It is very important that your gem reopens the modules ActiveJob and ActiveJob::QueueAdapters instead of defining them. Because their proper definition lives in Active Job. Furthermore, if the project reloads, you do not want any of ActiveJob or ActiveJob::QueueAdapters to be reloaded. Bottom line, Zeitwerk should not be managing those namespaces. Active Job owns them and defines them. Your gem needs to reopen them.