Add Washington premium assistance (Cascade Care Savings)

[DTrim99]

Washington Cascade Care Savings (state premium assistance)

Summary

Closes #9222.

Adds Washington's Cascade Care Savings (CCS) program — the state premium assistance program administered by the Washington Health Benefit Exchange (WAHBE) under RCW 43.71.110. CCS pays a flat per-member-per-month (PMPM) subsidy toward marketplace premiums, applied after federal APTC, for enrollees with ACA MAGI up to 250% of the federal poverty level. This PR encodes the enacted plan year 2026 amounts (effective January 1, 2026), following the CO/NM/MD state premium assistance pattern.

New variables: wa_cascade_care_savings (TaxUnit, added to healthcare_benefit_value), wa_cascade_care_savings_eligible, and person-level wa_cascade_care_savings_member_eligible / wa_cascade_care_savings_member_amount.

Survey correction

The issue's survey figures were wrong. Issue #9222 reported "$55/month per adult, $30/month per child." There is no age split — the PY2026 maximum PMPM amounts are split by federal-subsidy status:

Group	Definition	2025 max PMPM	2026 max PMPM
Group 1	Receives federal subsidies (APTC)	$155	$55
Groups 2/3	No federal subsidies (incl. 1332-waiver immigrants)	$250	$250

Verified against WAHBE's final PMPM release (page 1: "Customers with federal subsidies: $55 per member, per month (PMPM)"; "Customers without federal subsidies: $250 PMPM"; 2025-vs-2026 exhibit on page 7). No $30 child amount exists anywhere in the enacted policy, and none is encoded here.

Regulatory authority

Source	What it establishes
RCW 43.71.110	Establishes the premium assistance program; WAHBE sets amounts annually within the omnibus appropriations act
WAHBE Final PY2026 CCS Maximum PMPM Methodology (Sept. 30, 2025)	Enacted $55/$250 amounts (#page=1, exhibit #page=7); Group 1/2/3 definitions (#page=5); county lowest-cost Cascade Silver cap (#page=6)
WAHBE PY2026 Final Cascade Care Savings Policy	Eligibility §4 (#page=8), amount methodology §5 incl. benchmark premium expectation (#page=11), APTC-first ordering and general-welfare (non-taxable) treatment (#page=12)

All #page= anchors were verified against locally extracted PDF text. Note the policy PDF is a redlined final draft with dual-numbering artifacts (e.g., "14.15." for definition 15, "20265" for 2026); citations use the final numbering.

Eligibility

A tax unit is eligible when the program is in effect, ACA MAGI is at or below 250% FPL (aca_magi_fraction <= 2.5, per 26 U.S.C. §36B(d)(2) at the tax-filing-household level; Policy §4(1)(c), §4(11)), and at least one member falls in a modeled group:

• Group 1 (federally subsidized): is_aca_ptc_eligible — this single gate embeds the §4(1)(e)-(g) and §4(2)(a) tests (accepts all APTC; ineligible for MEC via Medicaid/Medicare; no other affordable MEC via ESHI), mirroring the CO premium assistance pattern.
• Group 3 (1332-waiver population): immigration_status == UNDOCUMENTED and not wa_apple_health_expansion_eligible — the Apple Health Expansion exclusion implements §4(1)(f) and prevents double-counting with the existing Medicaid-cost proxy (Expansion covers undocumented adults up to 138% FPL, so modeled Group 3 is the undocumented 138–250% FPL population).
• Group 2 (lawfully present but APTC-ineligible) is not separately identifiable in the model — it requires filing-behavior and sub-100%-FPL APTC-denial facts the model does not represent. Documented in the variable docstrings; see Not modeled.

All four variables use defined_for = StateCode.WA.

Benefit calculation

Per Policy §5(1) (#page=11), with APTC applied first (§5(3), #page=12):

1. Per-member PMPM build-up: each eligible member contributes $55 (with federal subsidy) or $250 (without); the household base is the sum, annualized ×12.
2. Benchmark premium expectation (new for PY2026, §5(1)(e)): a per-member monthly minimum contribution of $0 (≤150% FPL), $10 (>150–200%), or $15 (>200% FPL), encoded as a single-amount bracket over aca_magi_fraction starting 2026-01-01. It enters only the cap, never the base amount.
3. Cap: min_(12 × PMPM sum, max_(0, slcsp_annual − aca_ptc − 12 × Σ per-member expectation)), floored at zero.

