Strange Locale Switching Behavior with NEXT_LOCALE Cookie - Need Help with Debugging

[FrontLeejonghun]

Hello everyone,

First, let me share the repository structure:
https://github.com/FrontLeejonghun/next-intl-replace-bug

Current Issue:
Unfortunately, this issue cannot be reproduced in the repository above, but it consistently occurs in my production application. The next-intl and Next.js versions are identical between both environments.

Steps to Reproduce (in my production app):

1. Access the root path /
2. Automatically redirects to /ko based on locale detection
3. User switches locale to en using the locale switcher
4. NEXT_LOCALE cookie changes: ko → en → ko, and in the Network tab, I see a 307 redirect → /ko_rsc111

Environment Details:

• The issue occurs only in my production application
• Cannot reproduce in a clean repository with identical versions
• next-intl configuration is 100% identical between both environments
• Next.js and next-intl versions match exactly

Important Note:
The application works perfectly fine with next-intl: ^3.11.3, but this issue appears with newer versions.

Question:
Has anyone experienced similar behavior with locale switching where the NEXT_LOCALE cookie oscillates between values and generates unexpected redirect URLs with suffixes like _rsc111?

Any insights into what might cause this discrepancy between environments would be greatly appreciated. I'm particularly curious about what could cause the cookie to flip back and forth and generate those unusual redirect URLs.

Additional Context:

• The behavior is consistent and reproducible in the production environment
• Clean reproduction case works as expected
• Works fine with next-intl v3.11.3
• Looking for debugging strategies or known issues that might cause this

Thank you for your time and any suggestions you might have!

[amannn]

I see a 307 redirect → /ko_rsc111

That sounds very odd to me. Are you sure you don't mean /ko?_rsc=111?. That's a normal client-side navigation in Next.js.

But it's still strange that the cookie value changes back to ko. Is the user also on the ko page, or does only the cookie change?

Any chance you could share a link to your app in case it's public? Some way to reproduce it would certainly help, currently there's a lot of guessing here.

I'd really try to break it down as much as possible, e.g. by removing most of your code and see what the minimal case is where you can still see the issue. From there, looking at each request in isolation until you find the invalid one might help—e.g. if you can break it down to a single curl statement, that should make it straightforward to debug.

[FrontLeejonghun]

#1942
Maybe this one explains it better and is easier to understand
And
if this issue is not a bug but rather a Q&A.
Let me know, and I’ll move it accordingly

[FrontLeejonghun]

That sounds very odd to me. Are you sure you don't mean /ko?_rsc=111?. That's a normal client-side navigation in Next.js.

Yap Sorry for makes confused

But it's still strange that the cookie value changes back to ko. Is the user also on the ko page, or does only the cookie change?

At the moment the cookie changes from ko → en → ko, the user always remains on the ko page.