# Invoice math contract v1.0

This is an explicit software-testing contract, not a national tax rulebook.

The input is only `invoice` plus this contract. Do not feed expected labels, family, mutation or provenance to a model being evaluated. A challenge file strips these fields. `country` is research metadata; currency precision determines calculation.

1. Currency precision: SAR/AED/QAR two decimals; KWD/BHD/OMR three. Precision facts checked against SIX on 2026-09-30. These are the benchmark's arithmetic precision, not a statement that every e-invoice standard permits that precision in every field.
2. All quantities/prices/allowances/charges/rates are nonnegative; quantity > 0. Rate is an explicitly supplied fraction from 0 to 1; no jurisdiction-specific tax rate is inferred. Negative line net is forbidden.
3. Line net = round-half-up(quantity × unit_price − allowance + charge, currency precision).
4. Line tax = round-half-up(line net × rate, currency precision). Tax is rounded per line, NOT on an invoice group or subtotal. This choice is part of the synthetic contract and may differ from a real tax standard.
5. Net = sum(line net). Tax = sum(line tax). Gross = net + tax. Due = gross − prepaid. Prepaid must be an exact nonnegative currency amount no greater than gross.
6. Credit notes contain positive line magnitudes and zero prepaid. Negate the final net/tax/gross/due for credit notes. This sign convention is explicitly synthetic.
7. Output all four canonical ASCII decimal strings with exactly currency precision. `valid` means all claimed totals exactly equal these strings numerically. `error_fields` lists mismatched claimed fields in net,tax,gross,due order. A wrong net claim does not cascade into the expected gross: recompute each from original lines.
8. Numeric styles are ASCII with dot decimal, Arabic-Indic digits with U+066B decimal, and either with three-digit grouping (comma or U+066C). In grouped style the leading group is one to three digits and later groups exactly three. Minus is ASCII U+002D. No mixed digit alphabets, whitespace, bidi controls, exponent notation, currency symbols, or NaN/Infinity. These excluded parser cases are tested by controls, not claimed as coverage families in the dataset.
9. Ten designed families cover ties, per-line rounding, allowance, charge, prepayment, credit note, large precision stress, zero rate, mixed rates and zero due. Large-value stress is intentionally unrealistic to reveal binary floating-point errors. It is not an estimate of GCC invoice values.
10. Every positive/negative pair differs in one claimed total by one minor unit. Keep pairs together in splits. There are five parameter variations per currency/family, not 600 independent business processes. No dialect or natural-language authenticity claim.

The benchmark excludes XML/UBL schemas, signing, QR codes, identification, real tax semantics, regulatory certification, OCR, exchange rates, and payment reconciliation.