FA-58471 / Loan amortization schedules / Open access

Odd first period day count: interest rounding stage · case 01

Stub interest drifts by several cents on long first periods.

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

ROOT CAUSE

The per-diem amount is rounded before being multiplied by the day count.

VERIFIED REPAIR

Round once, after multiplying by the days.

Unsuccessful approach: Rounding before dividing by the basis truncates the final result.

Case contract

x = {'principal', 'rate_bp', 'convention': '30/360'|'act/365'|'act/360', 'start': [y,m,d], 'first_due': [y,m,d]}. 30/360: a start day of 31 becomes 30; then a due day of 31 becomes 30 when the (adjusted) start day is 30; days = 360*dy + 30*dm + dd. Actual conventions count calendar days, basis 365 or 360 (365 even in leap years). If days <= 0 return {'error': 'due_not_after_start'}. Interest = round_half_up(P*bp*days/(10000*basis)). Return {'days', 'basis', 'interest'}.

Why this case matters

Amortization engines drive borrower statements, payoff quotes and investor remittances; a misplaced rounding step, boundary or ordering rule compounds across hundreds of periods.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
import datetime
N = 1
observations = []
def solve(x):
    def rnd(n, d):
        q, r = divmod(n, d)
        return q + (1 if 2 * r >= d else 0)
    y1, m1, d1 = x['start']
    y2, m2, d2 = x['first_due']
    conv = x['convention']
    if conv == '30/360':
        if d1 == 31:
            d1 = 30
        if d2 == 31 and d1 == 30:
            d2 = 30
        days = 360 * (y2 - y1) + 30 * (m2 - m1) + (d2 - d1)
        basis = 360
    else:
        days = (datetime.date(y2, m2, d2) - datetime.date(y1, m1, d1)).days
        basis = 365 if conv == 'act/365' else 360
    if days <= 0:
        return {'error': 'due_not_after_start'}
    interest = rnd(x['principal'] * x['rate_bp'], 10000 * basis) * days
    return {'days': days, 'basis': basis, 'interest': interest}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[['regression: interest rounding stage', {'principal': 12345, 'rate_bp': 365, 'convention': 'act/360', 'start': [2023, 3, 15], 'first_due': [2024, 3, 1]}, {'days': 352, 'basis': 360, 'interest': 441}], ['control 1', {'principal': 100000, 'rate_bp': 365, 'convention': 'act/360', 'start': [2024, 11, 30], 'first_due': [2024, 12, 1]}, {'days': 1, 'basis': 360, 'interest': 10}], ['control 2', {'principal': 36500, 'rate_bp': 1200, 'convention': '30/360', 'start': [2023, 3, 29], 'first_due': [2023, 3, 31]}, {'days': 2, 'basis': 360, 'interest': 24}], ['control 3', {'principal': 1000000, 'rate_bp': 365, 'convention': 'act/365', 'start': [2023, 8, 30], 'first_due': [2023, 8, 30]}, {'error': 'due_not_after_start'}], ['control 4', {'principal': 100000, 'rate_bp': 500, 'convention': 'act/365', 'start': [2024, 12, 31], 'first_due': [2024, 12, 31]}, {'error': 'due_not_after_start'}], ['control 5', {'principal': 1000000, 'rate_bp': 1200, 'convention': 'act/360', 'start': [2024, 9, 30], 'first_due': [2024, 9, 30]}, {'error': 'due_not_after_start'}], ['control 