evilution kills 1,088 of 1,093 mutants in Rails AI test

The piece builds its contract table on Design by Contract, splitting rules into invariants, preconditions, and postconditions. It gives concrete evidence that invariants alone are not enough: a spec checking only that the total stays at 10 let a swap bug survive, where available_stock and reserved_stock were exchanged. Adding the postcondition that available drops by 3 and reserved rises by 3 killed that mutant, and adding boundary values just below, at, and just above the reservation quantity took the inventory model from 62 survivors out of 150 to all 150 killed.

On tooling, the author compared Ruby mutation testers directly. mutant's README restricts free use to public repositories, with $30 per developer per month or $250 per year for commercial use, so it was avoided for a private app. The archived mutest fork failed to install. evilution, under MIT, was chosen, and the author notes both evilution and the also-MIT mutineer only appeared in 2026, so changelogs and issues are worth reviewing before adoption.

The review division of labor is explicit: quality gates decide automatically whether the completion conditions are met, and humans check only whether each spec actually verifies the contract row named in its ID, whether surviving mutants are real test gaps, and whether disabled lines have genuine equivalence reasons. The author adds that stub usage in specs should be limited to two cases — failures unreachable from any real DB state, and external APIs — each with a written reason.