application.rb

# config/application.rb
config.middleware.insert_after ActionDispatch::Flash, Warden::Manager do |manager|
  manager.default_strategies :authentication_token
  manager.failure_app = UnauthorizedController
end

unauthorized_controller.rb

# app/controllers/unauthorized_controller.rb
class UnauthorizedController < ActionController::Metal
  def self.call(env)
    @respond ||= action(:respond)
    @respond.call(env)
  end

  def respond
    self.response_body = "Unauthorized Action"
    self.status = :unauthorized
  end
end