Building a financial engine that audits itself

The alarm was the bug

Spent the morning convinced a card was only capturing 6% of its spending. It was not. Two cards share one credit line — a supplementary plastic on the same account — and reconciling them separately made each card’s purchases look like the other’s missing data.

Merging them took the line to 93%. The lesson is dull and important: instrument the entity, not the identifier.

A rate nobody publishes

No provider carries an EGP to AED retail rate. But two consecutive credit limits differ by exactly what the card was charged, spread included, so the rate was sitting in the data the whole time — one subtraction away.

Measured against the reference rate, the bank takes about 3%. Across three currency pairs the number held.

Starting point

The dashboard reports a negative savings rate on a month that was comfortably positive. Cause is almost certainly that transfers are being counted as spending. Beginning here.