Source: https://guthlabs.ai/docs/instruments/student-loan-payoff-idr-estimate/implementation/

instrument implementation / implementation-contract

# Student Loan Payoff & IDR Estimate implementation

A source-bound implementation contract for Student Loan Payoff & IDR Estimate; 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**: t7-007

**Effect**: Computation or source lookup only

## Input and validation requirements

The literal standard amortization formula is undefined at a zero interest rate because its denominator becomes zero, and the simplified IDR formula has unspecified behavior for zero balances and negative discretionary income.

## 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_usd": 0,
    "annual_rate": 0.05,
    "term_months": 120,
    "annual_agi_usd": 20000,
    "family_size": 1,
    "plan_type": "simplified_10pct_150pct_poverty",
    "idr_percent": 0.1,
    "poverty_multiplier": 1.5
  },
  "expected_outputs": {
    "standard_monthly_payment_usd": 0,
    "estimated_idr_monthly_payment_usd": 0,
    "standard_payoff_time_months": 0
  },
  "reference_data": {
    "hhs_2024_poverty_guideline_usd_family_1": 15060,
    "source_convention": "HHS 2024 Poverty Guidelines, contiguous 48 states and District of Columbia"
  },
  "working": "Monthly rate = 0.05 / 12 = 0.0041666667. Standard payment = 0 × 0.0041666667 / [1 − (1.0041666667)^(-120)] = 0.00. Poverty threshold = 15060 × 1.5 = 22590. Annual discretionary income = max(0, 20000 − 22590) = 0. IDR payment = 0 × 0.10 / 12 = 0.00. With zero balance, payoff time = 0 months."
}
````

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/student-loan-payoff-idr-estimate/>)[02**Mechanism**](<https://guthlabs.ai/docs/instruments/student-loan-payoff-idr-estimate/mechanism/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/student-loan-payoff-idr-estimate/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`