Shadow-Comparing Agent Patches Can Catch Hidden Behavior Changes Before Merge

A software testing workflow published on DEV Community proposes that passing test suites alone are insufficient to validate agent-generated code patches, since such patches can silently alter encodings, error types, or edge-case handling without breaking existing assertions. The approach involves running both a frozen baseline and the candidate patch against an identical deterministic corpus, then comparing their outputs byte-for-byte after canonicalization. Any divergence found must be logged in a structured 'accepted-delta ledger' and explicitly classified before the patch is allowed to merge. The harness relies on three components — a pinned baseline artifact, the candidate patch, and the delta ledger — to ensure unspecified behavior is treated as implicitly contractual. The workflow is presented as a complement to, not a replacement for, manual code review, acting as an automated filter for behavioral drift that reviewers would otherwise miss.