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FA-58841 / Payroll withholding rules / Open access

Social security wage-base withholding: section 125 wage reduction · case 01

Employees contributing to a 401(k) have too little social security withheld.

Verified by executionVariant 1 · 10 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The FICA wage builder subtracts 401(k) deferrals as if they were cafeteria-plan deductions; elective deferrals remain FICA wages.

THE FAILURE

The FICA wage builder subtracts 401(k) deferrals as if they were cafeteria-plan deductions; elective deferrals remain FICA wages.

Unsuccessful approach: The attempt swaps which deduction is subtracted, so section 125 premiums now stop reducing FICA wages while 401(k) deferrals still do.

Case contract

Input {ytd, lines:[[kind, cents]], base, exempt}. Social security wages are reg+ot+bonus+tips minus sec125 (401k deferrals do not reduce them), floored at 0. Exempt employees owe nothing ([0,0]). Taxable = min(wages, max(0, base-ytd)); tax = 6.2% rounded half-up to the cent. Return [taxable, tax].

Why this case matters

Employee social security withholding must stop exactly at the annual wage base and use the FICA wage definition, not the income-tax one.

1 / The failure

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

N = 1
observations = []
def solve(x):
    lines = x['lines']
    fica = 0
    for kind, amt in lines:
        if kind in ('reg', 'ot', 'bonus', 'tips'):
            fica += amt
        elif kind in ('sec125', '401k'):
            fica -= amt
    fica = max(0, fica)
    if x['exempt']:
        return [0, 0]
    room = max(0, x['base'] - x['ytd'])
    taxable = min(fica, room)
    tax = (taxable * 62 + 500) // 1000
    return [taxable, tax]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('regression', {'ytd': 16856079, 'lines': [['ot', 1416], ['401k', 15045]], 'base': 16860000, 'exempt': False}, [1416, 88]), ('regression', {'ytd': 3381968, 'lines': [['401k', 4345], ['ot', 6225]], 'base': 17610000, 'exempt': False}, [6225, 386]), ('partial-repair probe (boundary)', {'ytd': 0, 'lines': [['sec125', 10000], ['reg', 5000]], 'base': 16860000, 'exempt': False}, [0, 0]), ('partial-repair probe', {'ytd': 17429873, 'lines': [['reg', 14135], ['sec125', 260961], ['ot', 187839]], 'base': 17610000, 'exempt': False}, [0, 0]), ('boundary control', {'ytd': 16860000, 'lines': [['reg', 100000]], 'base': 16860000, 'exempt': False}, [0, 0]), ('boundary control', {'ytd': 16800000, 'lines': [['reg', 100000], ['401k', 5000]], 'base': 16860000, 'exempt': False}, [60000, 3720]), ('normal control', {'ytd': 1002361, 'lines': [['bonus', 5955]], 'base': 1000000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 16598376, 'lines': [['401k', 17815], ['sec125', 6865]], 'base': 16860000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 17473948, 'lines': [['sec125', 1367], ['sec125', 805]], 'base': 17610000, 'exempt': True}, [0, 