mutant

What is 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.

What is an Alive Mutation?

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.

Author

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.

Quick Start

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

  def adult?
    @age >= 18
  end
end

ruby
# 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:

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

Mutant finds a surviving mutation indicating a shallow test:

diff
 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.

Session History

Mutant records every run to .mutant/results/. You can recall past results
without re-running mutation testing:

bash
# List past sessions (most recent first)
mutant session list

# Show full report from the latest session
mutant session show

# Show full report from a specific session
mutant session show --session-id 019cf6f1-77e8-74b6-82db-f8b5faf570cd

# List subjects with alive/total mutation counts
mutant session subject

# Show alive mutations for a specific subject
mutant session subject 'Foo::Bar#baz'

# Remove old or incompatible session files
mutant session gc --keep 50

Next Steps

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

Ruby Versions

Mutant is supported on Linux and macOS.

Version	Runtime	Syntax	Mutations
3.2	:heavy_check_mark:	:heavy_check_mark:	:heavy_check_mark:
3.3	:heavy_check_mark:	:heavy_check_mark:	:heavy_check_mark:
3.4	:heavy_check_mark:	:heavy_check_mark:	:heavy_check_mark:
4.0	:heavy_check_mark:	:heavy_check_mark:	:heavy_check_mark:

Rails

Mutant runs against Rails applications, including parallel-worker database
isolation (PostgreSQL and SQLite). The hook recipes are verified in CI on every
non-EOL Rails version:

Version	Status
7.2	:heavy_check_mark:
8.0	:heavy_check_mark:
8.1	:heavy_check_mark:

See the Rails Integration guide for setup, eager loading, and
per-worker database isolation. A runnable, CI-verified example lives in
rails_example.

Licensing

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

Commercial use requires a subscription ($30/month or $250/year per developer).
Enterprise — reach out directly.
See commercial licensing for pricing and details.