FA-62776 / Tax bracket computation / Open access

Joint filers do not receive the higher phase-out start · case 01

Married couples lose credit earlier than stipulated.

Verified by executionVariant 1 · 7 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The 6920 joint adjustment to the phase-out start is missing.

THE FAILURE

The 6920 joint adjustment to the phase-out start is missing.

Unsuccessful approach: Applying the joint adjustment only when there are qualifying children misses childless joint filers.

Case contract

solve(earned, agi, children, mfj, invest): stipulated earned-income credit, whole dollars. Investment income above 11600 disqualifies (0). Parameters by min(children, 3): phase-in rate %, max credit, phase-out rate %, phase-out start (0: 7.65, 600, 7.65, 9800; 1: 34, 3995, 15.98, 21560; 2: 40, 6604, 21.06, 21560; 3+: 45, 7430, 21.06, 21560); joint filers add 6920 to the start. Credit = min(rate_in*earned, max) reduced by rate_out*(max(earned, agi) - start) when positive, floored at 0. Return integer cents half-up.

Why this case matters

Tax computations hinge on which slice, threshold, ordering and rounding rule applies at each step; a misplaced boundary silently misstates liabilities.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
from fractions import Fraction
N = 1
observations = []
def solve(earned, agi, children, mfj, invest):
    def prog(x, br):
        tax, lower = Fraction(0), 0
        for upper, rate in br:
            top = x if upper is None else min(x, upper)
            if top > lower: tax += (top - lower) * Fraction(rate) / 100
            if upper is None or x <= upper: break
            lower = upper
        return tax
    def cents(v):
        v = v * 100
        return int(v + Fraction(1, 2)) if v >= 0 else -int(-v + Fraction(1, 2))
    
    P = {0: ['7.65', 7840, 600, '7.65', 9800], 1: ['34', 11750, 3995, '15.98', 21560], 2: ['40', 16510, 6604, '21.06', 21560], 3: ['45', 16510, 7430, '21.06', 21560]}
    if invest > 11600: return 0
    rin, cap, mx, rout, start = P[min(children, 3)]
    pass
    credit = min(Fraction(rin) * earned / 100, Fraction(mx))
    income = max(earned, agi)
    if income > start: credit -= Fraction(rout) * (income - start) / 100
    return cents(max(0, credit))
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
cases = [[('regression joint-offset 1', (30000, 30000, 2, True, 0), 628389),
  ('regression joint-offset 2', (12000, 12000, 0, True, 0), 60000),
  ('partial repair guard 2', (10000, 0, 0, True, 0), 60000), ('control: plateau', (10000, 10000, 1, False, 0), 340000),
  ('control: phase-out two children', (30000, 30000, 2, False, 0), 482654),
  ('control: agi drives phase-out', (12000, 30000, 1, False, 0), 264629),
  ('control: investment limit', (20000, 20000, 1, False, 11601), 0)],
 [('regression joint-offset 1', (16510, 30000, 1, True, 11600), 375210),
  ('regression joint-offset 2', (5000, 30000, 1, True, 0), 145710),
  ('partial repair guard 1', (10000, 0, 0, True, 11600), 60000),
  ('partial repair guard 2', (12000, 12000, 0, True, 0), 60000),
  ('control: investment limit', (20000, 20000, 1, False, 11601), 0),
  ('control: no children', (5000, 5000, 0, False, 0), 38250), ('control: joint', (30000, 30000, 2, True, 0), 628389),
  ('control: four children', (15000, 15000, 4, False, 0), 675000)],
 [('regression joint-offset 1', (40000, 10000, 5, True, 11600), 500389),
  ('regression joint-offset 2', (5000, 30000, 5, True, 0), 192989),
  ('partial repair guard 1', (10000, 0, 0, True, 0), 60000),
  ('partial repair guard 2', (10000, 0, 0, True, 11600), 60000),
  ('control: four children', (15000, 15000, 4, False, 0), 675000),
  ('control: phased to zero', (60000, 60000, 1, False, 0), 0),
  ('control: investment at limit', (15000, 15000, 3, True, 11600), 675000),
  ('control: plateau', (10000, 10000, 1, False, 0), 340000)],
 [('regression joint-offset 1', (5000, 30000, 3, True, 0), 192989),
  ('regression joint-offset 2', (23681, 10000, 5, True, 11600), 743000),
  ('partial repair guard 1', (12000, 12000, 0, True, 0), 60000),
  ('partial repair guard 2', (10000, 0, 0, True, 0), 60000), ('control: plateau', (10000, 10000, 1, False, 0), 340000),
  ('control: phase-out two children', (30000, 30000, 2, False, 0), 482654),
  ('control: agi drives phase-out', (12000, 30000, 1, False, 0), 264629),
  ('control: investment limit', (20000, 20000, 1, False, 11601), 0)],
 [('regression joint-offset 1', (25000, 0, 2, True, 11600), 660400),
  ('regression joint-offset 2', (40000, 30000, 2, True, 11600), 417789),
  ('partial repair guard 1', (10000, 0, 0, True, 11600), 60000),
  ('partial repair guard 2', (12000, 12000, 0, True, 0), 60000),
  ('control: investment limit', (20000, 20000, 1, False, 11601), 0),
  ('control: no children', (5000, 5000, 0, False, 0), 38250), ('control: joint', (30000, 30000, 2, True, 0), 628389),
  ('control: four children', (15000, 15000, 4, False, 0), 675000)]]
for label, args, expected in cases[N - 1]:
    check(label, 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 fixture	Actual	Expected	Outcome
regression joint-offset 1	`482654`	`628389`	Failed
regression joint-offset 2	`43170`	`60000`	Failed
partial repair guard 2	`58470`	`60000`	Failed
control: plateau	`340000`	`340000`	Passed
control: phase-out two children	`482654`	`482654`	Passed
control: agi drives phase-out	`264629`	`264629`	Passed
control: investment limit	`0`	`0`	Passed