6', {'principal': 1000000, 'rate_bp': 500, 'convention': 'act/365', 'start': [2023, 12, 31], 'first_due': [2024, 1, 30]}, {'days': 30, 'basis': 365, 'interest': 4110}]], [['regression: interest rounding stage', {'principal': 12345, 'rate_bp': 600, 'convention': '30/360', 'start': [2023, 7, 28], 'first_due': [2023, 8, 28]}, {'days': 30, 'basis': 360, 'interest': 62}], ['control 1', {'principal': 12345, 'rate_bp': 600, 'convention': '30/360', 'start': [2024, 8, 29], 'first_due': [2024, 8, 29]}, {'error': 'due_not_after_start'}], ['control 2', {'principal': 36500, 'rate_bp': 1200, 'convention': 'act/365', 'start': [2024, 10, 29], 'first_due': [2025, 9, 30]}, {'days': 336, 'basis': 365, 'interest': 4032}], ['control 3', {'principal': 36500, 'rate_bp': 600, 'convention': 'act/365', 'start': [2023, 5, 15], 'first_due': [2023, 6, 30]}, {'days': 46, 'basis': 365, 'interest': 276}], ['control 4', {'principal': 1000000, 'rate_bp': 365, 'convention': 'act/365', 'start': [2023, 10, 31], 'first_due': [2023, 10, 30]}, {'error': 'due_not_after_start'}], ['control 5', {'principal': 36500, 'rate_bp': 600, 'convention': 'act/365', 'start': [2024, 11, 30], 'first_due': [2024, 12, 15]}, {'days': 15, 'basis': 365, 'interest': 90}], ['control 6', {'principal': 100000, 'rate_bp': 500, 'convention': 'act/365', 'start': [2023, 11, 30], 'first_due': [2023, 11, 30]}, {'error': 'due_not_after_start'}]], [['regression: interest rounding stage', {'principal': 12345, 'rate_bp': 500, 'convention': 'act/360', 'start': [2023, 11, 28], 'first_due': [2024, 10, 31]}, {'days': 338, 'basis': 360, 'interest': 580}], ['control 1', {'principal': 12345, 'rate_bp': 500, 'convention': '30/360', 'start': [2024, 1, 31], 'first_due': [2024, 1, 28]}, {'error': 'due_not_after_start'}], ['control 2', {'principal': 1000000, 'rate_bp': 365, 'convention': 'act/365', 'start': [2023, 12, 1], 'first_due': [2023, 12, 15]}, {'days': 14, 'basis': 365, 'interest': 1400}], ['control 3', {'principal': 1000000, 'rate_bp': 600, 'convention': 'act/360', 'start': [2023, 10, 30], 'first_due': [2023, 10, 31]}, {'days': 1, 'basis': 360, 'interest': 167}], ['control 4', {'principal': 36500, 'rate_bp': 1200, 'convention': 'act/365', 'start': [2024, 3, 29], 'first_due': [2025, 3, 31]}, {'days': 367, 'basis': 365, 'interest': 4404}], ['control 5', {'principal': 1000000, 'rate_bp': 600, 'convention': 'act/365', 'start': [2023, 4, 29], 'first_due': [2023, 4, 15]}, {'error': 'due_not_after_start'}], ['control 6', {'principal': 36500, 'rate_bp': 1200, 'convention': 'act/360', 'start': [2023, 6, 29], 'first_due': [2023, 7, 1]}, {'days': 2, 'basis': 360, 'interest': 24}]], [['regression: interest rounding stage', {'principal': 100000, 'rate_bp': 1200, 'convention': '30/360', 'start': [2023, 4, 1], 'first_due': [2024, 3, 1]}, {'days': 330, 'basis': 360, 'interest': 11000}], ['regression: interest rounding stage, partial-repair probe', {'principal': 12345, 'rate_bp': 1200, 'convention': 'act/365', 'start': [2024, 2, 29], 'first_due': [2024, 4, 29]}, {'days': 60, 'basis': 365, 'interest': 244}], ['control 