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FA-58906 / Payroll withholding rules / Open access

W-4 step adjustment withholding: credit de-annualization · case 01

Any employee claiming dependents has withholding wiped out entirely each paycheck.

Verified by executionVariant 1 · 9 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The annual step 3 credit is subtracted in full every pay period instead of being divided by the number of periods.

THE FAILURE

The annual step 3 credit is subtracted in full every pay period instead of being divided by the number of periods.

Unsuccessful approach: The attempt divides by twelve regardless of pay frequency, so weekly and biweekly employees get oversized credits.

Case contract

Input {wage, periods, status single|mfj|hoh, step2, credits, other_income, deductions, extra}; annual amounts in cents except wage and extra (per period). Annual = wage*periods + other_income. Deduction = base standard (single 8,600, mfj 12,900, hoh 8,600 dollars; halved when step2) + itemized deductions (never halved). Thresholds 7,000/30,000/90,000 dollars at 10/12/22% scaled by status unit (single 2, mfj 4, hoh 3)/2, or /4 when step2. Tentative per-period = annual tax/periods half-up; credits/periods half-up is subtracted, floored at 0, then extra is added.

Why this case matters

Modern W-4 withholding combines annual adjustments with per-period ones, and the order of flooring and adding extra withholding matters.

1 / The failure

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(x):
    p = x['periods']
    annual = x['wage'] * p + x['other_income']
    base_std = {'single': 860000, 'mfj': 1290000, 'hoh': 860000}[x['status']]
    ded = (base_std // 2 if x['step2'] else base_std) + x['deductions']
    taxable = max(0, annual - ded)
    unit = {'single': 2, 'mfj': 4, 'hoh': 3}[x['status']]
    div = 4 if x['step2'] else 2
    bounds = [700000 * unit // div, 3000000 * unit // div, 9000000 * unit // div]
    rates = [10, 12, 22]
    tax = 0
    for i, lo in enumerate(bounds):
        hi = bounds[i + 1] if i + 1 < len(bounds) else taxable
        if taxable > lo:
            tax += (min(taxable, hi) - lo) * rates[i]
    tentative = (tax * 2 + 100 * p) // (200 * p)
    credit_pp = x['credits']
    wh = max(0, tentative - credit_pp) + x['extra']
    return [tentative, wh]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('regression', {'wage': 232685, 'periods': 52, 'status': 'mfj', 'step2': False, 'credits': 200000, 'other_income': 0, 'deductions': 479252, 'extra': 0}, [18839, 14993]), ('regression', {'wage': 492438, 'periods': 12, 'status': 'single', 'step2': True, 'credits': 400000, 'other_income': 131536, 'deductions': 0, 'extra': 0}, [59948, 26615]), ('partial-repair probe', {'wage': 244499, 'periods': 26, 'status': 'hoh', 'step2': False, 'credits': 305778, 'other_income': 170107, 'deductions': 0, 'extra': 0}, [18656, 6895]), ('partial-repair probe', {'wage': 142479, 'periods': 52, 'status': 'hoh', 'step2': True, 'credits': 551126, 'other_income': 0, 'deductions': 0, 'extra': 3806}, [14670, 7877]), ('boundary control', {'wage': 0, 'periods': 26, 'status': 'single', 'step2': False, 'credits': 400000, 'other_income': 0, 'deductions': 0, 'extra': 5000}, [0, 5000]), ('normal control', {'wage': 4942, 'periods': 12, 'status': 'single', 'step2': False, 'credits': 400000, 'other_income': 1831505, 'deductions': 0, 'extra': 8860}, [2757, 8860]), ('normal control', {'wage': 111690, 'periods': 26, 'status': 'mfj', 'step2': False, 'credits': 0, 'other_income': 0, 'deductions': 0, 'extra': 