SHA-256 / 78883c807f321b347f771da6881db1f6921b39c497d5b4580f7b7bc0df3237a1

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
from fractions import Fraction
N = 1
observations = []
def solve(earned, agi, children, mfj, invest):
    def prog(x, br):
        tax, lower = Fraction(0), 0
        for upper, rate in br:
            top = x if upper is None else min(x, upper)
            if top > lower: tax += (top - lower) * Fraction(rate) / 100
            if upper is None or x <= upper: break
            lower = upper
        return tax
    def cents(v):
        v = v * 100
        return int(v + Fraction(1, 2)) if v >= 0 else -int(-v + Fraction(1, 2))
    
    P = {0: ['7.65', 7840, 600, '7.65', 9800], 1: ['34', 11750, 3995, '15.98', 21560], 2: ['40', 16510, 6604, '21.06', 21560], 3: ['45', 16510, 7430, '21.06', 21560]}
    if invest > 11600: return 0
    rin, cap, mx, rout, start = P[min(children, 3)]
    if mfj and children: start += 6920
    credit = min(Fraction(rin) * earned / 100, Fraction(mx))
    income = max(earned, agi)
    if income > start: credit -= Fraction(rout) * (income - start) / 100
    return cents(max(0, credit))
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
cases = [[('regression joint-offset 1', (30000, 30000, 2, True, 0), 628389),
  ('regression joint-offset 2', (12000, 12000, 0, True, 0), 60000),
  ('partial repair guard 2', (10000, 0, 0, True, 0), 60000), ('control: plateau', (10000, 10000, 1, False, 0), 340000),
  ('control: phase-out two children', (30000, 30000, 2, False, 0), 482654),
  ('control: agi drives phase-out', (12000, 30000, 1, False, 0), 264629),
  ('control: investment limit', (20000, 20000, 1, False, 11601), 0)],
 [('regression joint-offset 1', (16510, 30000, 1, True, 11600), 375210),
  ('regression joint-offset 2', (5000, 30000, 1, True, 0), 145710),
  ('partial repair guard 1', (10000, 0, 0, True, 11600), 60000),
  ('partial repair guard 2', (12000, 12000, 0, True, 0), 60000),
  ('control: investment limit', (20000, 20000, 1, False, 11601), 0),
  ('control: no children', (5000, 5000, 0, False, 0), 38250), ('control: joint', (30000, 30000, 2, True, 0), 628389),
  ('control: four children', (15000, 15000, 4, False, 0), 675000)],
 [('regression joint-offset 1', (40000, 10000, 5, True, 11600), 500389),
  ('regression joint-offset 2', (5000, 30000, 5, True, 0), 192989),
  ('partial repair guard 1', (10000, 0, 0, True, 0), 60000),
  ('partial repair guard 2', (10000, 0, 0, True, 11600), 60000),
  ('control: four children', (15000, 15000, 4, False, 0), 675000),
  ('control: phased to zero', (60000, 60000, 1, False, 0), 0),
  ('control: investment at limit', (15000, 15000, 3, True, 11600), 675000),
  ('control: plateau', (10000, 10000, 1, False, 0), 340000)],
 [('regression joint-offset 1', (5000, 30000, 3, True, 0), 192989),
  ('regression joint-offset 2', (23681, 10000, 5, True, 11600), 743000),
  ('partial repair guard 1', (12000, 12000, 0, True, 0), 60000),
  ('partial repair guard 2', (10000, 0, 0, True, 0), 60000), ('control: plateau', (10000, 10000, 1, False, 0), 340000),
  ('control: phase-out two children', (30000, 30000, 2, False, 0), 482654),
  ('control: agi drives phase-out', (12000, 30000, 1, False, 0), 264629),
  ('control: investment limit', (20000, 20000, 1, False, 11601), 0)],
 [('regression joint-offset 1', (25000, 0, 2, True, 11600), 660400),
  ('regression joint-offset 2', (40000, 30000, 2, True, 11600), 417789),
  ('partial repair guard 1', (10000, 0, 0, True, 11600), 60000),
  ('partial repair guard 2', (12000, 12000, 0, True, 0), 60000),
  ('control: investment limit', (20000, 20000, 1, False, 11601), 0),
  ('control: no children', (5000, 5000, 0, False, 0), 38250), ('control: joint', (30000, 30000, 2, True, 0), 628389),
  ('control: four children', (15000, 15000, 4, False, 0), 675000)]]
for label, args, expected in cases[N - 1]:
    check(label, 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 fixture	Actual	Expected	Outcome
regression joint-offset 1	`628389`	`628389`	Passed
regression joint-offset 2	`43170`	`60000`	Failed
partial repair guard 2	`58470`	`60000`	Failed
control: plateau	`340000`	`340000`	Passed
control: phase-out two children	`482654`	`482654`	Passed
control: agi drives phase-out	`264629`	`264629`	Passed
control: investment limit	`0`	`0`	Passed

SHA-256 / e4e3dfa8010308bb6073e8e3a3cdd99d2db06c16d8248b5eee935a5567616190

HELD IN THE MEMBER ARCHIVE

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

This mechanism has 7 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.

Member access is invitation-based. Sign in with your invited account to inspect the repair.

Verification & scope

A deterministic, bounded teaching model with a stipulated toy contract; it makes no claim of conformance to any real regulation, standard, or institution's rules. 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 2026-09-29T14:47:07.981638+00:00.

Case digest / 856641ca875304aaad069c0d516e15a1afd2b56ce05449629e11be38a7d9723c