1', {'principal': 36500, 'rate_bp': 500, 'convention': 'act/365', 'start': [2023, 4, 28], 'first_due': [2023, 6, 28]}, {'days': 61, 'basis': 365, 'interest': 305}], ['control 2', {'principal': 12345, 'rate_bp': 365, 'convention': 'act/360', 'start': [2024, 4, 30], 'first_due': [2024, 5, 1]}, {'days': 1, 'basis': 360, 'interest': 1}], ['control 3', {'principal': 36500, 'rate_bp': 1200, 'convention': 'act/365', 'start': [2023, 4, 30], 'first_due': [2023, 4, 30]}, {'error': 'due_not_after_start'}], ['control 4', {'principal': 36500, 'rate_bp': 600, 'convention': 'act/365', 'start': [2023, 12, 31], 'first_due': [2024, 1, 28]}, {'days': 28, 'basis': 365, 'interest': 168}], ['control 5', {'principal': 36500, 'rate_bp': 1200, 'convention': 'act/365', 'start': [2024, 11, 1], 'first_due': [2025, 11, 1]}, {'days': 365, 'basis': 365, 'interest': 4380}]], [['regression: interest rounding stage', {'principal': 1000000, 'rate_bp': 600, 'convention': 'act/360', 'start': [2024, 6, 28], 'first_due': [2024, 7, 30]}, {'days': 32, 'basis': 360, 'interest': 5333}], ['regression: interest rounding stage, partial-repair probe', {'principal': 1000000, 'rate_bp': 500, 'convention': 'act/360', 'start': [2024, 1, 29], 'first_due': [2024, 1, 30]}, {'days': 1, 'basis': 360, 'interest': 139}], ['control 1', {'principal': 36500, 'rate_bp': 500, 'convention': 'act/365', 'start': [2024, 2, 29], 'first_due': [2024, 3, 29]}, {'days': 29, 'basis': 365, 'interest': 145}], ['control 2', {'principal': 100000, 'rate_bp': 365, 'convention': 'act/365', 'start': [2024, 6, 28], 'first_due': [2025, 5, 15]}, {'days': 321, 'basis': 365, 'interest': 3210}], ['control 3', {'principal': 12345, 'rate_bp': 365, 'convention': 'act/360', 'start': [2024, 7, 30], 'first_due': [2024, 7, 30]}, {'error': 'due_not_after_start'}], ['control 4', {'principal': 100000, 'rate_bp': 365, 'convention': 'act/365', 'start': [2024, 9, 28], 'first_due': [2025, 9, 30]}, {'days': 367, 'basis': 365, 'interest': 3670}], ['control 5', {'principal': 36500, 'rate_bp': 365, 'convention': '30/360', 'start': [2024, 6, 30], 'first_due': [2024, 6, 15]}, {'error': 'due_not_after_start'}]]]
for label, args, expected in fixtures[N-1]:
    try:
        actual = solve(args)
    except Exception as exc:
        actual = 'raised ' + type(exc).__name__
    check(label, actual, 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: interest rounding stage	`{'basis': 360, 'days': 352, 'interest': 352}`	`{'basis': 360, 'days': 352, 'interest': 441}`	Failed
control 1	`{'basis': 360, 'days': 1, 'interest': 10}`	`{'basis': 360, 'days': 1, 'interest': 10}`	Passed
control 2	`{'basis': 360, 'days': 2, 'interest': 24}`	`{'basis': 360, 'days': 2, 'interest': 24}`	Passed
control 3	`{'error': 'due_not_after_start'}`	`{'error': 'due_not_after_start'}`	Passed
control 4	`{'error': 'due_not_after_start'}`	`{'error': 'due_not_after_start'}`	Passed
control 5	`{'error': 'due_not_after_start'}`	`{'error': 'due_not_after_start'}`	Passed
control 6	`{'basis': 365, 'days': 30, 'interest': 4110}`	`{'basis': 365, 'days': 30, 'interest': 4110}`	Passed

