Lowercase "upgrade" value in Connection header may prevent connection upgrade

Start in Mint.WebSocket.Utils.headers/2, especially the HTTP/1 header list shown in the issue, and review how header casing is handled by Mint. Reproduce the websocket upgrade against the reported server or an equivalent HTTP/1 endpoint, then verify that the generated handshake is accepted without requiring a duplicate custom Connection header.

1. Description of issue

I wasted several hours today trying to figure out why MintWebSocket.upgrade/5 was failing when connecting to a websocket at wss://demo.rss.chat/

I noted that other websocket clients were succeeding and went to compare the headers that were sent at the upgrade request.

This particular server rejects the standard Mint.WebSocket.upgrade/5 call with a status 400 and a somewhat misleading message:

HTTP/1.1 400 Bad Request
Alt-Svc: h3=":443"; ma=2592000
Content-Type: text/html
Date: Mon, 17 Aug 2026 00:13:04 GMT
Server: Caddy
Transfer-Encoding: chunked

34
This HTTP server can't handle WebSocket connections.

I finally tracked down the issue to the fact that MintWebSocket.upgrade/5 sends a Connection header with the value "upgrade" in lowercase. When that is changed to "Upgrade" (capitalized), the socket is upgraded successfully.

2. Possible fix

Mint.WebSocket.Utils.headers/2 shows the following:

  # header names are written in their conventional casing: Mint lowercases them
  # unless the connection was opened with `case_sensitive_headers: true`
  def headers({:http1, nonce}, extensions) when is_binary(nonce) do
    [
      {"Upgrade", "websocket"},
      {"Connection", "upgrade"},
      {"Sec-WebSocket-Version", "13"},
      {"Sec-WebSocket-Key", nonce},
      {"Sec-WebSocket-Extensions", extension_string(extensions)}
    ]
    |> Enum.reject(fn {_k, v} -> v == "" end)
  end

When I changed {"Connection", "upgrade"} to {"Connection", "Upgrade"} (capitalized), the connection succeeded. All of the non-normative examples in RFC6445 section 1.1 and 1.2 show the handshake headers using the capitalized "Upgrade" and the lowercase "websocket".

I believe that Mint.WebSocket.Utils.headers/2 should return the capitalized version.

For this particular server I was able to connect by supplying my own {"Connection", "Upgrade"} header (so that both values, "upgrade" and "Upgrade", are sent to the server in that order), but I don't know if that will work generally or not.