[MaxGhenis]

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

Bug

ct_covered_connecticut_eligible compared the truncated aca_magi_fraction (np.floor(100 * magi / fpg) / 100) with <= 1.75. Truncation maps every income from 175.00% to 175.99% of the poverty line to 1.75, so all of them passed. C.G.S. § 19a-754c(b)(1)(C)(iv) (from June Sp. Sess. P.A. 21-2, § 16) covers adults who "have household income up to one hundred seventy-five per cent of the federal poverty level", and DSS applies it to "household income up to an including 175% of the Federal Poverty Level". No source prescribes truncating the percentage.

Executed on main (645fcd3; the file is unchanged at 740e1eb), single filer in Connecticut, 2026, 2025 one-person guideline $15,650, wages only (nothing supplied):

Income	Exact % of FPL	aca_magi_fraction	Eligible on main	Eligible here
$27,387.50	175.000%	1.75	yes	yes
$27,387.97	175.003%	1.75	yes	no
$27,543.06	175.994%	1.75	yes	no
$27,544.00	176.000%	1.76	no	no

Fix

Decide eligibility on the exact ratio max(aca_magi, 0) / tax_unit_fpg(period.last_year), the pattern of #10305. Nothing else in Covered Connecticut reads the fraction. The variable now also cites the codified statute.

Invariants

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

1. Band = statute. Eligible if and only if r <= 1.75, r = max(MAGI, 0) / FPG(prior year).
2. Only the sliver changes. Relative to main, eligibility can only be lost, and only for r in (1.75, 1.76), so ct_covered_connecticut can only fall. Deduced: floor(100r)/100 <= 1.75 holds exactly when r < 1.76.

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. The repo's test-design rules keep such cases in YAML.

Tests

• Every case in the three Covered Connecticut 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. Case 10, which supplied 1.7501 (a value truncation never produces), now supplies $17,501 against $10,000 and exercises the fix.
• New income-derived cases with the real guideline: $27,386.56 (174.994%) and $27,387.50 (175%) eligible; $27,387.97 (175.003%) and $27,543.06 (175.994%) ineligible.
• Mutation check: main's gate fails exactly Cases 10, 10c and 10d.
• 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 76 mismatches, all in the truncation sliver.
• Local run: the Connecticut DSS YAML folder, 245 passed.

Impact

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

Coordination

#10041 (draft) edits ct_covered_connecticut.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. Part of the aca_magi_fraction audit (table: #10329) begun with #10305; federal edge in #10312.

axiom: pending

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