SHA-256 / 3b3051e6630323e1b19b69665fb6ef91a8885c718ba91184f080370020118da1

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
import datetime
N = 1
observations = []
def solve(x):
    def rnd(n, d):
        q, r = divmod(n, d)
        return q + (1 if 2 * r >= d else 0)
    y1, m1, d1 = x['start']
    y2, m2, d2 = x['first_due']
    conv = x['convention']
    if conv == '30/360':
        if d1 == 31:
            d1 = 30
        if d2 == 31 and d1 == 30:
            d2 = 30
        days = 360 * (y2 - y1) + 30 * (m2 - m1) + (d2 - d1)
        basis = 360
    else:
        days = (datetime.date(y2, m2, d2) - datetime.date(y1, m1, d1)).days
        basis = 365 if conv == 'act/365' else 360
    if days <= 0:
        return {'error': 'due_not_after_start'}
    interest = rnd(x['principal'] * x['rate_bp'] * days, 10000) // basis
    return {'days': days, 'basis': basis, 'interest': interest}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[['regression: interest rounding stage', {'principal': 12345, 'rate_bp': 365, 'convention': 'act/360', 'start': [2023, 3, 15], 'first_due': [2024, 3, 1]}, {'days': 352, 'basis': 360, 'interest': 441}], ['control 1', {'principal': 100000, 'rate_bp': 365, 'convention': 'act/360', 'start': [2024, 11, 30], 'first_due': [2024, 12, 1]}, {'days': 1, 'basis': 360, 'interest': 10}], ['control 2', {'principal': 36500, 'rate_bp': 1200, 'convention': '30/360', 'start': [2023, 3, 29], 'first_due': [2023, 3, 31]}, {'days': 2, 'basis': 360, 'interest': 24}], ['control 3', {'principal': 1000000, 'rate_bp': 365, 'convention': 'act/365', 'start': [2023, 8, 30], 'first_due': [2023, 8, 30]}, {'error': 'due_not_after_start'}], ['control 4', {'principal': 100000, 'rate_bp': 500, 'convention': 'act/365', 'start': [2024, 12, 31], 'first_due': [2024, 12, 31]}, {'error': 'due_not_after_start'}], ['control 5', {'principal': 1000000, 'rate_bp': 1200, 'convention': 'act/360', 'start': [2024, 9, 30], 'first_due': [2024, 9, 30]}, {'error': 'due_not_after_start'}], ['control 6', {'principal': 1000000, 'rate_bp': 500, 'convention': 'act/365', 'start': [2023, 12, 31], 'first_due': [2024, 1, 30]}, {'days': 30, 'basis': 365, 'interest': 4110}]], [['regression: interest rounding stage', {'principal': 12345, 'rate_bp': 600, 'convention': '30/360', 'start': [2023, 7, 28], 'first_due': [2023, 8, 28]}, {'days': 30, 'basis': 360, 'interest': 62}], ['control 1', {'principal': 12345, 'rate_bp': 600, 'convention': '30/360', 'start': [2024, 8, 29], 'first_due': [2024, 8, 29]}, {'error': 'due_not_after_start'}], ['control 2', {'principal': 36500, 'rate_bp': 1200, 'convention': 'act/365', 'start': [2024, 10, 29], 'first_due': [2025, 9, 30]}, {'days': 336, 'basis': 365, 'interest': 4032}], ['control 3', {'principal': 36500, 'rate_bp': 600, 'convention': 'act/365', 'start': [2023, 5, 15], 'first_due': [2023, 6, 30]}, {'days': 46, 'basis': 365, 'interest': 276}], ['control 4', {'principal': 1000000, 'rate_bp': 365, 'convention': 'act/365', 'start': [2023, 10, 31], 'first_due': [2023, 10, 30]}, {'error': 'due_not_after_start'}], ['control 5', {'principal': 36500, 'rate_bp': 600, 'convention': 'act/365', 'start': [2024, 11, 30], 'first_due': [2024, 12, 15]}, {'days': 15, 'basis': 365, 'interest': 90}], ['control 6', {'principal': 100000, 'rate_bp': 500, 'convention': 'act/365', 'start': [2023, 11, 30], 'first_due': [2023, 11, 30]}, {'error': 'due_not_after_start'}]], [['regression: interest rounding stage', {'principal': 12345, 'rate_bp': 500, 'convention': 'act/360', 'start': [2023, 11, 28], 'first_due': [2024, 10, 31]}, {'days': 338, 'basis': 360, 'interest': 580}], ['control 1', {'principal': 12345, 'rate_bp': 500, 'convention': '30/360', 'start': [2024, 1, 31], 'first_due': [2024, 1, 28]}, {'error': 'due_not_after_start'}], ['control 2', {'principal': 1000000, 'rate_bp': 365, 'convention': 'act/365', 'start': [2023, 12, 1], 'first_due': [2023, 12, 15]}, {'days': 14, 'basis': 365, 'interest': 1400}], ['control 3', {'principal': 1000000, 'rate_bp': 600, 'convention': 'act/360', 'start': [2023, 10, 30], 'first_due': [2023, 10, 