Official-calculator oracles (mode 3): gov.uk and benefits-calculator household cases as a second UK oracle family

[vahid-ahmadi]

The mode-3 UK lane so far is UKMOD/EUROMOD-shaped: #41 (JRC connector) and the case schema + battery in #49. One oracle family is a single point of comparison — when PE and UKMOD disagree, nothing adjudicates. The UK has something the US mostly lacks: official and production-grade household calculators whose outputs are ground truth or near-ground-truth for a specified household:

• gov.uk estimators: HMRC's income-tax/NICs estimator, the State Pension and Child Benefit tax-charge (HICBC) calculators — official statements of liability for a given input set.
• Policy in Practice's Better Off Calculator / entitledto: production benefit calculators used for real claims, exercising UC (elements, taper, deductions), legacy benefits, and council tax support with local variation.
• Turn2us: a third independent benefits implementation.

Each case result is a per-household oracle diff in exactly the #49 schema — the case battery, closed-schema validation, and adjudication contract (tolerance/annotation requirements just landed in 838cc45) are reused as-is; only the connector and the oracle registration differ.

Why this earns a lane

• Adjudication: PE vs UKMOD vs an official calculator turns two-way disagreements into attributable defects (whose rules encoding is wrong). Feeds the Mode-3 case schema + UK battery: UKMOD/EUROMOD lane groundwork #49 battery's expected_focus rationale directly.
• Coverage where UKMOD is weak: UC deductions and the benefit cap (the UK UC deductions: external scores staged (FRR, JRF floor, historical cap costings) #21/UC deductions: FRR family re-harvested from primary sources + DB ingest #52 family) are modeled by production calculators in operational detail; a case battery over the FRR deduction priority order would validate the exact machinery the UC-deductions scores depend on.
• The two-child pair: the 2025/2026 abolition-boundary cases from Mode-3 case schema + UK battery: UKMOD/EUROMOD lane groundwork #49 get a second, independent expected-value source.

Constraints to respect (why this is scoped, not scraped)

Manual case entry or documented API/terms-compliant access only, one case at a time, provenance per result (calculator version/date screenshotted or archived) — these are oracle readings, not a harvest. Case inputs stay within the #49 closed schema so every oracle answers the identical household.

Steps

• Register the oracle vocabulary (oracle id + version/date per result row) in the Mode-3 case schema + UK battery: UKMOD/EUROMOD lane groundwork #49 schema — one field, no shape change
• 10-case starter battery: HICBC taper, NICs at each band boundary, UC taper + work allowance, two-child pair, benefit-cap binding case — each entered into ≥2 calculators
• Adjudication writeups where the three sides split (the SCHEMA.md contract now requires them)

Depends on #49 (schema, merged fixes). Complements #41; instance of #5.