Mutation Testing & Test Quality Protocol

Measure the actual defect-detection capability of your test suite beyond simple code coverage metrics.

1. The Mutation Testing Principle

1. High code coverage can be misleading if assertions are weak or missing.
2. Mutation testing intentionally injects mutants (faults) into production code:
   • Replaces comparison operators
   • Replaces arithmetic operators
   • Inverts boolean conditions
   • Removes function calls or return values
3. If tests still pass when code is mutated, the mutant survives (identifies an untested boundary).
4. If a test fails, the mutant is killed (confirms real assertion coverage).

2. Surviving Mutant Remediation

1. When a mutant survives, identify the missing edge-case test.
2. Add a targeted test case asserting the specific boundary condition until the mutant is killed.
3. Aim for a Mutation Score Indicator > 80% on mission-critical business logic.