31]}, {'days': 1, 'basis': 360, 'interest': 167}], ['control 4', {'principal': 36500, 'rate_bp': 1200, 'convention': 'act/365', 'start': [2024, 3, 29], 'first_due': [2025, 3, 31]}, {'days': 367, 'basis': 365, 'interest': 4404}], ['control 5', {'principal': 1000000, 'rate_bp': 600, 'convention': 'act/365', 'start': [2023, 4, 29], 'first_due': [2023, 4, 15]}, {'error': 'due_not_after_start'}], ['control 6', {'principal': 36500, 'rate_bp': 1200, 'convention': 'act/360', 'start': [2023, 6, 29], 'first_due': [2023, 7, 1]}, {'days': 2, 'basis': 360, 'interest': 24}]], [['regression: interest rounding stage', {'principal': 100000, 'rate_bp': 1200, 'convention': '30/360', 'start': [2023, 4, 1], 'first_due': [2024, 3, 1]}, {'days': 330, 'basis': 360, 'interest': 11000}], ['regression: interest rounding stage, partial-repair probe', {'principal': 12345, 'rate_bp': 1200, 'convention': 'act/365', 'start': [2024, 2, 29], 'first_due': [2024, 4, 29]}, {'days': 60, 'basis': 365, 'interest': 244}], ['control 1', {'principal': 36500, 'rate_bp': 500, 'convention': 'act/365', 'start': [2023, 4, 28], 'first_due': [2023, 6, 28]}, {'days': 61, 'basis': 365, 'interest': 305}], ['control 2', {'principal': 12345, 'rate_bp': 365, 'convention': 'act/360', 'start': [2024, 4, 30], 'first_due': [2024, 5, 1]}, {'days': 1, 'basis': 360, 'interest': 1}], ['control 3', {'principal': 36500, 'rate_bp': 1200, 'convention': 'act/365', 'start': [2023, 4, 30], 'first_due': [2023, 4, 30]}, {'error': 'due_not_after_start'}], ['control 4', {'principal': 36500, 'rate_bp': 600, 'convention': 'act/365', 'start': [2023, 12, 31], 'first_due': [2024, 1, 28]}, {'days': 28, 'basis': 365, 'interest': 168}], ['control 5', {'principal': 36500, 'rate_bp': 1200, 'convention': 'act/365', 'start': [2024, 11, 1], 'first_due': [2025, 11, 1]}, {'days': 365, 'basis': 365, 'interest': 4380}]], [['regression: interest rounding stage', {'principal': 1000000, 'rate_bp': 600, 'convention': 'act/360', 'start': [2024, 6, 28], 'first_due': [2024, 7, 30]}, {'days': 32, 'basis': 360, 'interest': 5333}], ['regression: interest rounding stage, partial-repair probe', {'principal': 1000000, 'rate_bp': 500, 'convention': 'act/360', 'start': [2024, 1, 29], 'first_due': [2024, 1, 30]}, {'days': 1, 'basis': 360, 'interest': 139}], ['control 1', {'principal': 36500, 'rate_bp': 500, 'convention': 'act/365', 'start': [2024, 2, 29], 'first_due': [2024, 3, 29]}, {'days': 29, 'basis': 365, 'interest': 145}], ['control 2', {'principal': 100000, 'rate_bp': 365, 'convention': 'act/365', 'start': [2024, 6, 28], 'first_due': [2025, 5, 15]}, {'days': 321, 'basis': 365, 'interest': 3210}], ['control 3', {'principal': 12345, 'rate_bp': 365, 'convention': 'act/360', 'start': [2024, 7, 30], 'first_due': [2024, 7, 30]}, {'error': 'due_not_after_start'}], ['control 4', {'principal': 100000, 'rate_bp': 365, 'convention': 'act/365', 'start': [2024, 9, 28], 'first_due': [2025, 9, 30]}, {'days': 367, 'basis': 365, 'interest': 3670}], ['control 5', {'principal': 36500, 'rate_bp': 365, 'convention': '30/360', 'start': [2024, 6, 30], 'first_due': [2024, 6, 15]}, {'error': 'due_not_after_start'}]]]
for label, args, expected in fixtures[N-1]:
    try:
        actual = solve(args)
    except Exception as exc:
        actual = 'raised ' + type(exc).__name__
    check(label, actual, 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: interest rounding stage	`{'basis': 360, 'days': 352, 'interest': 440}`	`{'basis': 360, 'days': 352, 'interest': 441}`	Failed
control 1	`{'basis': 360, 'days': 1, 'interest': 10}`	`{'basis': 360, 'days': 1, 'interest': 10}`	Passed
control 2	`{'basis': 360, 'days': 2, 'interest': 24}`	`{'basis': 360, 'days': 2, 'interest': 24}`	Passed
control 3	`{'error': 'due_not_after_start'}`	`{'error': 'due_not_after_start'}`	Passed
control 4	`{'error': 'due_not_after_start'}`	`{'error': 'due_not_after_start'}`	Passed
control 5	`{'error': 'due_not_after_start'}`	`{'error': 'due_not_after_start'}`	Passed
control 6	`{'basis': 365, 'days': 30, 'interest': 4109}`	`{'basis': 365, 'days': 30, 'interest': 4110}`	Failed

