Source: https://guthlabs.ai/docs/instruments/refinance-break-even/implementation/

instrument implementation / implementation-contract

# Refinance Break-Even implementation

A source-bound implementation contract for Refinance Break-Even; 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**: t4-020

**Effect**: Computation or source lookup only

## Input and validation requirements

The instrument produces an undefined breakeven when old and new payments are equal because the formula divides by zero. It also needs explicit handling of the zero-interest payment branch and an explicit savings sign convention.

## 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": {
    "old_monthly_payment_usd": 2400,
    "new_monthly_payment_usd": 2100,
    "closing_costs_usd": 6000,
    "remaining_horizon_months": 120
  },
  "expected_outputs": {
    "monthly_savings_usd": 300,
    "breakeven_months": 20,
    "lifetime_net_savings_usd": 30000
  },
  "working": "Monthly savings = 2,400 - 2,100 = 300 USD/month. Breakeven = 6,000 / 300 = 20 months. Gross payment savings over 120 months = 300 × 120 = 36,000 USD. Net lifetime savings = 36,000 - 6,000 = 30,000 USD."
}
````

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/refinance-break-even/>)[02**Mechanism**](<https://guthlabs.ai/docs/instruments/refinance-break-even/mechanism/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/refinance-break-even/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`