Source: https://guthlabs.ai/docs/instruments/lease-vs-buy-comparator/implementation/

instrument implementation / implementation-contract

# Lease vs Buy Comparator implementation

A source-bound implementation contract for Lease vs Buy Comparator; 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**: t5-146

**Effect**: Computation or source lookup only

## Input and validation requirements

The valid-looking discount-rate input r=-1 creates zero discount denominators and undefined NPVs unless explicitly rejected. A separate resale-value sign ambiguity can also reverse the recommendation if the interface does not specify whether resale is entered as positive proceeds or as a signed cash flow.

## 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": {
    "purchase_price": 30000,
    "buy_payments": [
      10000,
      10000,
      10000
    ],
    "buy_maintenance": [
      1000,
      2100,
      3200
    ],
    "resale_value": 13310,
    "lease_payments": [
      8000,
      8000,
      8000
    ],
    "discount_rate_per_period": 0.1,
    "periods": 3,
    "period_unit": "year"
  },
  "expected_outputs": {
    "buy_NPV": 19917.3554,
    "lease_NPV": 19894.8159,
    "lower_NPV": "lease"
  },
  "working": "Buy: t1=(10000+1000)/1.10=11000/1.10=10000.0000; t2=(10000+2100)/1.10^2=12100/1.21=10000.0000; t3 operating cash flow=(10000+3200)/1.10^3=13200/1.331=9917.3554; resale PV=13310/1.331=10000.0000; buy_NPV=10000.0000+10000.0000+9917.3554-10000.0000=19917.3554. Lease: 8000/1.10=7272.7273; 8000/1.21=6611.5702; 8000/1.331=6010.5184; lease_NPV=7272.7273+6611.5702+6010.5184=19894.8159."
}
````

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/lease-vs-buy-comparator/>)[02**Mechanism**](<https://guthlabs.ai/docs/instruments/lease-vs-buy-comparator/mechanism/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/lease-vs-buy-comparator/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-tranche5/tranche5-results.json`