What a system replacement does to measured error: three SNAP eligibility-system migrations in the quality-control record

Verifying encoded SNAP rules and simulating payment error under cost sharing. The QC files as a verification oracle — 6,081 in-scope fiscal 2024 cases across seven states reproduced exactly, zero dollar tolerance — a distributional error model reported with its validation failures, and the fiscal 2025 rates landing where the sampling-noise mechanism says realized years should: 18 of 53 jurisdictions changed cost-share tiers, only 7 of those changes beyond two-year sampling noise. This page embeds the revision-7 manuscript (updated 2026-08-13), which carries five rounds of adversarial review, archived unedited, and adds the engine-adoption accounting bounds, the official-vs-file wedge decomposition, and the pre-registered model-repair experiments.