Source: https://guthlabs.ai/docs/instruments/hsa-triple-tax-advantage-calculator/mechanism/
instrument mechanism / verified-with-binding-errata
# HSA Triple-Tax-Advantage Calculator mechanism
The inspectable method record for HSA Triple-Tax-Advantage Calculator, kept separate from implementation and qualification.
Status: **VERIFIED_WITH_ERRATA**Lifecycle: **verified-with-binding-errata**
## Model scope
The conceptual comparison is reasonable, but the instrument cannot be formula-verified as specified because the actual equations, contribution timing, taxable-account tax timing, and definition of tax savings are missing. The four vectors are valid only under the explicit simplified assumptions listed above. The honest educational-only label is appropriate but should also state that results depend on eligibility, contribution limits, qualified-use status, tax jurisdiction, and tax-law changes.
- The specification does not provide explicit equations for HSA future value or taxable-account future value.
- The contribution timing is unspecified; the test calculations assume equal contributions made at the end of each year.
- The taxable-account treatment is ambiguous: the calculations assume the taxable account receives the after-marginal-tax contribution, with capital-gains tax paid once at the end on total gains. This is not equivalent to an annual tax-drag model.
- Tax savings is undefined. The test calculations define it as HSA balance minus taxable-account balance, which is a balance advantage and not necessarily literal tax savings.
- HSA contributions are not universally tax-free: tax treatment depends on eligibility, contribution deductibility, payroll treatment, state rules, and whether withdrawals are for qualified medical expenses.
- The marginal tax rate is not necessarily the applicable rate for every contribution or withdrawal, and the model does not specify whether payroll taxes or state taxes are included.
## Units and conventions
- annual_contribution is a gross end-of-year contribution in currency units.
- years is a nonnegative integer.
- expected_return, marginal_tax_rate, and capital_gains_rate are decimal rates, so 6% is entered as 0.06, not 6.
- The assumed HSA formula is HSA_FV = C × ((1 + r)^n - 1) / r when r is not zero; for r = 0, HSA_FV = C × n.
- The taxable deposit is assumed to be C × (1 - marginal_tax_rate).
- The taxable pre-tax account value is taxable_deposit × ((1 + r)^n - 1) / r, with taxable_deposit × n when r = 0.
## Inspectable vector
One of 4 source-bound review vectors:
````
{
"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."
}
````
## Failure boundary
**BREAK_CLAIMED** · SELF_FLAGGED
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.
## Other records
[01**Overview**]()[03**Implementation**]()[04**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`
- `BOUNDARY_HAZARD_CATALOG.json`