FA-62191 / Currency rounding and FX conversion / Open access

The disclosed markup is rounded to nearest and can understate · case 01

A markup of 3.504% is disclosed as 3.50%.

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

ROOT CAUSE

The disclosure uses half-up rounding instead of rounding toward positive infinity.

VERIFIED REPAIR

Round the disclosed percentage with ROUND_CEILING.

Unsuccessful approach: Rounding away from zero matches ceiling for markups but overstates promotional discounts.

Case contract

solve(amount, ref_rate, markup_bps, card_exp): amount in merchant currency, ref_rate cardholder units per merchant unit, markup_bps an integer (negative for a promotional discount). The offered rate is ref*(1+bps/10000) rounded half-up to 6 decimals, and the card amount is amount*offered rounded half-up at card_exp. The disclosed markup percentage is (offered/ref - 1)*100 computed from the rounded offered rate and rounded toward positive infinity to 2 decimals so it never understates. Return [offered, card_amount, markup_pct] as strings.

Why this case matters

Currency amounts must be rounded at the right stage and in the right unit, or ledgers, quotes and settlements drift by minor units.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
from decimal import Decimal, ROUND_HALF_EVEN, ROUND_HALF_UP, ROUND_FLOOR, ROUND_CEILING, ROUND_DOWN, ROUND_UP, ROUND_HALF_DOWN
N = 1
observations = []
def solve(amount, ref_rate, markup_bps, card_exp):
    ref = Decimal(ref_rate)
    raw = ref * (1 + Decimal(markup_bps) / 10000)
    dcc = raw.quantize(Decimal('0.000001'), rounding=ROUND_HALF_UP)
    card = (Decimal(amount) * dcc).quantize(Decimal(1).scaleb(-card_exp), rounding=ROUND_HALF_UP)
    pct = ((dcc / ref - 1) * 100).quantize(Decimal('0.01'), rounding=ROUND_HALF_UP)
    return [format(dcc, 'f'), format(card, 'f'), format(pct, 'f')]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
cases = [[('regression disclosure-rounding-direction 1', ('5000', '0.00662', 350, 2), ['0.006852', '34.26', '3.51']),
  ('regression disclosure-rounding-direction 2', ('49.99', '151.3742', 275, 0), ['155.536991', '7775', '2.76']),
  ('partial repair guard 1', ('80.00', '0.913377', -50, 2), ['0.908810', '72.70', '-0.50']),
  ('partial repair guard 2', ('1171.26', '0.4683115', -50, 3), ['0.465970', '545.772', '-0.49']),
  ('control: euro to dollar', ('100.00', '1.083471', 300, 2), ['1.115975', '111.60', '3.00']),
  ('control: dinar card', ('250.00', '0.307133', 400, 3), ['0.319418', '79.855', '4.00']),
  ('control: zero markup', ('12.34', '1.2', 0, 2), ['1.200000', '14.81', '0.00'])],
 [('regression disclosure-rounding-direction 1', ('3353.43', '37.36650', 275, 0), ['38.394079', '128752', '2.76']),
  ('regression disclosure-rounding-direction 2', ('1171.26', '0.4683115', -50, 3), ['0.465970', '545.772', '-0.49']),
  ('partial repair guard 1', ('4003.90', '112.95238', -50, 2), ['112.387618', '449988.78', '-0.50']),
  ('partial repair guard 2', ('3826.89', '1.5416740', -50, 2), ['1.533966', '5870.32', '-0.49']),
  ('control: dinar card', ('250.00', '0.307133', 400, 3), ['0.319418', '79.855', '4.00']),
  ('control: promotional discount', ('80.00', '0.913377', -50, 2), ['0.908810', '72.70', '-0.50']),
