Using the guard gem to run tests etc in your Rails app normally requires some form of file system monitoring. The monitoring will be OS dependant and rb-fsevent is the gem for OSX. This can be added to the Gemfile conditionally with: gem 'rb-fsevent', :require => false if RUBY_PLATFORM =~ /darwin/i Unfortunately when you next push to Heroku you are likely to get an error along the lines of: This i