0}, [823, 823]), ('normal control', {'wage': 67447, 'periods': 24, 'status': 'mfj', 'step2': False, 'credits': 400000, 'other_income': 0, 'deductions': 0, 'extra': 2500}, [0, 2500]), ('normal control', {'wage': 69636, 'periods': 24, 'status': 'hoh', 'step2': True, 'credits': 877725, 'other_income': 1721162, 'deductions': 1014955, 'extra': 0}, [5927, 0])], [('regression', {'wage': 244499, 'periods': 26, 'status': 'hoh', 'step2': False, 'credits': 305778, 'other_income': 170107, 'deductions': 0, 'extra': 0}, [18656, 6895]), ('regression', {'wage': 232685, 'periods': 52, 'status': 'mfj', 'step2': False, 'credits': 200000, 'other_income': 0, 'deductions': 479252, 'extra': 0}, [18839, 14993]), ('partial-repair probe', {'wage': 142479, 'periods': 52, 'status': 'hoh', 'step2': True, 'credits': 551126, 'other_income': 0, 'deductions': 0, 'extra': 3806}, [14670, 7877]), ('partial-repair probe', {'wage': 667888, 'periods': 24, 'status': 'hoh', 'step2': False, 'credits': 200000, 'other_income': 923171, 'deductions': 893525, 'extra': 2500}, [74949, 69116]), ('boundary control', {'wage': 0, 'periods': 26, 'status': 'single', 'step2': False, 'credits': 400000, 'other_income': 0, 'deductions': 0, 'extra': 5000}, [0, 5000]), ('normal control', {'wage': 1041418, 'periods': 24, 'status': 'hoh', 'step2': False, 'credits': 0, 'other_income': 0, 'deductions': 0, 'extra': 2500}, [156854, 159354]), ('normal control', {'wage': 245742, 'periods': 24, 'status': 'single', 'step2': False, 'credits': 0, 'other_income': 0, 'deductions': 0, 'extra': 2500}, [19772, 22272]), ('normal control', {'wage': 196398, 'periods': 12, 'status': 'single', 'step2': False, 'credits': 0, 'other_income': 0, 'deductions': 0, 'extra': 0}, [6640, 6640]), ('normal control', {'wage': 64669, 'periods': 24, 'status': 'single', 'step2': False, 'credits': 0, 'other_income': 0, 'deductions': 0, 'extra': 2500}, [0, 2500])], [('regression', {'wage': 142479, 'periods': 52, 'status': 'hoh', 'step2': True, 'credits': 551126, 'other_income': 0, 'deductions': 0, 'extra': 3806}, [14670, 7877]), ('regression', {'wage': 244499, 'periods': 26, 'status': 'hoh', 'step2': False, 'credits': 305778, 'other_income': 170107, 'deductions': 0, 'extra': 0}, [18656, 6895]), ('partial-repair probe', {'wage': 452063, 'periods': 24, 'status': 'single', 'step2': False, 'credits': 200000, 'other_income': 0, 'deductions': 0, 'extra': 1399}, [48654, 41720]), ('partial-repair probe', {'wage': 190399, 'periods': 24, 'status': 'mfj', 'step2': True, 'credits': 200000, 'other_income': 1327070, 'deductions': 0, 'extra': 0}, [20842, 12509]), ('boundary control', {'wage': 0, 'periods': 26, 'status': 'single', 'step2': False, 'credits': 400000, 'other_income': 0, 'deductions': 0, 'extra': 5000}, [0, 5000]), ('normal control', {'wage': 79014, 'periods': 24, 'status': 'mfj', 'step2': False, 'credits': 400000, 'other_income': 1864011, 'deductions': 0, 'extra': 0}, [4460, 0]), ('normal control', {'wage': 322003, 'periods': 12, 'status': 'single', 'step2': False, 'credits': 0, 'other_income': 0, 'deductions': 0, 'extra': 2500}, [19207, 21707]), ('normal control', {'wage': 93316, 'periods': 12, 'status': 'mfj', 'step2': False, 'credits': 200000, 'other_income': 1111591, 'deductions': 0, 'extra': 1563}, [0, 1563]), ('normal control', {'wage': 285367, 'periods': 52, 'status': 'hoh', 'step2': True, 'credits': 0, 'other_income': 0, 'deductions': 1182238, 'extra': 2500}, [41104, 43604])], [('regression', {'wage': 667888, 'periods': 24, 'status': 'hoh', 'step2': False, 'credits': 200000, 'other_income': 923171, 'deductions': 