Usage You need to tell RuboCop to load the Thread Safety extension. There are three ways to do this: RuboCop configuration file Put this into your .rubocop.yml. plugins: rubocop-thread_safety Now you can run rubocop and it will automatically load the RuboCop Thread Safety cops together with the standard cops. Command line $ rubocop --plugin rubocop-thread_safety Rake task RuboCop::RakeTask.new do |task| task.requires << 'rubocop-thread_safety' end Installation Cops