0]), ('normal control', {'ytd': 16862120, 'lines': [['401k', 4852]], 'base': 16860000, 'exempt': False}, [0, 0])], [('regression', {'ytd': 3381968, 'lines': [['401k', 4345], ['ot', 6225]], 'base': 17610000, 'exempt': False}, [6225, 386]), ('regression', {'ytd': 17543362, 'lines': [['401k', 1330], ['ot', 1995], ['bonus', 366]], 'base': 17610000, 'exempt': False}, [2361, 146]), ('partial-repair probe', {'ytd': 17429873, 'lines': [['reg', 14135], ['sec125', 260961], ['ot', 187839]], 'base': 17610000, 'exempt': False}, [0, 0]), ('partial-repair probe', {'ytd': 8981578, 'lines': [['sec125', 35], ['ot', 4702], ['sec125', 74887], ['tips', 226998]], 'base': 17610000, 'exempt': False}, [156778, 9720]), ('boundary control', {'ytd': 0, 'lines': [['reg', 12345]], 'base': 16860000, 'exempt': True}, [0, 0]), ('boundary control', {'ytd': 16860000, 'lines': [['reg', 100000]], 'base': 16860000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 17365173, 'lines': [['401k', 1322]], 'base': 17610000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 1004946, 'lines': [['bonus', 48], ['ot', 3255]], 'base': 1000000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 16863463, 'lines': [['sec125', 37799], ['reg', 9785]], 'base': 16860000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 17613197, 'lines': [['sec125', 92109], ['ot', 11145], ['401k', 6145], ['tips', 1513]], 'base': 17610000, 'exempt': True}, [0, 0])], [('regression', {'ytd': 17543362, 'lines': [['401k', 1330], ['ot', 1995], ['bonus', 366]], 'base': 17610000, 'exempt': False}, [2361, 146]), ('regression', {'ytd': 16573248, 'lines': [['401k', 19585], ['sec125', 2886], ['bonus', 129728]], 'base': 16860000, 'exempt': False}, [126842, 7864]), ('partial-repair probe', {'ytd': 6205542, 'lines': [['ot', 2254], ['reg', 6550], ['tips', 10665], ['sec125', 2193]], 'base': 17610000, 'exempt': False}, [17276, 1071]), ('partial-repair probe', {'ytd': 3381968, 'lines': [['401k', 4345], ['ot', 6225]], 'base': 17610000, 'exempt': False}, [6225, 386]), ('boundary control', {'ytd': 16800000, 'lines': [['reg', 100000], ['401k', 5000]], 'base': 16860000, 'exempt': False}, [60000, 3720]), ('boundary control', {'ytd': 0, 'lines': [['reg', 12345]], 'base': 16860000, 'exempt': True}, [0, 0]), ('normal control', {'ytd': 1000562, 'lines': [['ot', 19155], ['sec125', 716], ['bonus', 9555], ['ot', 607]], 'base': 1000000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 888135, 'lines': [['reg', 4773]], 'base': 1000000, 'exempt': False}, [4773, 296]), ('normal control', {'ytd': 1007445, 'lines': [['reg', 945], ['ot', 220313], ['reg', 6015], ['reg', 18815]], 'base': 1000000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 16860733, 'lines': [['401k', 16515], ['bonus', 563], ['ot', 10025]], 'base': 16860000, 'exempt': False}, [0, 0])], [('regression', {'ytd': 16573248, 'lines': [['401k', 19585], ['sec125', 2886], ['bonus', 129728]], 'base': 16860000, 'exempt': False}, [126842, 