893525, 'extra': 2500}, [74949, 69116]), ('regression', {'wage': 142479, 'periods': 52, 'status': 'hoh', 'step2': True, 'credits': 551126, 'other_income': 0, 'deductions': 0, 'extra': 3806}, [14670, 7877]), ('partial-repair probe', {'wage': 292525, 'periods': 26, 'status': 'hoh', 'step2': True, 'credits': 400000, 'other_income': 645414, 'deductions': 0, 'extra': 8294}, [36467, 29376]), ('partial-repair probe', {'wage': 242233, 'periods': 26, 'status': 'hoh', 'step2': False, 'credits': 400000, 'other_income': 0, 'deductions': 0, 'extra': 0}, [17599, 2214]), ('boundary control', {'wage': 0, 'periods': 26, 'status': 'single', 'step2': False, 'credits': 400000, 'other_income': 0, 'deductions': 0, 'extra': 5000}, [0, 5000]), ('normal control', {'wage': 37313, 'periods': 52, 'status': 'mfj', 'step2': False, 'credits': 117737, 'other_income': 1320234, 'deductions': 0, 'extra': 5664}, [1097, 5664]), ('normal control', {'wage': 179669, 'periods': 52, 'status': 'single', 'step2': False, 'credits': 0, 'other_income': 0, 'deductions': 0, 'extra': 0}, [17076, 17076]), ('normal control', {'wage': 170108, 'periods': 12, 'status': 'single', 'step2': True, 'credits': 200000, 'other_income': 167672, 'deductions': 0, 'extra': 0}, [12373, 0]), ('normal control', {'wage': 124748, 'periods': 26, 'status': 'mfj', 'step2': True, 'credits': 0, 'other_income': 0, 'deductions': 0, 'extra': 0}, [7302, 7302])], [('regression', {'wage': 452063, 'periods': 24, 'status': 'single', 'step2': False, 'credits': 200000, 'other_income': 0, 'deductions': 0, 'extra': 1399}, [48654, 41720]), ('regression', {'wage': 667888, 'periods': 24, 'status': 'hoh', 'step2': False, 'credits': 200000, 'other_income': 923171, 'deductions': 893525, 'extra': 2500}, [74949, 69116]), ('partial-repair probe', {'wage': 617786, 'periods': 24, 'status': 'single', 'step2': True, 'credits': 400000, 'other_income': 1587260, 'deductions': 0, 'extra': 2500}, [125063, 110896]), ('partial-repair probe', {'wage': 344993, 'periods': 26, 'status': 'mfj', 'step2': False, 'credits': 200000, 'other_income': 0, 'deductions': 0, 'extra': 2500}, [25445, 20253]), ('boundary control', {'wage': 0, 'periods': 26, 'status': 'single', 'step2': False, 'credits': 400000, 'other_income': 0, 'deductions': 0, 'extra': 5000}, [0, 5000]), ('normal control', {'wage': 90292, 'periods': 52, 'status': 'hoh', 'step2': True, 'credits': 0, 'other_income': 0, 'deductions': 0, 'extra': 2500}, [7968, 10468]), ('normal control', {'wage': 250357, 'periods': 52, 'status': 'single', 'step2': False, 'credits': 0, 'other_income': 0, 'deductions': 0, 'extra': 2500}, [31632, 34132]), ('normal control', {'wage': 251742, 'periods': 52, 'status': 'hoh', 'step2': False, 'credits': 0, 'other_income': 0, 'deductions': 392790, 'extra': 0}, [23568, 23568]), ('normal control', {'wage': 290758, 'periods': 26, 'status': 'single', 'step2': False, 'credits': 0, 'other_income': 1100573, 'deductions': 944056, 'extra': 0}, [26644, 26644])]]
for i, (label, args, expected) in enumerate(fixtures[N-1]):
    check("%s %d" % (label, i), solve(args), expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)
Boundary fixtureActualExpectedOutcome
regression 0[18839, 0][18839, 14993]Failed
regression 1[59948, 0][59948, 26615]Failed
partial-repair probe 2[18656, 0][18656, 6895]Failed
partial-repair probe 3[14670, 3806][14670, 7877]Failed
boundary control 4[0, 5000][0, 5000]Passed
normal control 5[2757, 8860][2757, 8860]Passed
normal control 6[823, 823][823, 823]Passed
normal control 7[0, 2500][0, 2500]Passed
normal control 8[5927, 0][5927, 0]Passed