  ('control: zero markup', ('12.34', '1.2', 0, 2), ['1.200000', '14.81', '0.00']),
  ('control: euro to dollar', ('100.00', '1.083471', 300, 2), ['1.115975', '111.60', '3.00'])],
 [('regression disclosure-rounding-direction 1', ('157.22', '31.44890', 150, 3), ['31.920634', '5018.562', '1.51']),
  ('regression disclosure-rounding-direction 2', ('3157.31', '0.0891399', 150, 3), ['0.090477', '285.664', '1.51']),
  ('partial repair guard 1', ('1099.42', '0.8959289', -50, 3), ['0.891449', '980.077', '-0.50']),
  ('partial repair guard 2', ('839.76', '1.4166337', -50, 2), ['1.409551', '1183.68', '-0.49']),
  ('control: euro to dollar', ('100.00', '1.083471', 300, 2), ['1.115975', '111.60', '3.00']),
  ('control: yen to dollar', ('5000', '0.00662', 350, 2), ['0.006852', '34.26', '3.51']),
  ('control: to yen card', ('49.99', '151.3742', 275, 0), ['155.536991', '7775', '2.76']),
  ('control: dinar card', ('250.00', '0.307133', 400, 3), ['0.319418', '79.855', '4.00'])],
 [('regression disclosure-rounding-direction 1', ('508.07', '12.046349', 275, 0), ['12.377624', '6289', '2.76']),
  ('regression disclosure-rounding-direction 2', ('3826.89', '1.5416740', -50, 2), ['1.533966', '5870.32', '-0.49']),
  ('partial repair guard 1', ('2901.49', '0.0386239', -50, 3), ['0.038431', '111.507', '-0.49']),
  ('partial repair guard 2', ('304.69', '0.8083309', -50, 0), ['0.804289', '245', '-0.50']),
  ('control: dinar card', ('250.00', '0.307133', 400, 3), ['0.319418', '79.855', '4.00']),
  ('control: promotional discount', ('80.00', '0.913377', -50, 2), ['0.908810', '72.70', '-0.50']),
  ('control: zero markup', ('12.34', '1.2', 0, 2), ['1.200000', '14.81', '0.00']),
  ('control: euro to dollar', ('100.00', '1.083471', 300, 2), ['1.115975', '111.60', '3.00'])],
 [('regression disclosure-rounding-direction 1', ('419.33', '78.12165', 350, 2), ['80.855908', '33905.31', '3.51']),
  ('regression disclosure-rounding-direction 2', ('2378.17', '122.42736', 300, 2), ['126.100181', '299887.67', '3.01']),
  ('partial repair guard 1', ('635.41', '83.19332', -50, 2), ['82.777353', '52597.56', '-0.50']),
  ('partial repair guard 2', ('1155.57', '2.42284', -75, 0), ['2.404669', '2779', '-0.74']),
  ('control: euro to dollar', ('100.00', '1.083471', 300, 2), ['1.115975', '111.60', '3.00']),
  ('control: yen to dollar', ('5000', '0.00662', 350, 2), ['0.006852', '34.26', '3.51']),
  ('control: to yen card', ('49.99', '151.3742', 275, 0), ['155.536991', '7775', '2.76']),
  ('control: dinar card', ('250.00', '0.307133', 400, 3), ['0.319418', '79.855', '4.00'])]]
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 disclosure-rounding-direction 1	`['0.006852', '34.26', '3.50']`	`['0.006852', '34.26', '3.51']`	Failed
regression disclosure-rounding-direction 2	`['155.536991', '7775', '2.75']`	`['155.536991', '7775', '2.76']`	Failed
partial repair guard 1	`['0.908810', '72.70', '-0.50']`	`['0.908810', '72.70', '-0.50']`	Passed
partial repair guard 2	`['0.465970', '545.772', '-0.50']`	`['0.465970', '545.772', '-0.49']`	Failed
control: euro to dollar	`['1.115975', '111.60', '3.00']`	`['1.115975', '111.60', '3.00']`	Passed
control: dinar card	`['0.319418', '79.855', '4.00']`	`['0.319418', '79.855', '4.00']`	Passed
control: zero markup	`['1.200000', '14.81', '0.00']`	`['1.200000', '14.81', '0.00']`	Passed

