Sales Tax Calculator mechanism

Model scope

The core forward arithmetic is standard, and the reverse arithmetic is standard once pre_tax=total/(1+rate) is specified. NEEDS_REVIEW is required because the specification does not state the reverse formula, rate representation, currency-rounding policy, effective table date, locality scope, or taxable-item assumptions. The honest label is directionally appropriate but should replace 'as of bundle build' with an actual bundle date and identify whether rates are state-only or combined.

• The forward formulas tax=price*rate and total=price+tax are correct when rate is a decimal fraction and the entire price is taxable.
• The reverse formula is underspecified; the standard formula is pre_tax=total/(1+rate), with tax=total-pre_tax.
• The specification does not define currency rounding: whether tax is rounded before total, whether totals are rounded independently, or whether unrounded values are retained internally.
• A jurisdiction lookup must distinguish state-only rates from combined state/local rates and must apply the rate effective on a specified date.
• Taxability, exemptions, special product rates, caps, and destination-based sourcing are not defined; therefore the calculator must be labeled as a simple taxable-price estimator rather than a complete tax-compliance calculator.

Units and conventions

• Manual rates are entered as percentages, such as 7.25%, and converted to decimal fractions before multiplication: 7.25%=0.0725.
• Prices and totals are denominated in US dollars.
• Expected displayed currency values use ordinary nearest-cent rounding, with tax rounded first and total computed as price plus rounded tax.
• Lookup examples use the cited state base rate only; local sales taxes are excluded unless explicitly supplied.
• All prices in the examples are assumed fully taxable and no tax holidays, exemptions, special product rates, or tax caps apply.

Inspectable vector

One of 4 source-bound review vectors:

{
  "inputs": {
    "mode": "forward",
    "price_usd": 100,
    "rate_percent": 7.25,
    "jurisdiction": "California state base rate"
  },
  "expected_outputs": {
    "tax_usd": 7.25,
    "total_usd": 107.25
  },
  "reference_data": {
    "jurisdiction": "California",
    "rate_percent": 7.25,
    "rate_decimal": 0.0725,
    "source_convention": "widely tabulated standard value (California state base sales-tax rate; official state tax authority, 2025 reference)"
  },
  "working": "Convert the rate: 7.25/100=0.0725. Tax=100.00*0.0725=7.25. Total=100.00+7.25=107.25."
}

Failure boundary

BREAK_CLAIMED · SELF_FLAGGED

The reverse formula has an undefined input at tax_rate = -1.00 because its denominator becomes zero. The specification does not state validation behavior for this case, so an unguarded implementation would break or return an invalid numeric result. The manual rate interface also has a serious unit ambiguity when an entry such as 8 may mean 8 percent or decimal 8.0.

Other records

Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

• VERIFIED_LEDGER.json
• evidence-tranche7/tranche7-results.json
• BOUNDARY_HAZARD_CATALOG.json