SHA-256 / 61eb67a7f17f215364890e9d0145819a9df1531981b2de5544ae2817861385c7

2 / The unsuccessful fix

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(x):
    p = x['periods']
    annual = x['wage'] * p + x['other_income']
    base_std = {'single': 860000, 'mfj': 1290000, 'hoh': 860000}[x['status']]
    ded = (base_std // 2 if x['step2'] else base_std) + x['deductions']
    taxable = max(0, annual - ded)
    unit = {'single': 2, 'mfj': 4, 'hoh': 3}[x['status']]
    div = 4 if x['step2'] else 2
    bounds = [700000 * unit // div, 3000000 * unit // div, 9000000 * unit // div]
    rates = [10, 12, 22]
    tax = 0
    for i, lo in enumerate(bounds):
        hi = bounds[i + 1] if i + 1 < len(bounds) else taxable
        if taxable > lo:
            tax += (min(taxable, hi) - lo) * rates[i]
    tentative = (tax * 2 + 100 * p) // (200 * p)
    credit_pp = x['credits'] // 12
    wh = max(0, tentative - credit_pp) + x['extra']
    return [tentative, wh]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('regression', {'wage': 232685, 'periods': 52, 'status': 'mfj', 'step2': False, 'credits': 200000, 'other_income': 0, 'deductions': 479252, 'extra': 0}, [18839, 14993]), ('regression', {'wage': 492438, 'periods': 12, 'status': 'single', 'step2': True, 'credits': 400000, 'other_income': 131536, 'deductions': 0, 'extra': 0}, [59948, 26615]), ('partial-repair probe', {'wage': 244499, 'periods': 26, 'status': 'hoh', 'step2': False, 'credits': 305778, 'other_income': 170107, 'deductions': 0, 'extra': 0}, [18656, 6895]), ('partial-repair probe', {'wage': 142479, 'periods': 52, 'status': 'hoh', 'step2': True, 'credits': 551126, 'other_income': 0, 'deductions': 0, 'extra': 3806}, [14670, 7877]), ('boundary control', {'wage': 0, 'periods': 26, 'status': 'single', 'step2': False, 'credits': 400000, 'other_income': 0, 'deductions': 0, 'extra': 5000}, [0, 5000]), ('normal control', {'wage': 4942, 'periods': 12, 'status': 'single', 'step2': False, 'credits': 400000, 'other_income': 1831505, 'deductions': 0, 'extra': 8860}, [2757, 8860]), ('normal control', {'wage': 111690, 'periods': 26, 'status': 'mfj', 'step2': False, 'credits': 0, 'other_income': 0, 'deductions': 0, 'extra': 0}, [823, 823]), ('normal control', {'wage': 67447, 'periods': 24, 'status': 'mfj', 'step2': False, 'credits': 400000, 'other_income': 0, 'deductions': 0, 'extra': 2500}, [0, 2500]), ('normal control', {'wage': 69636, 'periods': 24, 'status': 'hoh', 'step2': True, 'credits': 877725, 'other_income': 1721162, 'deductions': 1014955, 'extra': 0}, [5927, 0])], [('regression', {'wage': 244499, 'periods': 26, 'status': 'hoh', 'step2': False, 'credits': 305778, 'other_income': 170107, 'deductions': 0, 'extra': 0}, [18656, 6895]), ('regression', {'wage': 232685, 'periods': 52, 'status': 'mfj', 'step2': False, 'credits': 200000, 'other_income': 0, 'deductions': 479252, 'extra': 0}, [18839, 14993]), ('partial-repair probe', {'wage': 142479, 'periods': 52, 'status': 'hoh', 'step2': True, 'credits': 551126, 'other_income': 0, 'deductions': 0, 'extra': 3806}, [14670, 7877]), ('partial-repair probe', {'wage': 667888, 'periods': 24, 'status': 'hoh', 'step2': False, 'credits': 200000, 'other_income': 923171, 'deductions': 893525, 'extra': 2500}, [74949, 69116]), ('boundary control', {'wage': 0, 'periods': 26, 'status': 'single', 'step2': False, 'credits': 400000, 'other_income': 0, 'deductions': 0, 'extra': 5000}, [0, 5000]), ('normal control', {'wage': 1041418, 