mutaruby

https://github.com/Atlas-Authorization/mutaruby

mutaruby changes your code in small ways (mutants) and runs your test command against each change. A mutant your tests do not notice is a gap in your tests. Works with any test command (rspec, minitest, rake), runs every mutant in a fresh process inside a private copy of the project, and reports a mutation score as text, JSON or JUnit XML.