The headers in Edge Function is collapsed

[unix]

Hello, this issue describes what may be a bug, but I'm not sure if the service is behaving as expected and may need your help 🙏

Description

In Vercel's Edge Function service, all headers will be collapsed, which is very unfriendly to set-cookie and may not be recognized by browsers.

Example

This is a simple example I created for a project with only one middleware using Edge Function, setting 2 cookies and doing nothing:

I tried to splice the cookies string manually using the headers.set / headers.append method, but I got the same result. This problem may be unrelated to the implementation of the cookie method.
The cookie domain constraint was also debugged and seems to be irrelevant.

export default (req: NextRequest) => {
  const res = NextResponse.next();
  res.cookie("key1", "value");
  res.cookie("key2", "value");
  return res;
};

set-cookie should be expanded when there is no middleware, and in fact, we get this when we run next.js locally:

# curl --head http://localhost:3000
HTTP/1.1 200 OK
set-cookie: key1=value; Path=/
set-cookie: key2=value; Path=/

When we run this example on Edge Function, the set-cookie fields will be collapsed together as follows:

# curl --head https://api-fold-cookies.vercel.app
HTTP/2 200
set-cookie: key1=value; Path=/, key2=value; Path=/

Impact

When accessing a collapsed cookie in a browser, the browser will only receive the first value(only key1). And the result on the Edge Function is not consistent with the local development, which may cause trouble for users to find the cause of the problem.

Also as a side note, I noticed that in RFC 2109 it states that the middleware can fold the headers, which seems reasonable, but is there a way we can work around this or make the results run the same locally as on the service?

RFC 2109
An origin server may include multiple Set-Cookie headers in a
response. Note that an intervening gateway could fold multiple such
headers into a single header.

View questions online

• Example repo
• Visit demo site and check cookies
• Run curl --head https://api-fold-cookies.vercel.app.