Bypass Grape and Rack::Response's response materialization

config.ru

require 'grape'

class ResponseHijack
  BODY_KEY = 'response_hijack.body'.freeze
  CONTENT_TYPE_KEY = 'response_hijack.content_type'.freeze

  def initialize(app)
    @app = app
  end

  def call(env)
    response = @app.call(env)
    if (body = env[BODY_KEY])
      content_type = env[CONTENT_TYPE_KEY]
      response[1].delete('Content-Length')
      response[1]['Content-Type'] = content_type if content_type
      response[2] = body
    end
    response
  end
end

class App < Grape::API
  format :json
  version 'v1', using: :path

  namespace do
    use ResponseHijack
    get do
      env[ResponseHijack::BODY_KEY] = ['foo', 'bar']
      env[ResponseHijack::CONTENT_TYPE_KEY] = 'text/plain'
      nil
    end
  end
end

run App