SHA-256 / a419d244c2e4841e6b9426d055e7ba940d5dc820950ae077ad81e0cfe2c51485

3 / The verified repair

Exit 0

"""Failure Map reference implementation. Python standard library only."""
import json
import datetime
N = 1
observations = []
def solve(x):
    def rnd(n, d):
        q, r = divmod(n, d)
        return q + (1 if 2 * r >= d else 0)
    y1, m1, d1 = x['start']
    y2, m2, d2 = x['first_due']
    conv = x['convention']
    if conv == '30/360':
        if d1 == 31:
            d1 = 30
        if d2 == 31 and d1 == 30:
            d2 = 30
        days = 360 * (y2 - y1) + 30 * (m2 - m1) + (d2 - d1)
        basis = 360
    else:
        days = (datetime.date(y2, m2, d2) - datetime.date(y1, m1, d1)).days
        basis = 365 if conv == 'act/365' else 360
    if days <= 0:
        return {'error': 'due_not_after_start'}
    interest = rnd(x['principal'] * x['rate_bp'] * days, 10000 * basis)
    return {'days': days, 'basis': basis, 'interest': interest}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[['regression: interest rounding stage', {'principal': 12345, 'rate_bp': 365, 'convention': 'act/360', 'start': [2023, 3, 15], 'first_due': [2024, 3, 1]}, {'days': 352, 'basis': 360, 'interest': 441}], ['control 1', {'principal': 100000, 'rate_bp': 365, 'convention': 'act/360', 'start': [2024, 11, 30], 'first_due': [2024, 12, 1]}, {'days': 1, 'basis': 360, 'interest': 10}], ['control 2', {'principal': 36500, 'rate_bp': 1200, 'convention': '30/360', 'start': [2023, 3, 29], 'first_due': [2023, 3, 31]}, {'days': 2, 'basis': 360, 'interest': 24}], ['control 3', {'principal': 1000000, 'rate_bp': 365, 'convention': 'act/365', 'start': [2023, 8, 30], 'first_due': [2023, 8, 30]}, {'error': 'due_not_after_start'}], ['control 4', {'principal': 100000, 'rate_bp': 500, 'convention': 'act/365', 'start': [2024, 12, 31], 'first_due': [2024, 12, 31]}, {'error': 'due_not_after_start'}], ['control 5', {'principal': 1000000, 'rate_bp': 1200, 'convention': 'act/360', 'start': [2024, 9, 30], 'first_due': [2024, 9, 30]}, {'error': 'due_not_after_start'}], ['control 6', {'principal': 1000000, 'rate_bp': 500, 'convention': 'act/365', 'start': [2023, 12, 31], 'first_due': [2024, 1, 30]}, {'days': 30, 'basis': 365, 'interest': 4110}]], [['regression: interest rounding stage', {'principal': 12345, 'rate_bp': 600, 'convention': '30/360', 'start': [2023, 7, 28], 'first_due': [2023, 8, 28]}, {'days': 30, 'basis': 360, 'interest': 62}], ['control 1', {'principal': 12345, 'rate_bp': 600, 'convention': '30/360', 'start': [2024, 8, 29], 'first_due': [2024, 8, 29]}, {'error': 'due_not_after_start'}], ['control 2', {'principal': 36500, 