Yeah use Ruby. I prefer to have well defined interfaces for the objects I use. I love Ruby to death, but it's frustrating to encounter code like this:

  [:thing, :other_thing].each do |msg|
    define_method(msg) do
      @json[msg].to_s
    end
  end

When this is far more clear and, for me, aesthetically pleasing:

  def thing
    @json[:thing].to_s
  end

  def other_thing
    @json[:other_thing].to_s
  end

I'm not sure how you'd use a ruby object that maps to an OpenAPI spec if you don't know what's in the API response until runtime... surely you or the user of your library has an idea what's in the API they are consuming?