`HttpClient.send` puts a header's string where `Response` says `Array String`

[gilramir]

Found against: gren 0.6.6, gren-lang/core 7.4.2, gren-lang/node 6.1.3, node 22
Reproduction: https://github.com/gilramir/gren-bug-reports/tree/main/2026-09-15-httpclient-header-values

HttpClient.Response's headers are a Dict String (Array String), but
HttpClient.send stores each header's value as a plain string. So for
content-type: text/plain, Array.length of the values is 10 and
Array.first is Just "t", and String.join ", " values throws inside
fetch's promise chain after the task has already succeeded: the program
silently stops, with nothing on stderr and exit status 0. A repeated
set-cookie also keeps only the last cookie.

Reproduction

gren.json dependencies: gren-lang/core 7.4.2, gren-lang/node 6.1.3. Save as src/Main.gren. Build and run with gren make Main --output=app && node app; the third row is never printed:

module Main exposing (main)

import Dict
import HttpClient
import Init
import Node
import Stream
import Task


main : Node.SimpleProgram a
main =
    Node.defineSimpleProgram <| \env ->
        Init.await HttpClient.initialize <| \http ->
            let
                print line =
                    Stream.writeLineAsBytes line env.stdout
                        |> Task.onError (\_ -> Task.succeed env.stdout)
            in
            Node.endSimpleProgram
                (HttpClient.get "data:text/plain,x"
                    |> HttpClient.send http
                    |> Task.andThen
                        (\response ->
                            let
                                values =
                                    Dict.get "content-type" response.headers |> Maybe.withDefault []
                            in
                            print "| call | result | expected |\n|---|---|---|"
                                |> Task.andThen (\_ -> print ("| `Array.length values` | " ++ String.fromInt (Array.length values) ++ " | 1 |"))
                                |> Task.andThen (\_ -> print ("| `Array.first values` | " ++ Debug.toString (Array.first values) ++ " | Just \"text/plain\" |"))
                                |> Task.andThen (\_ -> print ("| `String.join \", \" values` | " ++ String.join ", " values ++ " | text/plain |"))
                        )
                    |> Task.onError (\_ -> print "request failed")
                )

Output:

$ node app; echo "exit status $?"
| call | result | expected |
|---|---|---|
| `Array.length values` | 10 | 1 |
| `Array.first values` | Just "t" | Just "text/plain" |
exit status 0

Cause

_HttpClient_formatResponse in Gren/Kernel/HttpClient.js, which send uses:

for (const [key, value] of res.headers.entries()) {
  headerDict = A3(__Dict_set, key.toLowerCase(), value, headerDict);
}

Headers.entries() gives each value as a string. fetch already joins a
repeated header with ", ", except set-cookie, which entries() gives once
per cookie, so __Dict_set keeps only the last one. The streaming API's
_HttpClient_formatResponseLegacy reads res.headersDistinct, whose values are
arrays, and is not affected.