`next/font` interferes with header links from middleware.

[zenzen-sol]

`next/font` interferes with header links from middleware.

[amannn]

Hey @samcx, I hope it's ok for me to chime in.

I've tested with the latest canary and am observing the same behavior as you do: In development, only the React optimization header value for Link shows up and after a production build, only a manually set value for Link is returned (in this case set by next-intl).

There are two aspects here that worry me a bit:

1. Even when I bump reactMaxHeadersLength to a value like 999999999, the behavior is exactly the same. Could it be, that a logic for merging headers is missing here? That's at least what I'd hope to exist and I guess there's a need for both: proper i18n links as well as performance optimization.
2. A Link response header being overridden in development is quite unfortunate. While I'm happy that it still exists in production, I'd assume this to be rather confusion for developers who want to verify the header is there. Note that the Link response header in the i18n use case is not an optimization, but a necessary signal to search engines to link together multilingual pages. The absence of this header could result in search engines recognizing variants of pages as duplicates and de-indexing them. Therefore this is quite a crucial functionality.

Any chance we could get to the bottom of this and see if there's something to be adjusted in Next.js to work correctly?

[amannn]

Awesome, thanks a lot @samcx!

[samcx]

@zenzen-sol I am also seeing that on next dev.

I think what is probably happening is these i18n headers are working differently in next build. (locally using next build, it shows).

It's tough to say why it gets flipped and explain why next/font doesn't show on next build, but I do know React is making these decisions to best organize what to preload first.

In the meantime, I will be closing this issue!

[samcx]

@zenzen-sol This is likely because the headers size was limited, so next/font was taking precedence.

We expanded these headers →

• https://github.com/vercel/next.js/releases/tag/v15.0.0-canary.99
• #67715

I am taking a look to confirm!

[samcx]

@zenzen-sol

Despite the headers size increase, I only see the next-intl's link headers, which probably makes sense (React calculates what is the best items to add to the link headers).

[zenzen-sol]

@samcx I can confirm that I'm seeing the same when deploying to Vercel. At this point, I'll assume that the correct header will be set, although I haven't found any way to confirm it locally. (Still seeing only the font optimization headers when running in dev.)

Thanks for looking into this!

[samcx]

@amannn Thank you for the clarification! I should have also checked next-intl when next/font wasn't present. When next/font isn't present, the next-intl Link Headers do show up in next dev.

Not sure if it's getting "replaced," but I have shared this with our team!