Guard county-keyed lookups in VT renter credit, MN CCAP, WI Shares and MD CCS

[MaxGhenis]

Problem

defined_for masks a formula's output, but policyengine-core 3.32.15 runs the formula over the whole population and skips it only when the mask is all false. A county key missing from a county-keyed parameter table therefore raises ParameterNotFoundError whenever the simulation also holds a household the table doesn't cover. This follows #10113 (the WA property tax exemption) and comes from the same 51-state probe.

Variable	Crashes on main when
vt_renter_credit	any non-VT household shares the simulation; a VT household has county UNKNOWN; a tax unit has 9+ people (the tables cover sizes 1–8), even a lone VT household
mn_ccap_provider_rate	an MN household has county UNKNOWN
wi_shares_hourly_max_rate, wi_shares_monthly_max_rate	a WI household has county UNKNOWN
md_ccs_payment_rate	an MD household has county UNKNOWN

Changes

• County guard. Each formula masks the county with the md_flat_rate_county_tax.py pattern, where(in_state & (county != "UNKNOWN"), county, "<FIRST>_COUNTY_<ST>"). MN keeps its existing Hennepin fallback. VT switches from the county enum to county_str to match the other guards; the keys are the same.
• The UNKNOWN fallbacks are placeholders, not estimates. For an in-state household with county UNKNOWN, VT, WI and MD use their first-listed county and MN uses Hennepin. The sources prescribe the actual county's rate and give no fallback, so these return an arbitrary in-state value instead of crashing. A household with no county input takes the variable's default county instead (for MN, Aitkin), which can differ: an MN infant in 45 hours a week of family care gets $888.33 a month with county omitted and $1,083.33 with UNKNOWN. No household in the default dataset has an unknown or mismatched county.
• Size cap before 2023. The calculators document the size-8 income-limit cap for TY2023–2025. This PR applies the same cap to 2021–2022; no contrary rule was found for those years.
• VT household size. A new parameter, gov.states.vt.tax.income.credits.renter.max_household_size = 8, caps the table lookup. Tax units of 9 or more use the 8-person income limits and fair market rent.

Why 8 for Vermont

• The department caps at 8. Its Renter Credit Website Calculators for TY2023, TY2024 and TY2025 (cited in the parameter files) share these Calculator-sheet formulas:
  • G11 and G12, the partial and full credit income limits: MATCH(TEXT(IF(Household_Size>8,8,Household_Size),0),...).
  • G13, the credit amount: IF(Household_Size>4,4,Household_Size). PE's fair_market_rent table already repeats the 4-person value through size 8, so capping at 8 is exact.
  • G5, family size, is validated to 1–10.
• The calculator reproduces the expectations. I recalculated the TY2025 calculator in LibreOffice for the test households. It gives 1,162 (Chittenden, size 2), 846 (Addison, size 2), 976 (Chittenden, size 7) and 1,250 (Chittenden, sizes 8, 9 and 10), exactly the expectations in the new tests.
• The statute is silent on size. 32 V.S.A. § 6061(19)–(20) defines the limits as HUD's, and § 6066(b)(2) charges the Commissioner with calculating the credit. The TY2025 booklet (RCC-146, file pages 37–38) prints family sizes 1–8 only.
• Alternative reading, not adopted. HUD does define larger households. Its FY 2024 income-limit methodology adds 8% of the 4-person limit per person over 8, and its FMR convention (68 Fed. Reg. 56705) adds 15% per bedroom over 4. A literal reading of § 6061 could import those values. The department caps instead, and PE's tables already transcribe its 4-bedroom cap, so this PR follows the department.

Tests

New vectorized YAML cases put an in-state household, an in-state UNKNOWN-county household and (for the crash guards) an out-of-state household in one simulation, so the mask is not all false:

• vt_renter_credit.yaml test 11: Chittenden, New Hampshire, and VT UNKNOWN (falls back to Addison).
• vt_renter_credit.yaml test 12: VT tax units of 8, 9 and 12.
• mn_ccap_provider_rate.yaml Case 13, wi_shares_hourly_max_rate.yaml Case 9, wi_shares_monthly_max_rate.yaml Case 6, and the new md_ccs_payment_rate.yaml informal-provider case.