7864]), ('regression', {'ytd': 12812610, 'lines': [['401k', 2454], ['ot', 3955]], 'base': 16860000, 'exempt': False}, [3955, 245]), ('partial-repair probe', {'ytd': 16617084, 'lines': [['reg', 4712], ['reg', 365692], ['sec125', 340208]], 'base': 16860000, 'exempt': False}, [30196, 1872]), ('partial-repair probe', {'ytd': 17543362, 'lines': [['401k', 1330], ['ot', 1995], ['bonus', 366]], 'base': 17610000, 'exempt': False}, [2361, 146]), ('boundary control', {'ytd': 16860000, 'lines': [['reg', 100000]], 'base': 16860000, 'exempt': False}, [0, 0]), ('boundary control', {'ytd': 16800000, 'lines': [['reg', 100000], ['401k', 5000]], 'base': 16860000, 'exempt': False}, [60000, 3720]), ('normal control', {'ytd': 1003495, 'lines': [['tips', 16185], ['bonus', 19575]], 'base': 1000000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 17386310, 'lines': [['401k', 2353]], 'base': 17610000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 16860000, 'lines': [['reg', 7465], ['reg', 3855], ['401k', 4387], ['bonus', 5805]], 'base': 16860000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 17612007, 'lines': [['reg', 4615], ['ot', 12255]], 'base': 17610000, 'exempt': False}, [0, 0])], [('regression', {'ytd': 12812610, 'lines': [['401k', 2454], ['ot', 3955]], 'base': 16860000, 'exempt': False}, [3955, 245]), ('regression', {'ytd': 17609450, 'lines': [['bonus', 11635], ['401k', 226117]], 'base': 17610000, 'exempt': False}, [550, 34]), ('partial-repair probe', {'ytd': 4027320, 'lines': [['tips', 88262], ['bonus', 963], ['sec125', 1621], ['ot', 9375]], 'base': 17610000, 'exempt': False}, [96979, 6013]), ('partial-repair probe', {'ytd': 772136, 'lines': [['bonus', 91140], ['bonus', 2377], ['bonus', 1344], ['sec125', 189513]], 'base': 1000000, 'exempt': False}, [0, 0]), ('boundary control', {'ytd': 0, 'lines': [['reg', 12345]], 'base': 16860000, 'exempt': True}, [0, 0]), ('boundary control', {'ytd': 16860000, 'lines': [['reg', 100000]], 'base': 16860000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 1000000, 'lines': [['tips', 285990], ['bonus', 1566], ['401k', 16295]], 'base': 1000000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 16863311, 'lines': [['tips', 19505]], 'base': 16860000, 'exempt': True}, [0, 0]), ('normal control', {'ytd': 17612457, 'lines': [['tips', 6928], ['401k', 1357], ['tips', 4125], ['bonus', 143712]], 'base': 17610000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 4871884, 'lines': [['401k', 9525]], 'base': 17610000, 'exempt': False}, [0, 0])]]
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[0, 0][1416, 88]Failed
regression 1[1880, 117][6225, 386]Failed
partial-repair probe (boundary) 2[0, 0][0, 0]Passed
partial-repair probe 3[0, 0][0, 0]Passed
boundary control 4[0, 0][0, 0]Passed
boundary control 5[60000, 3720][60000, 3720]Passed
normal control 6[0, 0][0, 0]Passed
normal control 7[0, 0][0, 0]Passed
normal control 8[0, 0][0, 0]Passed
normal control 9[0, 0][0, 0]Passed

