Source: https://guthlabs.ai/docs/instruments/credit-card-payoff-planner/implementation/

instrument implementation / implementation-contract

# Credit Card Payoff Planner implementation

A source-bound implementation contract for Credit Card Payoff Planner; 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**: t4-056

**Effect**: Computation or source lookup only

## Input and validation requirements

The literal logarithmic payoff formula breaks at APR = 0 through a 0/0 expression, and the under-minimum-payment branch produces no finite payoff. The specification also leaves APR convention ambiguous, allowing materially different results.

## 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": {
    "balance": 1000,
    "apr": 0.12,
    "monthly_payment": 100
  },
  "expected_outputs": {
    "analytical_months_to_payoff": 10.5886,
    "scheduled_months_to_payoff": 11,
    "total_interest": 58.9849,
    "min_payment_trap_warning": false
  },
  "working": "Monthly rate: r = 0.12/12 = 0.01. Interest accrued each month initially: 1000 x 0.01 = 10, so P = 100 > 10. Analytical months: n = ln(100/(100 - 0.01 x 1000)) / ln(1.01) = ln(100/90) / ln(1.01) = 0.1053605/0.00995033 = 10.5886. After 10 full payments: B10 = 1000(1.01)^10 - 100[((1.01)^10 - 1)/0.01] = 58.4009. Final payment after one more month of interest: 58.4009 x 1.01 = 58.9849. Total paid = 10 x 100 + 58.9849 = 1058.9849. Total interest = 1058.9849 - 1000 = 58.9849."
}
````

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/credit-card-payoff-planner/>)[02**Mechanism**](<https://guthlabs.ai/docs/instruments/credit-card-payoff-planner/mechanism/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/credit-card-payoff-planner/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-tranche4/tranche4-results.json`