mutant

AI writes your code. AI writes your tests. But who tests the tests?

Copilot, Claude, and ChatGPT generate code faster than humans can review it. They'll even write tests that pass. But passing tests aren't the same as meaningful tests.

Mutant is mutation testing for Ruby. It systematically modifies your code and verifies your tests actually catch each change.

The more code AI writes for you, the more you need verification you can trust.

Each surviving (alive) mutation is a call to action with exactly one of two options:

• Keep the mutated code - your tests already specify the correct semantics, and the original code is redundant. Accept the mutation as a simplification.
• Add the missing test - the original code is correct, but the tests don't verify the behavior the mutation removed.

Mutant was created and is developed by Markus Schirp. It is the subject of IEEE-published research and is included in the Trail of Bits Ruby Security Field Guide.

# lib/person.rb
class Person
  def initialize(age:)
    @age = age
  end

  def adult?
    @age >= 18
  end
end

# spec/person_spec.rb
RSpec.describe Person do
  describe '#adult?' do
    it 'returns true for age 19' do
      expect(Person.new(age: 19).adult?).to be(true)
    end

    it 'returns false for age 17' do
      expect(Person.new(age: 17).adult?).to be(false)
    end
  end
end

Tests pass. But run mutant:

gem install mutant-rspec
mutant run --use rspec --usage opensource --require ./lib/person 'Person#adult?'

Mutant finds a surviving mutation indicating a shallow test:

 def adult?
-  @age >= 18
+  @age > 18
 end

Your tests don't cover age == 18. The mutation from >= to > doesn't break them.

This is just one of many mutation operators. Mutant also mutates arithmetic, logical, bitwise operators, removes statements, modifies return values, and more.

A full working example is available in the quick_start directory.

1. Learn the nomenclature (subjects, mutations, operators)
2. Set up your integration: RSpec or Minitest
3. Run mutant on CI in incremental mode

Mutant is supported on Linux and macOS.

Free for open source. Use --usage opensource for public repositories.

Commercial use requires a subscription ($90/month per developer). Enterprise — reach out directly. See commercial licensing for pricing and details.

• Configuration
• RSpec Integration
• Minitest Integration
• Incremental Mode
• Reading Reports
• Concurrency
• AST Pattern Matching
• Hooks
• Sorbet
• Nomenclature
• Limitations
• Mutant in the Wild

• Discord
• GitHub Issues
• Release Announcements

See CONTRIBUTING.md for development setup and guidelines.

• Contributors
• The mutant-minitest integration was sponsored by Arkency

Parts of Mutant are being incrementally rewritten in Rust for improved performance. This is currently opt-in and requires no changes for existing users. See RUST.md for details.