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

[zenzen-sol]

Link to the code that reproduces this issue

https://github.com/zenzen-sol/next-reproduction-template/tree/sol/repro-001-next-15-broken

To Reproduce

1. pnpm i && pnpm run dev
2. Reload http://localhost:3000 in a browser.
3. Check the response header links in dev tools.

Examples:

• ✅ Last working release (NextJS 14.3.0-canary.42): https://github.com/zenzen-sol/next-reproduction-template/tree/sol/repro-001-next-14-3-0-canary-42-working
• 🙅🏻 First broken release (NextJS 14.3.0-canary.43): https://github.com/zenzen-sol/next-reproduction-template/tree/sol/repro-001-next-14-3-0-canary-43-broken
• ✅ Working without next/font (NextJS 15.0.0-rc.0): https://github.com/zenzen-sol/next-reproduction-template/tree/sol/repro-001-next-15-working

Current vs. Expected behavior

The next-intl library's middleware injects "alternate" links into the response header for SEO purposes. At the same time, next/font injects "preload" header links into the response header. Both sets of links should appear in the response header. This was working correctly in Next 14.

Since NextJS 14.3.0-canary.43 and 15.0.0-canary.0, the "alternate" links from next-intl are no longer present when next/font is used.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 23.6.0: Fri Jul  5 17:53:24 PDT 2024; root:xnu-10063.141.1~2/RELEASE_ARM64_T6020
  Available memory (MB): 32768
  Available CPU cores: 12
Binaries:
  Node: 20.11.1
  npm: 10.2.4
  Yarn: 3.6.1
  pnpm: 9.7.1
Relevant Packages:
  next: 15.0.0-rc.0 // Latest available version is detected (15.0.0-rc.0).
  eslint-config-next: N/A
  react: 19.0.0-rc-19bd26be-20240815
  react-dom: 19.0.0-rc-19bd26be-20240815
  typescript: 5.5.4
Next.js Config:
  output: N/A

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

Font (next/font), Internationalization (i18n), Metadata, Middleware

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

next dev (local), Vercel (Deployed)

Additional context

I tested my reproduction against canary releases back to 14.3.0-canary.0. This issue first appears in v14.3.0-canary.43, and is not present in 14.3.0-canary.42.

If I had to guess, the issue is related to the changes made to packages/next/src/server/app-render/app-render.tsx in that prerelease.

[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
• [ppr] Configuration for max react headers length #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]

@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!

[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?