SHA-256 / a01bd102eaa725944c3b8ccd30e1bfef04a73cbd2243e3aeb41a3a924b88e41f

2 / The unsuccessful fix

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

N = 1
observations = []
def solve(x):
    lines = x['lines']
    fica = 0
    for kind, amt in lines:
        if kind in ('reg', 'ot', 'bonus', 'tips'):
            fica += amt
        elif kind == '401k':
            fica -= amt
    fica = max(0, fica)
    if x['exempt']:
        return [0, 0]
    room = max(0, x['base'] - x['ytd'])
    taxable = min(fica, room)
    tax = (taxable * 62 + 500) // 1000
    return [taxable, tax]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('regression', {'ytd': 16856079, 'lines': [['ot', 1416], ['401k', 15045]], 'base': 16860000, 'exempt': False}, [1416, 88]), ('regression', {'ytd': 3381968, 'lines': [['401k', 4345], ['ot', 6225]], 'base': 17610000, 'exempt': False}, [6225, 386]), ('partial-repair probe (boundary)', {'ytd': 0, 'lines': [['sec125', 10000], ['reg', 5000]], 'base': 16860000, 'exempt': False}, [0, 0]), ('partial-repair probe', {'ytd': 17429873, 'lines': [['reg', 14135], ['sec125', 260961], ['ot', 187839]], 'base': 17610000, 'exempt': False}, [0, 0]), ('boundary control', {'ytd': 16860000, 'lines': [['reg', 100000]], 'base': 16860000, 'exempt': False}, [0, 0]), ('boundary control', {'ytd': 16800000, 'lines': [['reg', 100000], ['401k', 5000]], 'base': 16860000, 'exempt': False}, [60000, 3720]), ('normal control', {'ytd': 1002361, 'lines': [['bonus', 5955]], 'base': 1000000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 16598376, 'lines': [['401k', 17815], ['sec125', 6865]], 'base': 16860000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 17473948, 'lines': [['sec125', 1367], ['sec125', 805]], 'base': 17610000, 'exempt': True}, [0, 0]), ('normal control', {'ytd': 16862120, 'lines': [['401k', 4852]], 'base': 16860000, 'exempt': False}, [0, 0])], [('regression', {'ytd': 3381968, 'lines': [['401k', 4345], ['ot', 6225]], 'base': 17610000, 'exempt': False}, [6225, 386]), ('regression', {'ytd': 17543362, 'lines': [['401k', 1330], ['ot', 1995], ['bonus', 366]], 'base': 17610000, 'exempt': False}, [2361, 146]), ('partial-repair probe', {'ytd': 17429873, 'lines': [['reg', 14135], ['sec125', 260961], ['ot', 187839]], 'base': 17610000, 'exempt': False}, [0, 0]), ('partial-repair probe', {'ytd': 8981578, 'lines': [['sec125', 35], ['ot', 4702], ['sec125', 74887], ['tips', 226998]], 'base': 17610000, 'exempt': False}, [156778, 9720]), ('boundary control', {'ytd': 0, 'lines': [['reg', 12345]], 'base': 16860000, 'exempt': True}, [0, 0]), ('boundary control', {'ytd': 16860000, 'lines': [['reg', 100000]], 'base': 16860000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 17365173, 'lines': [['401k', 1322]], 'base': 17610000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 1004946, 'lines': [['bonus', 48], ['ot', 3255]], 'base': 1000000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 16863463, 'lines': [['sec125', 37799], ['reg', 9785]], 'base': 16860000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 17613197, 'lines': [['sec125', 92109], ['ot', 11145], ['401k', 6145], ['tips', 1513]], 'base': 17610000, 'exempt': True}, [0, 0])], [('regression', {'ytd': 17543362, 'lines': [['401k', 1330], ['ot', 1995], ['bonus', 366]], 'base': 17610000, 'exempt': False}, [2361, 146]), ('regression', {'ytd': 16573248, 'lines': [['401k', 19585], ['sec125', 2886], ['bonus', 129728]], 'base': 16860000, 'exempt': False}, [126842, 7864]), ('partial-repair probe', {'ytd': 6205542, 'lines': [['ot', 2254], ['reg', 6550], ['tips', 10665], ['sec125', 2193]], 'base': 17610000, 'exempt': False}, [17276, 1071]), ('partial-repair probe', {'ytd': 3381968, 'lines': [['401k', 4345], ['ot', 6225]], 'base': 17610000, 'exempt': False}, [6225, 386]), ('boundary control', {'ytd': 16800000, 'lines': [['reg', 100000], ['401k', 5000]], 'base': 16860000, 'exempt': False}, [60000, 3720]), ('boundary control', {'ytd': 0, 'lines': [['reg', 12345]], 'base': 16860000, 'exempt': True}, [0, 