basic example on how to use xml api with grape

api.rb

module API
  class Base < Grape::API
    mount API::V1::ApplicationV1 => "/api"
  end
end

application_v1.rb

module API
  module V1
    class ApplicationV1 < Grape::API
      require 'builder'
      format :xml
      default_format :xml
      unless Rails.env.development?
        rescue_from :all do |message, backtrace, options, env|
          Rails.logger.info message.inspect
          Rails.logger.info backtrace.inspect
          Rails.logger.info options.inspect
          Rails.logger.info env.inspect
          Rack::Response.new([{message: "Invalid Request"}.to_xml(root: :error)], 400, { "Content-type" => "application/xml" }).finish
        end
      end

      get "/action" do
        @data = Data.get(params)
        XMLResponses::Data::generate(@data)
      end
    end
  end
end

data.rb

module API
  module V1
    module XMLResponses::Data
      def self.generate(data)
        xml = Builder::XmlMarkup.new(:indent => 2)
        xml.Response do |resp|
          resp.APIVersion "1.0"
          resp.DataField data.field
        end
      end
    end
  end
end