Penny-accurate: testing a billing engine against 3 years of edge cases

A billing engine that is right 99% of the time is a billing engine that is wrong every week. So before this one ever sent a real invoice, we made it re-run three years of history and prove itself against what actually happened.

The client had every past invoice, every credit note, every awkward proration and mid-cycle plan change. We fed the lot through the new engine and compared the output, to the penny, against the real ledger. The mismatches were the gold — each one a rule nobody had written down, a rounding convention, an edge case that only existed because a customer once did something nobody designed for.

We fixed until the new engine reproduced three years of reality exactly, then ran the edge cases forward as an automated suite that runs on every change. The engine went live having already billed three years correctly in rehearsal.

That is what we mean by penny-accurate. Not that the maths looks right, but that it has been proven against the messiest evidence available — your own past — before it touches a single live customer.