I stopped testing business logic in TypeScript and moved it into Postgres

382 migrations. Most of the rules that matter aren't in my app code at all — they're triggers, constraints, RLS policies and SQL functions, tested with pgTAP.

The reasoning is uncomfortable but simple: anything enforced in the client is a suggestion. My product sells tamper-evident approval records. If a sign-off's immutability lives in a React component, it isn't immutable, it's polite.

So the update and delete triggers on a completed sign-off refuse for everyone, table owner included. Deposit gating withholds deliverable files at the point where the signed download URL is minted, not in the UI, and it fails closed on error. Seat limits are a trigger on the members table, not a check in a hook.

That pushes testing down with it. Vitest covers 531 files' worth of app logic. The rules live in pgTAP suites that run against a real Postgres in CI, because a mocked trigger proves nothing.

Two things I learned the hard way.

A test suite can be dark. I used mnemonic UUIDs in fixtures for readability. A UUID is hex, and letters like n, y, i and p aren't. Postgres rejected the insert — but the insert was in the fixtures, so the suite aborted before its first assertion and reported no failure and no result. 24 suites were in that state. Not randomly distributed: milestone billing, currency consolidation, the dunning ladder, money webhook events. The features people adopt the product for were the ones with no executing coverage. 125 literals repaired, and the gate now also asserts every suite declares a plan and calls finish(), so a suite can't ship unable to fail.

Global coverage numbers lie in a specific direction. My repo-wide floor is 22% lines, which is honest for a codebase dominated by page components and useless for lateFee.ts. An average lets a 100%-covered tax module and a 0%-covered page swap places without moving the number. So the arithmetic that lands on an invoice has per-module floors — invoice composition, tax, late fees, deposits, retainers, receivables — and the check also fails when a new money-shaped module appears in src/lib with no floor, because a hand-maintained list of important files rots by addition.