15.4.1 regression - using server side cookies component with middleware results in uncaught error server-action-reducer.js:104

[gladykov]

Link to the code that reproduces this issue

https://github.com/gladykov/nextjs-15.4.1-bug

To Reproduce

1. npm install
2. npm run dev
3. http://localhost:3000/
4. Click the button
5. Check the browser web dev console

Current vs. Expected behavior

Result:

server-action-reducer.js:104 Uncaught (in promise) Error: An unexpected response was received from the server.
    at fetchServerAction (server-action-reducer.js:104:37)

Recovery steps:

• Downgrade to next 15.4.0
• OR remove middleware.ts

Provide environment information

/bin/sh: line 1: pnpm: command not found

Operating System:
  Platform: linux
  Arch: x64
  Version: #1 ZEN SMP PREEMPT_DYNAMIC Tue, 19 Aug 2025 05:00:54 +0000
  Available memory (MB): 15853
  Available CPU cores: 8
Binaries:
  Node: 22.14.0
  npm: 11.5.2
  Yarn: 1.22.22
  pnpm: N/A
Relevant Packages:
  next: 15.4.1 // There is a newer version (15.5.0) available, upgrade recommended! 
  eslint-config-next: N/A
  react: 19.1.0
  react-dom: 19.1.0
  typescript: 5.9.2
Next.js Config:
  output: N/A

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

Middleware

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

next start (local), next dev (local)

Additional context

I can reproduce in v15.5.1-canary.4 as well

[icyJoseph]

🤔 I think this might be a combination of things - I was just documenting that this echoing thing:

    const headers = new Headers(request.headers);
    return NextResponse.next({ headers });

Is probably not a good idea - cuz the framework/host might inject headers - and then you echo that back to the client.

In this case, I think the problem is not the cookies setting, but that the headers in the request sent by the browser have a content-type header, and that overrides the server action's content-type header downstream

So the server action responds with text/x-component, but the request has text/plain - and in doing return NextResponse.next({ headers }); things the text/plain wins.

• next.js/packages/next/src/client/components/router-reducer/reducers/server-action-reducer.ts

  Lines 163 to 166 in e02f667

  	// Handle invalid server action responses.
  	// A valid response must have `content-type: text/x-component`, unless it's an external redirect.
  	// (external redirects have an 'x-action-redirect' header, but the body is an empty 'text/plain')
  	if (!isRscResponse && !redirectLocation) {

[gladykov]

Hi and thank you for fast investigation. Middleware is a part of this workaround:

    export function middleware(request: NextRequest) {
        // Add a new header x-current-path which passes the path to downstream components
        const headers = new Headers(request.headers);
        headers.set('x-current-path', request.nextUrl.pathname);
        return NextResponse.next({ headers });
}

which then I retrieve in code:

    const pathname = decodeURIComponent(headerList.get('x-current-path'));

[icyJoseph]

Where is const pathname = decodeURIComponent(headerList.get('x-current-path')); called? in a server component? Then you wanna do:

return NextResponse.next({ request: { headers } })

And in general, it is better to narrow down the amount of headers and such, I'd say create an empty headers instance, and append/set only the headers you need downstream.

[gladykov]

Yes, I use decodeURIComponent in server component.

Thank you, this worked :)

[icyJoseph]

Awesome, if you agree that this is sorted out, please don't forget to close. I'll refer this issue in the docs changing I have upcoming.

[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.