api.rb

module MyApp
  module API
    class Railtie < Rails::Railtie
      class Command
        attr_accessor :user

        def initialize(user = nil)
          @user = user || User.first
        end

        %i(get post patch put delete head).each do |method|
          define_method(method) do |path, parameters = {}, headers_or_env = {}|
            app.__send__ method, path, default_params.merge(parameters), default_headers.merge(headers_or_env)
            ActiveSupport::JSON.decode app.response.body
          end
        end

        private

        def default_params
          {}.tap do |h|
            h[:api_key] = @user.api_key if @user
          end
        end

        def default_headers
          {}.tap do |h|
            h['Accept'] = 'application/json'
          end
        end

        def app
          TOPLEVEL_BINDING.eval('app')
        end
      end

      module ClassMethods
        def api
          @api ||= Command.new
        end
      end

      console do
        TOPLEVEL_BINDING.eval('self').extend ClassMethods
      end
    end
  end
end

console

api.get 'users' #=> [{"id"=>1, "name"=>"foo"}, {"id"=>2, "name"=>"bar"}]
api.user = nil
api.get 'users' #=> {"status"=>401, "message"=>"Unauthorized"}

api_key はそれぞれのサービスの認証に必要なパラメータ名に変えてください。