Documented approximations:

• SLCSP dual-proxy: county-level plan premiums are unavailable, so SLCSP proxies both the enrolled plan (cap i) and the county lowest-cost Cascade Silver plan (cap ii). Under one proxy, cap (ii) always binds and the statute's two caps collapse to the single expression above. SLCSP ≥ lowest-cost Silver by construction, so the cap is slightly generous on that margin. Wakely confirms the cap is "the lowest cost silver Cascade Care rate in a county" (final PMPM #page=6).
• Cascade-plan takeup: enrollment in a standard Cascade Silver/Gold plan is unobservable; every otherwise-eligible marketplace enrollee is treated as enrolled (see Not modeled).
• Full-year enrollment (×12); per-person lowest-cost Silver net premiums computed at household level.

The benefit is not taxable (general welfare exclusion, §5(5)) and involves no reconciliation — it is a monthly premium payment to issuers, not a tax credit (§7). in_effect turns on 2026-01-01 with no end date: RCW 43.71.110 is permanent law funded by annual appropriations ($55M for PY2026, $5M reserved contingent on the 1332 waiver), so PY2027+ carries the PY2026 amounts forward as the current-law assumption.

Requirements coverage

43/43 tests pass across four files (wa_cascade_care_savings_eligible.yaml 10, wa_cascade_care_savings.yaml 17, integration.yaml 9, household/healthcare_benefit_value.yaml 7).

Requirement	Citation	Status
REQ-1 WA residency	Policy §4(1)(a); RCW 43.71.110(4)	defined_for = StateCode.WA; non-WA tests
REQ-2 Income ≤ 250% FPL (ACA MAGI)	Policy §4(1)(c), §4(11)	fpl_limit = 2.5; 2.49/2.5/2.51 boundary tests
REQ-3 Group 1 gate	Policy §4(1)(e)-(g), §4(2)(a)	is_aca_ptc_eligible; tested
REQ-4 Group 3 gate (1332 waiver)	Policy §4(1)(b)	immigration_status == UNDOCUMENTED; tested
REQ-5 Apple Health Expansion exclusion	Policy §4(1)(f)	~wa_apple_health_expansion_eligible; overlap tests
REQ-6 Cascade-plan enrollment	Policy §3 def. 14/15, §4(13)	Takeup approximation, documented
REQ-7 §4(2)(b)-(c) filing-behavior exclusions	Policy §4(2)	Documented as not modeled
REQ-8 Tax-unit eligibility variable	—	wa_cascade_care_savings_eligible; tested
REQ-9 $55 / $250 PMPM parameters	Final PMPM p.1, p.7	2026-01-01, monthly; tested
REQ-10 Per-member build-up ×12	Policy §5(1)(c)	Mixed-household tests
REQ-11/13 Net-premium cap (SLCSP proxy)	Policy §5(1)(d); final PMPM p.6	Cap-binds, cap=0, floor-order, tie tests
REQ-12 Benchmark expectation $0/$10/$15	Policy §5(1)(e)	Bracket parameter; 150%/200% boundary tests
REQ-14 healthcare_benefit_value wiring; not taxable	Policy §5(5)	In adds; wiring test
REQ-15 No reconciliation	Policy §7	Documented
REQ-20 in_effect 2026-01-01, open-ended	Policy §2; RCW 43.71.110	2025 off-year and 2027 carry-forward tests
REQ-22 Tests + programs.yaml + changelog	—	wa_cascade_care_savings entry, verified_start_year: 2026

REQ-21 (2025 backdate: $155/$250, no benchmark expectation, enrollment-freeze caveat) was deliberately not taken in this PR.

