HSA Triple-Tax-Advantage Calculator implementation

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-with-binding-errata

Evidence ID

t5-211

Effect

Computation or source lookup only

Input and validation requirements

The zero-return input makes the ordinary annuity expression 0/0 and therefore undefined unless a special limiting-case branch is implemented. The specification also does not state contribution timing or how losses are treated, creating additional result-changing hazards.

Required corrections

[
  {
    "vector": 4,
    "field": "hsa_balance/taxable/tax_savings",
    "wrong": "11000 / 8463 / 2537",
    "correct": "10000 / 7800 / 2200 under her own v1-v3 end-of-year convention",
    "cause": "v4 switched to annuity-due timing, contradicting v1-v3"
  }
]

Inspectable interface fixture

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

{
  "inputs": {
    "annual_contribution": 4000,
    "years": 10,
    "expected_return": 0.06,
    "marginal_tax_rate": 0.24,
    "capital_gains_rate": 0.15
  },
  "expected_outputs": {
    "hsa_balance": 52723.18,
    "taxable_account_balance": 38619.17,
    "tax_savings": 14104.01
  },
  "working": "Annuity factor = ((1.06^10) - 1) / 0.06 = (1.7908476965 - 1) / 0.06 = 13.1807949417. HSA balance = 4000 × 13.1807949417 = 52723.18. Taxable deposit = 4000 × (1 - 0.24) = 3040. Taxable pre-tax value = 3040 × 13.1807949417 = 40069.62. Taxable gain = 40069.62 - (3040 × 10) = 40069.62 - 30400 = 9669.62. Capital-gains tax = 9669.62 × 0.15 = 1450.44. Taxable balance = 40069.62 - 1450.44 = 38619.17. Tax savings/balance advantage = 52723.18 - 38619.17 = 14104.01."
}

Other records

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
• tranche errata record