'periods': 24, 'status': 'hoh', 'step2': False, 'credits': 0, 'other_income': 0, 'deductions': 0, 'extra': 2500}, [156854, 159354]), ('normal control', {'wage': 245742, 'periods': 24, 'status': 'single', 'step2': False, 'credits': 0, 'other_income': 0, 'deductions': 0, 'extra': 2500}, [19772, 22272]), ('normal control', {'wage': 196398, 'periods': 12, 'status': 'single', 'step2': False, 'credits': 0, 'other_income': 0, 'deductions': 0, 'extra': 0}, [6640, 6640]), ('normal control', {'wage': 64669, 'periods': 24, 'status': 'single', 'step2': False, 'credits': 0, 'other_income': 0, 'deductions': 0, 'extra': 2500}, [0, 2500])], [('regression', {'wage': 142479, 'periods': 52, 'status': 'hoh', 'step2': True, 'credits': 551126, 'other_income': 0, 'deductions': 0, 'extra': 3806}, [14670, 7877]), ('regression', {'wage': 244499, 'periods': 26, 'status': 'hoh', 'step2': False, 'credits': 305778, 'other_income': 170107, 'deductions': 0, 'extra': 0}, [18656, 6895]), ('partial-repair probe', {'wage': 452063, 'periods': 24, 'status': 'single', 'step2': False, 'credits': 200000, 'other_income': 0, 'deductions': 0, 'extra': 1399}, [48654, 41720]), ('partial-repair probe', {'wage': 190399, 'periods': 24, 'status': 'mfj', 'step2': True, 'credits': 200000, 'other_income': 1327070, 'deductions': 0, 'extra': 0}, [20842, 12509]), ('boundary control', {'wage': 0, 'periods': 26, 'status': 'single', 'step2': False, 'credits': 400000, 'other_income': 0, 'deductions': 0, 'extra': 5000}, [0, 5000]), ('normal control', {'wage': 79014, 'periods': 24, 'status': 'mfj', 'step2': False, 'credits': 400000, 'other_income': 1864011, 'deductions': 0, 'extra': 0}, [4460, 0]), ('normal control', {'wage': 322003, 'periods': 12, 'status': 'single', 'step2': False, 'credits': 0, 'other_income': 0, 'deductions': 0, 'extra': 2500}, [19207, 21707]), ('normal control', {'wage': 93316, 'periods': 12, 'status': 'mfj', 'step2': False, 'credits': 200000, 'other_income': 1111591, 'deductions': 0, 'extra': 1563}, [0, 1563]), ('normal control', {'wage': 285367, 'periods': 52, 'status': 'hoh', 'step2': True, 'credits': 0, 'other_income': 0, 'deductions': 1182238, 'extra': 2500}, [41104, 43604])], [('regression', {'wage': 667888, 'periods': 24, 'status': 'hoh', 'step2': False, 'credits': 200000, 'other_income': 923171, 'deductions': 893525, 'extra': 2500}, [74949, 69116]), ('regression', {'wage': 142479, 'periods': 52, 'status': 'hoh', 'step2': True, 'credits': 551126, 'other_income': 0, 'deductions': 0, 'extra': 3806}, [14670, 7877]), ('partial-repair probe', {'wage': 292525, 'periods': 26, 'status': 'hoh', 'step2': True, 'credits': 400000, 'other_income': 645414, 'deductions': 0, 'extra': 8294}, [36467, 29376]), ('partial-repair probe', {'wage': 242233, 'periods': 26, 'status': 'hoh', 'step2': False, 'credits': 400000, 'other_income': 0, 'deductions': 0, 'extra': 0}, [17599, 2214]), ('boundary control', {'wage': 0, 'periods': 26, 'status': 'single', 'step2': False, 'credits': 400000, 'other_income': 0, 'deductions': 0, 'extra': 5000}, [0, 5000]), ('normal control', {'wage': 37313, 'periods': 52, 'status': 'mfj', 'step2': False, 'credits': 117737, 'other_income': 1320234, 'deductions': 0, 'extra': 5664}, [1097, 5664]), ('normal control', {'wage': 179669, 'periods': 52, 'status': 'single', 'step2': False, 'credits': 0, 'other_income': 0, 'deductions': 0, 'extra': 0}, [17076, 17076]), ('normal control', {'wage': 170108, 'periods': 12, 'status': 'single', 