Mutation Testing with Mutant

When to Activate

• The user asks to run mutation testing.
• The user has alive mutations to fix.
• The user asks to verify mutation coverage on a subject.

When Not to Use

• The task does not involve mutation testing.

Inputs

• Alive mutation output to act on (paste the evil: block).
• Optional: subject expression to scope the run.

Outputs (Fixed Order)

1. Mutation results (alive count, coverage percentage).
2. Clear action for each alive mutation: add test or simplify code.

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Reading Mutation Output

An alive mutation looks like:

evil:YourClass#method:YourClass#method:lib/your_class.rb:42:abc12
@@ -1,3 +1,3 @@
 def method
-  @value >= threshold
+  @value > threshold
 end

• evil means no test killed this mutation.
• The diff shows original (-) and mutated (+) code.

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Reporting Format (BLUF)

Lead with the verdict, bottom line up front:

If unkillable:

Unkillable. Both forms are equivalent because [reason]. Add to ignore list.

If killable:

Killable.

Option A, add test:

(test diff)

Option B, simplify code:

(source diff)

Always present both options so the user can choose. Include evidence: if you cannot think of a test that would kill the mutation, say so, that is valuable signal toward unkillable.

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Usage

1. Run Mutant

bundle exec mutant run --fail-fast

When the subject is known, scope the run to avoid testing unrelated subjects:

bundle exec mutant run --fail-fast 'Entities::CreateService#call'

If the command succeeds, coverage is 100%, done. If it fails, find the evil: line in the output, it has the subject name, file path, and line number. The diff block immediately after shows the original and mutated code.

2. Investigate

Read the source file and existing spec file for the subject. Ask: "Is the mutated code acceptable for all valid inputs?"