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Using Mutation Testing to Safely Refactor Legacy Code Without Trusted Tests

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A passing test suite can conceal refactoring bugs when its tests assert little of substance, a problem the author says is worsened by free coding models that generate plausible tests quickly. The recommended method splits the work: a model proposes risky inputs, while expected outputs are recorded from the current code's behavior, which serves as the specification. The author demonstrates this on a legacy discount calculator with no prior tests, storing 20 to 60 inputs as data and serializing the current outputs as golden values. A mutation gate is then used to judge whether the resulting suite is strong enough before the function is changed. The article is a disclosed product-outreach piece promoting MonkeyCode's free model access.

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