'step2': True, 'credits': 200000, 'other_income': 167672, 'deductions': 0, 'extra': 0}, [12373, 0]), ('normal control', {'wage': 124748, 'periods': 26, 'status': 'mfj', 'step2': True, 'credits': 0, 'other_income': 0, 'deductions': 0, 'extra': 0}, [7302, 7302])], [('regression', {'wage': 452063, 'periods': 24, 'status': 'single', 'step2': False, 'credits': 200000, 'other_income': 0, 'deductions': 0, 'extra': 1399}, [48654, 41720]), ('regression', {'wage': 667888, 'periods': 24, 'status': 'hoh', 'step2': False, 'credits': 200000, 'other_income': 923171, 'deductions': 893525, 'extra': 2500}, [74949, 69116]), ('partial-repair probe', {'wage': 617786, 'periods': 24, 'status': 'single', 'step2': True, 'credits': 400000, 'other_income': 1587260, 'deductions': 0, 'extra': 2500}, [125063, 110896]), ('partial-repair probe', {'wage': 344993, 'periods': 26, 'status': 'mfj', 'step2': False, 'credits': 200000, 'other_income': 0, 'deductions': 0, 'extra': 2500}, [25445, 20253]), ('boundary control', {'wage': 0, 'periods': 26, 'status': 'single', 'step2': False, 'credits': 400000, 'other_income': 0, 'deductions': 0, 'extra': 5000}, [0, 5000]), ('normal control', {'wage': 90292, 'periods': 52, 'status': 'hoh', 'step2': True, 'credits': 0, 'other_income': 0, 'deductions': 0, 'extra': 2500}, [7968, 10468]), ('normal control', {'wage': 250357, 'periods': 52, 'status': 'single', 'step2': False, 'credits': 0, 'other_income': 0, 'deductions': 0, 'extra': 2500}, [31632, 34132]), ('normal control', {'wage': 251742, 'periods': 52, 'status': 'hoh', 'step2': False, 'credits': 0, 'other_income': 0, 'deductions': 392790, 'extra': 0}, [23568, 23568]), ('normal control', {'wage': 290758, 'periods': 26, 'status': 'single', 'step2': False, 'credits': 0, 'other_income': 1100573, 'deductions': 944056, 'extra': 0}, [26644, 26644])]]
for i, (label, args, expected) in enumerate(fixtures[N-1]):
    check("%s %d" % (label, i), solve(args), expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)
Boundary fixtureActualExpectedOutcome
regression 0[18839, 2173][18839, 14993]Failed
regression 1[59948, 26615][59948, 26615]Passed
partial-repair probe 2[18656, 0][18656, 6895]Failed
partial-repair probe 3[14670, 3806][14670, 7877]Failed
boundary control 4[0, 5000][0, 5000]Passed
normal control 5[2757, 8860][2757, 8860]Passed
normal control 6[823, 823][823, 823]Passed
normal control 7[0, 2500][0, 2500]Passed
normal control 8[5927, 0][5927, 0]Passed

SHA-256 / 2b075ead2f6e02b297148b962b5e93b0c73be25af1aea0f54aa959a3274c3791

HELD IN THE MEMBER ARCHIVE

The verified repair and its recorded checks are member-only.

This mechanism has 9 recorded checks per implementation. The open-access tier publishes the failure and the unsuccessful fix; the repaired source that passes every check, and the observations that prove it, are available to members.

Every case sharing this mechanism uses the same contract and the same repair, so this one record is held back for all of them.

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Verification & scope

A deterministic teaching model of a stipulated payroll rule with toy thresholds and rates. It makes no claim of conformance to any tax authority, statute or jurisdiction and is not payroll software. This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.

Observations recorded using Python 3.12.14 at 2026-09-29T14:46:31.179142+00:00.

Case digest / 3e86fde453527a7b31236afb4a319ae172decc244500a5228136af2d90582673