[BUG] Multiple cookies are not set properly

[blitss]

Describe the bug

When I try to set multiple cookies inside middleware, they are getting folded into one Set-Cookie header, and therefore browser doesn't set them properly.

According to cloudflare docs and the IETF RFC, the set-cookie header should not fold.

I narrowed down issue to opennext because it worked for me on other environments:

• next-on-pages
• next dev
• Just using cloudflare workers, e.g. this code:

export default {
  async fetch(request, env, ctx) {
    const headers = new Headers();
    headers.append("Set-Cookie", "test1=hello; Path=/; HttpOnly");
    headers.append("Set-Cookie", "test2=world; Path=/; HttpOnly");
    
    return new Response('Hello World!', { headers });
  },
};

Steps to reproduce

repro: https://github.com/blitss/opennext-set-cookies-issue

1. Create a perfectly normal nextjs project, can just use npm create cloudflare@latest -- my-next-app --framework=next --experimental
2. Create middleware.ts
3. Use multiple set-cookie header inside middleware:

import { NextRequest, NextResponse, NextFetchEvent } from "next/server";

export function middleware(request: NextRequest, event: NextFetchEvent) {
  // Create a response instance
  const response = new NextResponse('testing cookies');

  // Append multiple cookies using the Set-Cookie header
  response.headers.append(
    "Set-Cookie",
    "test1=value1; Path=/; HttpOnly; Secure; SameSite=Strict"
  );
  response.headers.append(
    "Set-Cookie",
    "test2=value2; Path=/; HttpOnly; Secure; SameSite=Strict"
  );

  return response;
}

export const config = {
  matcher: ["/test"],
};

A result:

Expected behavior

The set cookie should not fold

@opennextjs/cloudflare version

0.5.12

Wrangler version

4.3.0

next info output

% bunx next info

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.3.0: Thu Jan  2 20:24:23 PST 2025; root:xnu-11215.81.4~3/RELEASE_ARM64_T6020
  Available memory (MB): 16384
  Available CPU cores: 12
Binaries:
  Node: 20.10.0
  npm: 10.2.3
  Yarn: 1.22.19
  pnpm: 8.14.0
Relevant Packages:
  next: 15.2.3 // There is a newer version (15.2.4) available, upgrade recommended! 
  eslint-config-next: 15.1.6
  react: 19.0.0
  react-dom: 19.0.0
  typescript: 5.7.3
Next.js Config:
  output: N/A
 ⚠ There is a newer version (15.2.4) available, upgrade recommended! 
   Please try the latest canary version (`npm install next@canary`) to confirm the issue still exists before creating a new issue.
   Read more - https://nextjs.org/docs/messages/opening-an-issue

Additional context

No response

[blitss]

it looks like this issue is mitigated by this PR: #497
so for now it's fixable with bun install https://pkg.pr.new/@opennextjs/cloudflare@958f322
feel free to close this issue