How AI Refactoring Tools Can Silently Break Legacy Code — And How to Catch It

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A software developer demonstrated a workflow for safely using AI-generated code refactors on legacy functions that lack formal specifications. The approach involves first capturing the existing function's behavior by recording inputs and outputs as a 'ground truth' baseline, then prompting a free AI coding model to refactor the code while keeping behavior identical. Characterization and differential tests are then run against both the original and refactored versions to detect any divergence. In one test case, the AI changed a single comparison operator — from 'less than or equal to' to 'less than' — which silently flipped a shipping charge from zero to $4.99. The article concludes that AI-generated diffs should be treated as hypotheses requiring test-based verification, not trusted on appearance alone.

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