Source: https://guthlabs.ai/docs/instruments/sales-tax-calculator/implementation/

instrument implementation / implementation-contract

# Sales Tax Calculator implementation

A source-bound implementation contract for Sales Tax Calculator; deployment and executable qualification remain separate.

Status: **VERIFIED_CLEAN**Lifecycle: **implementation-contract**

## Binding contract

This record describes what an executable surface must bind and validate. It does not claim that a public calculator, package, or production route is deployed.

**Record owner**: Guth Instruments

**Lifecycle**: verified-clean

**Evidence ID**: t7-037

**Effect**: Computation or source lookup only

## Input and validation requirements

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.

## Required corrections

No binding erratum is attached to this unique-name record. Implementers must still preserve the declared units, domain, and failure behavior.

## Inspectable interface fixture

The sample below is evidence for interface and validation tests, not a production implementation.

````
{
  "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."
}
````

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/sales-tax-calculator/>)[02**Mechanism**](<https://guthlabs.ai/docs/instruments/sales-tax-calculator/mechanism/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/sales-tax-calculator/qualification/>)

## 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`