Implement SC H.4216 comprehensive tax reform

[DTrim99]

Summary

Implements South Carolina H.4216 as a contributed reform. This comprehensive income tax reform bill passed the SC Senate 39-5 and includes:

1. EITC Cap at $200 (Section 7)

The bill caps the SC EITC (125% of federal) at a maximum of $200.

2. SC Income Adjusted Deduction (SCIAD) (Section 3)

Replaces federal standard/itemized deductions with a new state-specific deduction:

Filing Status	Base Amount	Phase-out Start	Phase-out End
Single	$15,000	$40,000	$95,000
Married Filing Separately	$15,000	$40,000	$95,000
Head of Household	$22,500	$60,000	$142,500
Married Filing Jointly	$30,000	$80,000	$190,000
Surviving Spouse	$30,000	$80,000	$190,000

Formula: SCIAD = base_amount × (1 - (AGI - phase_out_start) / phase_out_width)

3. New Tax Rates (Section 1)

Bracket	Threshold	Rate
1	$0	1.99%
2	$30,000	5.21%

Key Implementation Details

• Federal standard/itemized deductions are eliminated for SC purposes (Section 2)
• The reform starts from AGI and applies SCIAD instead of federal deductions
• SC additions and subtractions still apply

Closes #7493

Test Plan

• 24 YAML tests covering:
  • EITC cap ($200 limit)
  • SCIAD for all 5 filing statuses
  • Phase-out boundary conditions (at start, mid, end)
  • Rate bracket boundaries ($30k threshold)
  • Zero/low income edge cases
  • Verification that federal deductions are ignored
  • Non-SC resident handling

🤖 Generated with Claude Code

[DTrim99]

Implementation Notes

Files Added

Parameters (gov/contrib/states/sc/h4216/):

• in_effect.yaml - Master switch (default: false)
• eitc/max.yaml - $200 cap
• sciad/amount.yaml - Base amounts by filing status
• sciad/phase_out/start.yaml - Phase-out thresholds
• sciad/phase_out/width.yaml - Phase-out denominators
• rates.yaml - New 1.99%/5.21% rate structure

Reform Code (reforms/states/sc/h4216/):

• Overrides sc_eitc to apply $200 cap
• Creates new sc_sciad variable with phase-out formula
• Overrides sc_taxable_income to use AGI + SCIAD (not federal taxable income)
• Overrides sc_income_tax_before_non_refundable_credits with new rates

Tests (24 total):

• 2 EITC cap tests
• 10 SCIAD tests (all filing statuses, phase-out boundaries)
• 4 rate bracket tests
• 3 standard/itemized deduction elimination tests
• 3 zero/low income edge cases
• 2 boundary tests (phase-out start/end exact values)

References

• Bill text (Feb 24, 2026 amendment)
• Issue #7493

[DTrim99]

Rebased on upstream/main and added changelog fragment in changelog.d/sc-h4216-tax-reform.added.md to comply with the new towncrier-based changelog system.

[DTrim99]

Review Fixes Applied

Added the following based on code review:

1. SCIAD Rounding Rule (Critical Fix)

Per H.4216 Section 3, SCIAD reductions must round down to nearest $10. Added:

raw_sciad = max_(base_amount * (1 - phase_out_fraction), 0)
# Per H.4216 Section 3, round down to nearest $10
return np.floor(raw_sciad / 10) * 10

2. Updated Tests

Adjusted expected values for 4 tests to reflect the rounding rule:

• Case 15: $14,999.7 → $14,990
• Case 17: $0.3 → $0
• Case 28: SCIAD $12,272.7 → $12,270, taxable income updated accordingly
• Case 38: SCIAD $12,273 → $12,270, taxable income updated accordingly

3. Documented Unmodeled Provisions

Added note in rates.yaml that H.4216 Items 2-3 (automatic rate reductions beginning 2027 based on revenue projections ≥5% YoY growth) are not currently modeled.

All 38 tests pass ✅