0]), ('normal control', {'ytd': 1000562, 'lines': [['ot', 19155], ['sec125', 716], ['bonus', 9555], ['ot', 607]], 'base': 1000000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 888135, 'lines': [['reg', 4773]], 'base': 1000000, 'exempt': False}, [4773, 296]), ('normal control', {'ytd': 1007445, 'lines': [['reg', 945], ['ot', 220313], ['reg', 6015], ['reg', 18815]], 'base': 1000000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 16860733, 'lines': [['401k', 16515], ['bonus', 563], ['ot', 10025]], 'base': 16860000, 'exempt': False}, [0, 0])], [('regression', {'ytd': 16573248, 'lines': [['401k', 19585], ['sec125', 2886], ['bonus', 129728]], 'base': 16860000, 'exempt': False}, [126842, 7864]), ('regression', {'ytd': 12812610, 'lines': [['401k', 2454], ['ot', 3955]], 'base': 16860000, 'exempt': False}, [3955, 245]), ('partial-repair probe', {'ytd': 16617084, 'lines': [['reg', 4712], ['reg', 365692], ['sec125', 340208]], 'base': 16860000, 'exempt': False}, [30196, 1872]), ('partial-repair probe', {'ytd': 17543362, 'lines': [['401k', 1330], ['ot', 1995], ['bonus', 366]], 'base': 17610000, 'exempt': False}, [2361, 146]), ('boundary control', {'ytd': 16860000, 'lines': [['reg', 100000]], 'base': 16860000, 'exempt': False}, [0, 0]), ('boundary control', {'ytd': 16800000, 'lines': [['reg', 100000], ['401k', 5000]], 'base': 16860000, 'exempt': False}, [60000, 3720]), ('normal control', {'ytd': 1003495, 'lines': [['tips', 16185], ['bonus', 19575]], 'base': 1000000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 17386310, 'lines': [['401k', 2353]], 'base': 17610000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 16860000, 'lines': [['reg', 7465], ['reg', 3855], ['401k', 4387], ['bonus', 5805]], 'base': 16860000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 17612007, 'lines': [['reg', 4615], ['ot', 12255]], 'base': 17610000, 'exempt': False}, [0, 0])], [('regression', {'ytd': 12812610, 'lines': [['401k', 2454], ['ot', 3955]], 'base': 16860000, 'exempt': False}, [3955, 245]), ('regression', {'ytd': 17609450, 'lines': [['bonus', 11635], ['401k', 226117]], 'base': 17610000, 'exempt': False}, [550, 34]), ('partial-repair probe', {'ytd': 4027320, 'lines': [['tips', 88262], ['bonus', 963], ['sec125', 1621], ['ot', 9375]], 'base': 17610000, 'exempt': False}, [96979, 6013]), ('partial-repair probe', {'ytd': 772136, 'lines': [['bonus', 91140], ['bonus', 2377], ['bonus', 1344], ['sec125', 189513]], 'base': 1000000, 'exempt': False}, [0, 0]), ('boundary control', {'ytd': 0, 'lines': [['reg', 12345]], 'base': 16860000, 'exempt': True}, [0, 0]), ('boundary control', {'ytd': 16860000, 'lines': [['reg', 100000]], 'base': 16860000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 1000000, 'lines': [['tips', 285990], ['bonus', 1566], ['401k', 16295]], 'base': 1000000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 16863311, 'lines': [['tips', 19505]], 'base': 16860000, 'exempt': True}, [0, 0]), ('normal control', {'ytd': 17612457, 'lines': [['tips', 6928], ['401k', 1357], ['tips', 4125], ['bonus', 143712]], 'base': 17610000, 'exempt': False}, [0, 0]), ('normal control', {'ytd': 4871884, 'lines': [['401k', 9525]], 'base': 17610000, 'exempt': False}, [0, 0])]]
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[0, 0][1416, 88]Failed
regression 1[1880, 117][6225, 386]Failed
partial-repair probe (boundary) 2[5000, 310][0, 0]Failed
partial-repair probe 3[180127, 11168][0, 0]Failed
boundary control 4[0, 0][0, 0]Passed
boundary control 5[60000, 3720][60000, 3720]Passed
normal control 6[0, 0][0, 0]Passed
normal control 7[0, 0][0, 0]Passed
normal control 8[0, 0][0, 0]Passed
normal control 9[0, 0][0, 0]Passed

SHA-256 / a803cdb682236babba0c27531507f004a7f4b483410806dd9c6e9cc9b0d42458

HELD IN THE MEMBER ARCHIVE

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

This mechanism has 10 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 2026-09-29T14:46:30.525062+00:00.

Case digest / c016585fe9a9fd38b42c4db8acba2910519d13b0341a90c338e3a9864030c465