The honest day-1 product: we'll tell you the moment Aurora, CO city sales-tax rate changes. You can't reconstruct a fee an agency already overwrote — but you can be the first to know when it moves next, with a dated, hashed record to back it.
For agents / programmatic: poll the diff API — GET /api/diffs (global change feed) or GET /api/field/aurora-co:sales-tax-rate (per-field history + latest alert). Free preview; the full signed history is x402-gated for evidence use. See /developers.
For humans: alerts ship via the diff feed today; an email/webhook subscription endpoint is the next add. Bookmark the field page — the change log updates in place.