Hanami Ruby

# app/actions/pixel/show.rb
module MyApp
  module Actions
    module Pixel
      class Show < MyApp::Action
        GIF = "R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==".unpack1("m0").freeze

        def handle(request, response)
          # Rack's content_type= sets the exact header; format= would append "; charset=utf-8".
          response.content_type = "image/gif"
          response.headers["Cache-Control"] = "no-store"
          response.body = GIF
        end
      end
    end
  end
end

# config/routes.rb
module MyApp
  class Routes < Hanami::Routes
    get "/pixel.gif", to: "pixel.show"
  end
end