Not modeled

• REQ-6 Cascade Silver/Gold standard-plan enrollment: plan choice is unobservable; all otherwise-eligible QHP enrollees are treated as Cascade-enrolled (takeup approximation).
• REQ-7 §4(2)(b)-(c) exclusions for non-intent-to-file and failure-to-reconcile: filing-behavior facts not represented in the model.
• REQ-16 COFA Islander Premium Assistance ineligibility test: no COFA variable exists; population is small.
• REQ-17 Takeup counts: Wakely's best estimate is ~72,240 Group 1 + ~8,090 Group 2/3 recipients vs ~248,290 total Exchange enrollment, so the model overstates participation absent a microdata takeup adjustment.
• REQ-18 Enrollment mechanics: SEPs, partial-month proration, the up-to-10% reserve, and the §11 low-funds contingency (activated PY2025, assumed off for PY2026).
• REQ-19 Group 2 (lawfully present, APTC-ineligible): not separately identifiable; would use the same $250 parameter as Group 3 if it becomes identifiable.

Files added

changelog.d/
└── wa-premium-assistance.added.md
policyengine_us/parameters/gov/states/wa/wahbe/cascade_care_savings/
├── amount/
│   ├── with_federal_subsidy.yaml       # $55 PMPM (2026)
│   └── without_federal_subsidy.yaml    # $250 PMPM (2026)
├── benchmark_expectation.yaml          # $0 / $10 / $15 by FPL band
├── fpl_limit.yaml                      # 2.5
└── in_effect.yaml                      # true from 2026-01-01
policyengine_us/variables/gov/states/wa/wahbe/cascade_care_savings/
├── wa_cascade_care_savings.py
├── wa_cascade_care_savings_eligible.py
├── wa_cascade_care_savings_member_amount.py
└── wa_cascade_care_savings_member_eligible.py
policyengine_us/tests/policy/baseline/gov/states/wa/wahbe/cascade_care_savings/
├── integration.yaml
├── wa_cascade_care_savings.yaml
└── wa_cascade_care_savings_eligible.yaml

Modified: policyengine_us/programs.yaml (program registry entry), policyengine_us/variables/household/healthcare_benefit_value.py (adds wa_cascade_care_savings), policyengine_us/tests/policy/baseline/household/healthcare_benefit_value.yaml (wiring test).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (eb465c8) to head (9d655b3).
⚠️ Report is 15 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
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  Coverage   100.00%   100.00%           
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  Files            7         5    -2     
  Lines          108        86   -22     
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- Hits           108        86   -22     

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[DTrim99]

Review fixes applied (/review-program → /fix-pr)

Full multi-agent review (regulatory, references, code patterns, test coverage, PDF value audit against the PY2026 Final CCS Policy + PMPM Methodology). 0 critical, 0 value mismatches — the $55/$250 PMPM split by federal-subsidy status (confirming the "$30/child" survey figure was wrong), the $0/$10/$15 benchmark-expectation tiers, the ≤250% FPL limit, and the SLCSP−APTC−expectation cap all verified; the redlined-PDF page anchors (§4 body p.8, §5 p.11) confirmed correct; and the round-1 Group 3 fix (Apple Health Kids exclusion + Group 1/Group 3 mutual exclusivity) confirmed correct and complete. Applied all should-address items and suggestions:

References / docs

• in_effect.yaml: the PMPM #page=1 title over-attributed the "$5M Section 1332 reserve" to a page that only supports the $55M appropriation — trimmed the title and added a CCS Policy #page=5 footnote citation that actually documents the reserve; de-duplicated the description.

Tests (77/77 passing)

• Two derived-from-raw-income integration scenarios (single Group-1 filer → $660; couple → $1,320) with the ACA chain (aca_magi_fraction, slcsp, aca_ptc) derived from wages.
• An MFS exclusion case driven by a real filing_status: SEPARATE, a 2.0001 benchmark-tier float-boundary pin (correctly $10, not $15), an exactly-2.5-FPL-ceiling case, and a two-tax-unit healthcare_benefit_value aggregation case.
• New dedicated unit test files for the two Person-level helpers — wa_cascade_care_savings_member_eligible.yaml (8 cases) and wa_cascade_care_savings_member_amount.yaml (7 cases) — covering the Group 1/3 gates, the Apple Health Kids exclusion, and Group 1/Group 3 mutual exclusivity ($55, not $55+$250).

No parameter values or formula logic changed. Rebased onto current main; CI green.

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