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 is due to Heroku not allowing conditions in the Gemfile, even in the dev group.

The alternative is to put the gem in it’s own group:

And on non Mac systems run bundle install --without darwin (this only needs to be run once, the without setting is remembered for future bundle installs). Then for Heroku run heroku config:add BUNDLE_WITHOUT="development test darwin"

Don’t forget to add –remote remote_name if you are pushing to a remote other than heroku (e.g. heroku config:add BUNDLE_WITHOUT="development test darwin" --remote staging) and don’t forget to merge your amended Gemfile into master before running git push heroku master.