Say what correct means first
If nobody can describe how an output would be checked, the rules have not been agreed yet — that is a finding, not a tooling problem. Every disputed rule gets settled in writing before anything is built.
Make results reproducible
Contractor OS ships a demo dataset that produces byte-identical figures on the embedded WebAssembly Postgres used locally and the managed Postgres in production — same revenue, same margin, balance sheet out by exactly 0.00 on both. Two environments agreeing to the cent is a check anyone can rerun.
Check against the real thing
When Shopflooring came off WordPress, all 36,786 legacy URLs were mapped before the DNS cutover, and the mapping was checked against production, 300 for 300 — not "the mapping code looks right".
Keep the checks running
Asisly carries six hundred and forty-three backend tests and thirteen scheduled jobs on a quarter-hour cadence, because scheduled work is where systems rot quietly. Where the rules run out, a person stands at one gate rather than a reviewer skimming everything.