SHA-256 / c28718af0b176906ed6e7ff3916c4f32d111ef7f531e53c20394a27c8b2bdcda

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
from decimal import Decimal, ROUND_HALF_EVEN, ROUND_HALF_UP, ROUND_FLOOR, ROUND_CEILING, ROUND_DOWN, ROUND_UP, ROUND_HALF_DOWN
N = 1
observations = []
def solve(amount, ref_rate, markup_bps, card_exp):
    ref = Decimal(ref_rate)
    raw = ref * (1 + Decimal(markup_bps) / 10000)
    dcc = raw.quantize(Decimal('0.000001'), rounding=ROUND_HALF_UP)
    card = (Decimal(amount) * dcc).quantize(Decimal(1).scaleb(-card_exp), rounding=ROUND_HALF_UP)
    pct = ((dcc / ref - 1) * 100).quantize(Decimal('0.01'), rounding=ROUND_UP)
    return [format(dcc, 'f'), format(card, 'f'), format(pct, 'f')]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
cases = [[('regression disclosure-rounding-direction 1', ('5000', '0.00662', 350, 2), ['0.006852', '34.26', '3.51']),
  ('regression disclosure-rounding-direction 2', ('49.99', '151.3742', 275, 0), ['155.536991', '7775', '2.76']),
  ('partial repair guard 1', ('80.00', '0.913377', -50, 2), ['0.908810', '72.70', '-0.50']),
  ('partial repair guard 2', ('1171.26', '0.4683115', -50, 3), ['0.465970', '545.772', '-0.49']),
  ('control: euro to dollar', ('100.00', '1.083471', 300, 2), ['1.115975', '111.60', '3.00']),
  ('control: dinar card', ('250.00', '0.307133', 400, 3), ['0.319418', '79.855', '4.00']),
  ('control: zero markup', ('12.34', '1.2', 0, 2), ['1.200000', '14.81', '0.00'])],
 [('regression disclosure-rounding-direction 1', ('3353.43', '37.36650', 275, 0), ['38.394079', '128752', '2.76']),
  ('regression disclosure-rounding-direction 2', ('1171.26', '0.4683115', -50, 3), ['0.465970', '545.772', '-0.49']),
  ('partial repair guard 1', ('4003.90', '112.95238', -50, 2), ['112.387618', '449988.78', '-0.50']),
  ('partial repair guard 2', ('3826.89', '1.5416740', -50, 2), ['1.533966', '5870.32', '-0.49']),
  ('control: dinar card', ('250.00', '0.307133', 400, 3), ['0.319418', '79.855', '4.00']),
  ('control: promotional discount', ('80.00', '0.913377', -50, 2), ['0.908810', '72.70', '-0.50']),
  ('control: zero markup', ('12.34', '1.2', 0, 2), ['1.200000', '14.81', '0.00']),
  ('control: euro to dollar', ('100.00', '1.083471', 300, 2), ['1.115975', '111.60', '3.00'])],
 [('regression disclosure-rounding-direction 1', ('157.22', '31.44890', 150, 3), ['31.920634', '5018.562', '1.51']),
  ('regression disclosure-rounding-direction 2', ('3157.31', '0.0891399', 150, 3), ['0.090477', '285.664', '1.51']),
  ('partial repair guard 1', ('1099.42', '0.8959289', -50, 3), ['0.891449', '980.077', '-0.50']),
  ('partial repair guard 2', ('839.76', '1.4166337', -50, 2), ['1.409551', '1183.68', '-0.49']),
  ('control: euro to dollar', ('100.00', '1.083471', 300, 2), ['1.115975', '111.60', '3.00']),
  ('control: yen to dollar', ('5000', '0.00662', 350, 2), ['0.006852', '34.26', '3.51']),
  ('control: to yen card', ('49.99', '151.3742', 275, 0), ['155.536991', '7775', '2.76']),
  ('control: dinar card', ('250.00', '0.307133', 400, 3), ['0.319418', '79.855', '4.00'])],
 [('regression disclosure-rounding-direction 1', ('508.07', '12.046349', 275, 0), ['12.377624', '6289', '2.76']),
  ('regression disclosure-rounding-direction 2', ('3826.89', '1.5416740', -50, 2), ['1.533966', '5870.32', '-0.49']),
  ('partial repair guard 1', ('2901.49', '0.0386239', -50, 3), ['0.038431', '111.507', '-0.49']),
  ('partial repair guard 2', ('304.69', '0.8083309', -50, 0), ['0.804289', '245', '-0.50']),
  ('control: dinar card', ('250.00', '0.307133', 400, 3), ['0.319418', '79.855', '4.00']),
  ('control: promotional discount', ('80.00', '0.913377', -50, 2), ['0.908810', '72.70', '-0.50']),
  ('control: zero markup', ('12.34', '1.2', 0, 2), ['1.200000', '14.81', '0.00']),
  ('control: euro to dollar', ('100.00', '1.083471', 300, 2), ['1.115975', '111.60', '3.00'])],
 [('regression disclosure-rounding-direction 1', ('419.33', '78.12165', 350, 2), ['80.855908', '33905.31', '3.51']),
  ('regression disclosure-rounding-direction 2', ('2378.17', '122.42736', 300, 2), ['126.100181', '299887.67', '3.01']),
  ('partial repair guard 1', ('635.41', '83.19332', -50, 2), ['82.777353', '52597.56', '-0.50']),
  ('partial repair guard 2', ('1155.57', '2.42284', -75, 0), ['2.404669', '2779', '-0.74']),
  ('control: euro to dollar', ('100.00', '1.083471', 300, 2), ['1.115975', '111.60', '3.00']),
  ('control: yen to dollar', ('5000', '0.00662', 350, 2), ['0.006852', '34.26', '3.51']),
  ('control: to yen card', ('49.99', '151.3742', 275, 0), ['155.536991', '7775', '2.76']),
  ('control: dinar card', ('250.00', '0.307133', 400, 3), ['0.319418', '79.855', '4.00'])]]
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 disclosure-rounding-direction 1	`['0.006852', '34.26', '3.51']`	`['0.006852', '34.26', '3.51']`	Passed
regression disclosure-rounding-direction 2	`['155.536991', '7775', '2.76']`	`['155.536991', '7775', '2.76']`	Passed
partial repair guard 1	`['0.908810', '72.70', '-0.51']`	`['0.908810', '72.70', '-0.50']`	Failed
partial repair guard 2	`['0.465970', '545.772', '-0.50']`	`['0.465970', '545.772', '-0.49']`	Failed
control: euro to dollar	`['1.115975', '111.60', '3.00']`	`['1.115975', '111.60', '3.00']`	Passed
control: dinar card	`['0.319418', '79.855', '4.00']`	`['0.319418', '79.855', '4.00']`	Passed
control: zero markup	`['1.200000', '14.81', '0.00']`	`['1.200000', '14.81', '0.00']`	Passed

