Hash the Side-Effect Ledger Before You Accept a Cleanup Refactor

Messy modules rarely break because a pure helper returns the wrong integer on a tidy fixture. They break because three functions share a temporary CSV path, an environment flag, and a cache nobody named. A coding agent then proposes a cleanup that deletes dead branches, renames locals, and still satisfies every existing assertion. The next production export fails because the implicit file layout moved while the return payload stayed identical. That failure mode is the reason this workflow exists, and it is not a style problem. The first commit should freeze a ledger of hidden couplings and store a hash beside it. Only after that hash is in source control should you allow one structural change. The cleanup is legitimate only when the recorded hash remains identical. Cleanup diffs fail differently than feature diffs Feature work usually changes an observable on purpose, so reviewers know which assertions must move. Cleanup work is sold as behavior-preserving, which trains people to trust deletions and rename-only hunks. Coding agents amplify that bias because they optimize for shorter files, conventional names, and green unit tests. Reviewers then accept large deletions that would look suspicious inside a feature pull request. Return-value tests are the wrong gate for that class of change. The public function can still return {"ok": true, "rows": 12} while the working directory quietly shifts. Downstream jobs that glob files or catch a named exception will fail after merge. Those hidden couplings remain part of the contract even when no unit test mentions them. Build a side-effect ledger instead of another unit test Treat the messy module as a black box that emits more than a return value. A ledger is a canonical JSONL file with one record per fixture and fully sorted keys. Side-effect entries need stable ordering so the serialized bytes stay deterministic across reruns. The SHA-256 digest of that file is the only number that must remain constant. Each record should c