Fails on main, passes here. With the five formulas restored from upstream/main and the new tests kept, the six new cases fail with ParameterNotFoundError (keys …thirty_percent.9, the out-of-state VT key, …weekly.CHILD_CARE_CENTER.UNKNOWN, …licensed_group.hourly.UNKNOWN ×2, …informal.rates.UNKNOWN). The 44 existing cases in those files pass. With the fix, all 50 pass.

Broader runs. All VT, MN, WI and MD baseline YAML tests plus household/state_property_tax_credits pass (1,920). code_health/test_parameter_yaml_keys.py, code_health/test_reference_urls.py and test_md_ccs_service_unit_enum_default.py pass (69).

51-state probe (2026). Five multi-household simulations: 51 states with real counties; the same plus UNKNOWN in every state; plus VT tax units of 9 and 12; a lone VT unit of 9; and every county of VT (14), MN (87), WI (72) and MD (24) plus UNKNOWN and out-of-state. Every scenario raised for at least one variable on main; all five variables compute in every scenario here. Script and outputs: ~/reviews/county-lookup-guards-2026-10/evidence/.

Invariants

The probe runs every county of the four states and checks that each variable computes. It does not itself assert main/head equality, size-cap equality, out-of-state zeros or calculator agreement; the YAML cases assert representative values for those:

• No crash. Each variable computes for any mix of households whose state and county agree, including UNKNOWN. (A VT household paired with an NH county still raises.)
• No change where main computed. In-state households with a real county use the same table key as before; VT sizes 1–8 use the same size key.
• Size cap. The VT credit for any size n ≥ 8 equals the size-8 credit.
• Out of state is zero. Out-of-state output is 0 (defined_for).
• Differential. The VT credit equals the department calculator for sizes 2, 7, 8, 9 and 10.

No Python test is added, per the repo's YAML-first rule; the YAML cases cover each path.

Hub microsimulation impact (current head)

A real microsimulation on the default dataset, main at the merge base 065d0e9560 against this PR's head 8fde198e14, for 2025 and 2026. The runs went one at a time under the shared heavy-job lock. Sums are weighted.

Output	2025 change	2026 change	2025 records changed (weighted)	2026 records changed (weighted)
state_income_tax	+0	+0	0 (0)	0 (0)
income_tax	+0	+0	0 (0)	0 (0)
household_net_income	+0	+0	0 (0)	0 (0)
household_benefits	+0	+0	0 (0)	0 (0)

Run on the standard outputs only: on main, requesting vt_renter_credit or the MN/WI/MD child-care rate variables across the whole population crashes (the crash this PR fixes), so a base run that names them cannot complete. No record changes in either year.

Comparison files are in fixes/10116-impact/ on the hub host.

Impact

Only cases that crashed on main get new values. In the default dataset every household has a real county matching its state. vt_renter_credit's only consumer is taxsim_state_property_tax_credit, through gov.states.household.state_property_tax_credits.

axiom: us-vt/statute/32/6066 TheAxiomFoundation/rulespec-us#1678 queued (corpus ingest TheAxiomFoundation/axiom-corpus#816; the rulespec-us corpus pin predates the VT § 6061/§ 6066 rows). The county guards are n/a: pure crash guards.

Follow-up (not in this PR)

The TY2025 booklet prints Washington County's 7-person partial-credit limit as $88,880; the calculator has $88,990. PE uses the booklet value.

🤖 Generated with Claude Code

[MaxGhenis]

The CI run on this pull request was cancelled at 23:27Z on 2026-10-11 to free the organization's shared GitHub runners for an urgent fix in another repository. The US + core hub will re-run it automatically (cancelled jobs only; jobs that had passed are kept). Nothing in this pull request changed. Please don't push or reopen just to restart CI.