SHA-256 / caea889c9a6c6fc8ac44dfd87a839dc0c3a1fdde129d8e23bfabf2156de40ce1

3 / The verified repair

Exit 0

"""Failure Map reference implementation. Python standard library only."""
import json
from decimal import Decimal, ROUND_HALF_EVEN, ROUND_HALF_UP, ROUND_FLOOR, ROUND_CEILING, ROUND_DOWN, ROUND_UP, ROUND_HALF_DOWN
N = 1
observations = []
def solve(amount, ref_rate, markup_bps, card_exp):
    ref = Decimal(ref_rate)
    raw = ref * (1 + Decimal(markup_bps) / 10000)
    dcc = raw.quantize(Decimal('0.000001'), rounding=ROUND_HALF_UP)
    card = (Decimal(amount) * dcc).quantize(Decimal(1).scaleb(-card_exp), rounding=ROUND_HALF_UP)
    pct = ((dcc / ref - 1) * 100).quantize(Decimal('0.01'), rounding=ROUND_CEILING)
    return [format(dcc, 'f'), format(card, 'f'), format(pct, 'f')]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
cases = [[('regression disclosure-rounding-direction 1', ('5000', '0.00662', 350, 2), ['0.006852', '34.26', '3.51']),
  ('regression disclosure-rounding-direction 2', ('49.99', '151.3742', 275, 0), ['155.536991', '7775', '2.76']),
  ('partial repair guard 1', ('80.00', '0.913377', -50, 2), ['0.908810', '72.70', '-0.50']),
  ('partial repair guard 2', ('1171.26', '0.4683115', -50, 3), ['0.465970', '545.772', '-0.49']),
  ('control: euro to dollar', ('100.00', '1.083471', 300, 2), ['1.115975', '111.60', '3.00']),
  ('control: dinar card', ('250.00', '0.307133', 400, 3), ['0.319418', '79.855', '4.00']),
  ('control: zero markup', ('12.34', '1.2', 0, 2), ['1.200000', '14.81', '0.00'])],
 [('regression disclosure-rounding-direction 1', ('3353.43', '37.36650', 275, 0), ['38.394079', '128752', '2.76']),
  ('regression disclosure-rounding-direction 2', ('1171.26', '0.4683115', -50, 3), ['0.465970', '545.772', '-0.49']),
  ('partial repair guard 1', ('4003.90', '112.95238', -50, 2), ['112.387618', '449988.78', '-0.50']),
  ('partial repair guard 2', ('3826.89', '1.5416740', -50, 2), ['1.533966', '5870.32', '-0.49']),
  ('control: dinar card', ('250.00', '0.307133', 400, 3), ['0.319418', '79.855', '4.00']),
  ('control: promotional discount', ('80.00', '0.913377', -50, 2), ['0.908810', '72.70', '-0.50']),
  ('control: zero markup', ('12.34', '1.2', 0, 2), ['1.200000', '14.81', '0.00']),
  ('control: euro to dollar', ('100.00', '1.083471', 300, 2), ['1.115975', '111.60', '3.00'])],
 [('regression disclosure-rounding-direction 1', ('157.22', '31.44890', 150, 3), ['31.920634', '5018.562', '1.51']),
  ('regression disclosure-rounding-direction 2', ('3157.31', '0.0891399', 150, 3), ['0.090477', '285.664', '1.51']),
  ('partial repair guard 1', ('1099.42', '0.8959289', -50, 3), ['0.891449', '980.077', '-0.50']),
  ('partial repair guard 2', ('839.76', '1.4166337', -50, 2), ['1.409551', '1183.68', '-0.49']),
  ('control: euro to dollar', ('100.00', '1.083471', 300, 2), ['1.115975', '111.60', '3.00']),
