` and `/src/app` exist. This will cause unexpected behavior.

✅ Auto-Resolve in Development Mode
If src/app/ has pages but app/ does not, Next.js could default to src/app/ instead of breaking.

✅ Manual Workaround for Now

• If your app breaks unexpectedly with 404s, check for an unintended app/ directory:

  ls -l

• Prevent accidental app/ creation in CI/CD by adding a check:

  [ -d "app" ] && echo "⚠️ Warning: 'app' folder exists. This may break Next.js routes."

Conclusion

✔ Next.js should throw an error or warning when both folders exist.
✔ Until this is fixed, manually check for unintended app/ directories if you encounter sudden 404s.
✔ A GitHub issue should be created to request an official fix.