Methodology · Divergences
Where we differ from the JCC-certified reference, and why.
Every non-"ok" fixture in our parity report is attributed to a specific root cause. This page is the catalog of root causes — the residuals — with statute citations, IRS authority cross-checks, and per-residual evidence. Each residual has a stable anchor so attribution columns can link directly here.
Our calculator's authority footing is independent of the reference: we match the AOC §4055 oracle 154/154 on the cartesian-y2025 corpus (AOC was decertified for OBBBA tax-data reasons, but its §4055 implementation is still current law and is our regression check), and our §4059 net-disposable-income pipeline matches IRS Rev. Proc. 2024-40 + OBBBA §70101 standard deduction + CA FTB Schedule X exactly to the cent across simple-wage single-filer scenarios at $5k / $6k / $8k / $12k monthly gross. Our complex tax modules (AMT §55, NIIT §1411, SE §1401, QBI §199A, Additional Medicare §3101(b)(2)) match IRS authority on 12 of 12 probed scenarios. Federal credit stacking (CTC, ACTC, EITC, HoH std) matches IRS Schedule 8812 + Pub 596 + OBBBA §70205. CA refundable credits (CalEITC, YCTC, FYTC) match the FTB Form 3514 Booklet 2025.
Current corpus parity: 120/140 (86%) within 1%, 20/140 (14%) within 1–5%, no fixtures over 5%. Max single-fixture Δ is 3.0%.
Active divergences
| # | Description | Status | Magnitude |
|---|---|---|---|
| 1 | OBBBA §70203 qualified-vehicle-loan interest not implemented in reference | reference gap | $6–$42/mo on 3 obbba-auto-* fixtures (0.8–2.5%) |
| 2 | Itemizer CA-state-tax in §4059 net — we follow §4059(a)(1) literal, reference follows Schedule-A coupling | interpretation choice (ours) | ~$20–$50/mo on itm-* and high-SALT obbba-salt-* fixtures |
Residual #1 — §70203 auto-loan-interest deduction
reference gap · OBBBA not yet implemented
OBBBA §70203 (qualified passenger vehicle loan interest, above-the-line deduction up to $10,000/year with $100k/$200k AGI phase-out) reduces federal tax for the parent paying support. Lower federal tax → smaller §4059(a)(1) deduction → higher net disposable income → higher CS.
We verified that the reference calculator does not implement §70203 at
all: DOM-walking every accordion section on the reference's calculator
page yields no field labeled auto, vehicle, car, loan interest, or any OBBBA/§70203
wording, and the reference's audit trail contains no §70203 line on
captured obbba-auto fixtures. Affected fixtures show the expected
direction (ours higher than reference by 0.8–2.5%):
| fixture | ours | reference | Δ |
|---|---|---|---|
obbba-auto-k1-n5000-auto200-o3000-t20 | $784 | $778 | +$6 (0.8%) |
obbba-auto-k1-n9000-auto750-phaseout-o4000-t20 | $1,343 | $1,321 | +$22 (1.6%) |
obbba-auto-k2-n7000-auto500-o4000-t20 | $1,684 | $1,642 | +$42 (2.5%) |
Our implementation lives in shared/tax/federal/obbba-deductions.ts and follows PL 119-21 §70203 verbatim. Not actionable on our side; will
close when the reference implements §70203.
Residual #2 — itemizer state-tax-in-§4059-net
interpretation choice (ours)
For itemizing filers, our calculator subtracts CA state tax from net per §4059(a)(1) ("the state and federal income tax liability") regardless of whether the federal deduction came from the standard or itemized path. The reference suppresses CA state tax from §4059 net for itemizers (matching the legacy DCSS / pre-OBBBA AOC convention) on the rationale that SALT (§164(b)(6) as amended by OBBBA §70120) already reduced federal taxable income via Schedule A. Both readings are defensible. Ours is literal-text; the reference's is Schedule-A-coupled. Gap is ~$20–$50/mo on itm-* and high-SALT obbba-salt-* fixtures, scaling with CA state tax. AOC matches us on these scenarios.
Authority backing summary
- AOC §4055 oracle: 154/154 cartesian-y2025 fixtures match ours exactly.
- IRS Rev. Proc. 2024-40 + OBBBA §70101: federal tax + std deduction match to the cent on simple-wage scenarios.
- CA FTB Schedule X: state tax matches to the cent.
- IRS Topic 751 / Schedule SE / Form 6251 / Form 8960: FICA / SE / AMT / NIIT match to the cent.
- Treasury Reg §1.199A: QBI deduction matches authority.
- IRS Schedule 8812 + Pub 596: CTC / ACTC / EITC stacking matches authority.
- FTB Form 3514 Booklet 2025: CalEITC / YCTC / FYTC parameters verified.
- IRS Notice 2025-68: §530A "Trump Account" non-deductibility confirmed.