  ('control: yen to dollar', ('5000', '0.00662', 350, 2), ['0.006852', '34.26', '3.51']),
  ('control: to yen card', ('49.99', '151.3742', 275, 0), ['155.536991', '7775', '2.76']),
  ('control: dinar card', ('250.00', '0.307133', 400, 3), ['0.319418', '79.855', '4.00'])],
 [('regression disclosure-rounding-direction 1', ('508.07', '12.046349', 275, 0), ['12.377624', '6289', '2.76']),
  ('regression disclosure-rounding-direction 2', ('3826.89', '1.5416740', -50, 2), ['1.533966', '5870.32', '-0.49']),
  ('partial repair guard 1', ('2901.49', '0.0386239', -50, 3), ['0.038431', '111.507', '-0.49']),
  ('partial repair guard 2', ('304.69', '0.8083309', -50, 0), ['0.804289', '245', '-0.50']),
  ('control: dinar card', ('250.00', '0.307133', 400, 3), ['0.319418', '79.855', '4.00']),
  ('control: promotional discount', ('80.00', '0.913377', -50, 2), ['0.908810', '72.70', '-0.50']),
  ('control: zero markup', ('12.34', '1.2', 0, 2), ['1.200000', '14.81', '0.00']),
  ('control: euro to dollar', ('100.00', '1.083471', 300, 2), ['1.115975', '111.60', '3.00'])],
 [('regression disclosure-rounding-direction 1', ('419.33', '78.12165', 350, 2), ['80.855908', '33905.31', '3.51']),
  ('regression disclosure-rounding-direction 2', ('2378.17', '122.42736', 300, 2), ['126.100181', '299887.67', '3.01']),
  ('partial repair guard 1', ('635.41', '83.19332', -50, 2), ['82.777353', '52597.56', '-0.50']),
  ('partial repair guard 2', ('1155.57', '2.42284', -75, 0), ['2.404669', '2779', '-0.74']),
  ('control: euro to dollar', ('100.00', '1.083471', 300, 2), ['1.115975', '111.60', '3.00']),
  ('control: yen to dollar', ('5000', '0.00662', 350, 2), ['0.006852', '34.26', '3.51']),
  ('control: to yen card', ('49.99', '151.3742', 275, 0), ['155.536991', '7775', '2.76']),
  ('control: dinar card', ('250.00', '0.307133', 400, 3), ['0.319418', '79.855', '4.00'])]]
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 disclosure-rounding-direction 1	`['0.006852', '34.26', '3.51']`	`['0.006852', '34.26', '3.51']`	Passed
regression disclosure-rounding-direction 2	`['155.536991', '7775', '2.76']`	`['155.536991', '7775', '2.76']`	Passed
partial repair guard 1	`['0.908810', '72.70', '-0.50']`	`['0.908810', '72.70', '-0.50']`	Passed
partial repair guard 2	`['0.465970', '545.772', '-0.49']`	`['0.465970', '545.772', '-0.49']`	Passed
control: euro to dollar	`['1.115975', '111.60', '3.00']`	`['1.115975', '111.60', '3.00']`	Passed
control: dinar card	`['0.319418', '79.855', '4.00']`	`['0.319418', '79.855', '4.00']`	Passed
control: zero markup	`['1.200000', '14.81', '0.00']`	`['1.200000', '14.81', '0.00']`	Passed

SHA-256 / 2a84f1c845ba1756dfddffa68311ab7f89c06491660a20640b06273b71d6431d

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 2026-09-29T14:47:02.337369+00:00.

Case digest / 17b0cbefc55a091a0144ff7d9909f4e4954f6e8cbbab92cbd66a9d336be0a344