Cowboy lowercases all the headers by default. This stream handler can be used to capitalize the headers. The implementation is in Elixir, and a configuration example showing its use in a Phoenix application is shown.

capitalize_headers.ex

defmodule AppWeb.CapitalizeHeaders do
  def init(stream_id, req, opts) do
    :cowboy_stream_h.init(stream_id, req, opts)
  end

  def data(stream_id, is_fin, data, state) do
    :cowboy_stream_h.data(stream_id, is_fin, data, state)
  end

  def info(stream_id, {:response, _status, headers, _body} = info, state) do
    info = put_elem(info, 2, capitalize_headers(headers))
    :cowboy_stream_h.info(stream_id, info, state)
  end

  def terminate(stream_id, reason, state) do
    :cowboy_stream_h.terminate(stream_id, reason, state)
  end

  def early_error(stream_id, reason, partial_req, resp, opts) do
    :cowboy_stream_h.early_error(stream_id, reason, partial_req, resp, opts)
  end

  defp capitalize_headers(headers) when is_map(headers) do
    Map.new(headers, fn {key, value} -> {capitalize_header(key), value} end)
  end

  defp capitalize_header("etag"), do: "ETag"
  defp capitalize_header("www-authenticate"), do: "WWW-Authenticate"
  defp capitalize_header(header), do: :cowboy_bstr.capitalize_token(header)
end

config.exs

config :app, AppWeb.Endpoint,
  http: [
    stream_handlers: [
      AppWeb.CapitalizeHeaders,
      :cowboy_telemetry_h,
      :cowboy_stream_h,
    ]
  ]