HSA Triple-Tax-Advantage Calculator mechanism

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

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