Mutant

Mutant is a mutation testing tool for Ruby, created and developed by Markus Schirp. It systematically modifies the code and verifies that the tests catch each change, to check that passing tests are also meaningful tests.

Each surviving mutation is a call to action: either the mutated code is accepted as a simplification, because the tests already specify the correct behaviour, or a missing test is added. Mutant works with RSpec and Minitest and can be limited to the code changed since a given Git revision for incremental use in CI. It is the subject of IEEE-published research.

Mutant is distributed under an end-user licence agreement: it is free for open source projects in public repositories, and use in an organisation requires a paid subscription per developer.