Middleware: Setting new request headers breaks headers()

[sommeeeer]

Link to the code that reproduces this issue

https://github.com/sommeeeer/next-middleware-headerslist-issue

To Reproduce

1. pnpm build && pnpm start
2. curl http://localhost:3000/issue
3. go to http://localhost:3000/issue-page

Current vs. Expected behavior

I have taken the example code from https://nextjs.org/docs/app/building-your-application/routing/middleware#setting-headers and put it in my middleware.ts. This breaks headers() in any route:

You will get an error saying:

I expect headers()to work, even though I have modified the request headers in middleware.ts

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #47~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Wed Oct  2 16:16:55 UTC 2
  Available memory (MB): 14800
  Available CPU cores: 16
Binaries:
  Node: 20.18.0
  npm: 11.1.0
  Yarn: N/A
  pnpm: 9.15.4
Relevant Packages:
  next: 15.2.4 // Latest available version is detected (15.2.4).
  eslint-config-next: 15.2.4
  react: 19.1.0
  react-dom: 19.1.0
  typescript: 5.8.2
Next.js Config:
  output: N/A

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

Headers, Middleware

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

next dev (local), next build (local), next start (local)

Additional context

No response

[sommeeeer]

Can confirm I get the same behavior on version 14.2.26 aswell

[sommeeeer]

I closed this as I was a bit pre-early on making it. I guess thats just a limitation of headers(). You can't console.log(await headers()) on it. After doing some more testing I found out that what you have in your middleware.ts doesn't matter.

I was able to use the getters and setters on headers() fine no matter what I had in my middleware.ts.

[github-actions]

This closed issue has been automatically locked because it had no new activity for 2 weeks. If you are running into a similar issue, please create a new issue with the steps to reproduce. Thank you.