[MaxGhenis]

Draft. Done: the fix and YAML cases. Pending: full-file impact, Axiom parity line, GPT-6.1 Sol review.

Bug

co_premium_assistance_eligible compared the truncated aca_magi_fraction (np.floor(100 * magi / fpg) / 100) with <= fpl_limit.upper (4.0). Truncation maps every income from 400.00% to 400.99% of the poverty line to 4.00, so all of them passed. Amended Regulation 4-2-78 (3 CCR 702-4), Section 4.C defines an "eligible enrollee" as one "whose household income is no greater than 400% of the Federal Poverty Level". (Section 5.B's heading reads "Households Below 400% FPL"; the definition, not the heading, sets the test.) No source prescribes truncating the percentage.

Under current law the gate also requires is_aca_ptc_eligible, whose income test ends at 400% (2026 onward), so incomes above 400% already failed end to end. The Colorado ceiling decides alone when a reform keeps the federal credit above 400%, such as extending the enhanced credit.

Executed on main (645fcd3; the file is unchanged at 740e1eb), single filer in Colorado, 2026, 2025 one-person guideline $15,650:

Income	Exact % of FPL	aca_magi_fraction	Gate alone on main (is_aca_ptc_eligible supplied)	Here
$62,600	400.000%	4.00	yes	yes
$62,600.47	400.003%	4.00	yes	no
$62,755.56	400.994%	4.00	yes	no
$62,756.50	401.000%	4.01	no	no

Fix

Decide the ceiling on the exact ratio max(aca_magi, 0) / tax_unit_fpg(period.last_year), the pattern of #10305. Nothing else in Colorado Premium Assistance reads the fraction.

Invariants

For every input, with the program in effect and an APTC-eligible member:

1. Band = regulation. Eligible if and only if r <= 4, r = max(MAGI, 0) / FPG(prior year); no floor.
2. Only the sliver changes. Relative to main, eligibility can only be lost, and only for r in (4.00, 4.01). Under current law that set is already federally ineligible, so baseline results should not move; the impact run is the check.

No Hypothesis test is added: the gate is one comparison on a ratio, and the YAML cases hit the limit, 0.003 and 0.994 points above it, plus a reform case. The repo's test-design rules keep such cases in YAML.

Tests

• Every case in the three Colorado Premium Assistance YAML files that supplied aca_magi_fraction now supplies aca_magi against a prior-year tax_unit_fpg of $10,000, so it still drives the gate.
• New income-derived cases with the real guideline: $62,599 (399.994%) and $62,600 (400%) eligible; $62,600.47 (400.003%) and $62,755.56 (400.994%) ineligible.
• Case 17 sets gov.aca.ptc_income_eligibility[2].amount: true (federal credit above 400%) and computes is_aca_ptc_eligible: at $62,600.47 the household is federally eligible and Colorado-ineligible.
• Mutation check: main's gate fails exactly Cases 15, 16 and 17.
• Local differential sweep (a script, not committed, per the repo's test-design rules): 256 tax units per year, 60% with income in cents within $2 of an edge and the rest uniform, guidelines $10,000-$80,000 including $20 multiples, comparing the eligibility gate with an exact-rational reading of the source on the float32 values the model receives. 0 mismatches here; on main the same draws give 83 mismatches, all in the truncation sliver.
• Local run: the Colorado Premium Assistance YAML folder, 53 passed.

Impact

Pending: full-file microsimulation via the US hub impact queue, 2026 and 2027.

Coordination

#10041 (draft) edits co_premium_assistance.yaml and changes aca_magi_fraction's denominator to a new aca_fpg; if it lands, this gate's exact ratio should use the same denominator. #10312 makes exactly 400% federally eligible; the two are independent (the new cases here supply is_aca_ptc_eligible or sit above 400%). Part of the aca_magi_fraction audit (table: #10329) begun with #10305.

axiom: pending

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