'rate_bp': 1200, 'convention': 'act/365', 'start': [2024, 10, 29], 'first_due': [2025, 9, 30]}, {'days': 336, 'basis': 365, 'interest': 4032}], ['control 3', {'principal': 36500, 'rate_bp': 600, 'convention': 'act/365', 'start': [2023, 5, 15], 'first_due': [2023, 6, 30]}, {'days': 46, 'basis': 365, 'interest': 276}], ['control 4', {'principal': 1000000, 'rate_bp': 365, 'convention': 'act/365', 'start': [2023, 10, 31], 'first_due': [2023, 10, 30]}, {'error': 'due_not_after_start'}], ['control 5', {'principal': 36500, 'rate_bp': 600, 'convention': 'act/365', 'start': [2024, 11, 30], 'first_due': [2024, 12, 15]}, {'days': 15, 'basis': 365, 'interest': 90}], ['control 6', {'principal': 100000, 'rate_bp': 500, 'convention': 'act/365', 'start': [2023, 11, 30], 'first_due': [2023, 11, 30]}, {'error': 'due_not_after_start'}]], [['regression: interest rounding stage', {'principal': 12345, 'rate_bp': 500, 'convention': 'act/360', 'start': [2023, 11, 28], 'first_due': [2024, 10, 31]}, {'days': 338, 'basis': 360, 'interest': 580}], ['control 1', {'principal': 12345, 'rate_bp': 500, 'convention': '30/360', 'start': [2024, 1, 31], 'first_due': [2024, 1, 28]}, {'error': 'due_not_after_start'}], ['control 2', {'principal': 1000000, 'rate_bp': 365, 'convention': 'act/365', 'start': [2023, 12, 1], 'first_due': [2023, 12, 15]}, {'days': 14, 'basis': 365, 'interest': 1400}], ['control 3', {'principal': 1000000, 'rate_bp': 600, 'convention': 'act/360', 'start': [2023, 10, 30], 'first_due': [2023, 10, 31]}, {'days': 1, 'basis': 360, 'interest': 167}], ['control 4', {'principal': 36500, 'rate_bp': 1200, 'convention': 'act/365', 'start': [2024, 3, 29], 'first_due': [2025, 3, 31]}, {'days': 367, 'basis': 365, 'interest': 4404}], ['control 5', {'principal': 1000000, 'rate_bp': 600, 'convention': 'act/365', 'start': [2023, 4, 29], 'first_due': [2023, 4, 15]}, {'error': 'due_not_after_start'}], ['control 6', {'principal': 36500, 'rate_bp': 1200, 'convention': 'act/360', 'start': [2023, 6, 29], 'first_due': [2023, 7, 1]}, {'days': 2, 'basis': 360, 'interest': 24}]], [['regression: interest rounding stage', {'principal': 100000, 'rate_bp': 1200, 'convention': '30/360', 'start': [2023, 4, 1], 'first_due': [2024, 3, 1]}, {'days': 330, 'basis': 360, 'interest': 11000}], ['regression: interest rounding stage, partial-repair probe', {'principal': 12345, 'rate_bp': 1200, 'convention': 'act/365', 'start': [2024, 2, 29], 'first_due': [2024, 4, 29]}, {'days': 60, 'basis': 365, 'interest': 244}], ['control 1', {'principal': 36500, 'rate_bp': 500, 'convention': 'act/365', 'start': [2023, 4, 28], 'first_due': [2023, 6, 28]}, {'days': 61, 'basis': 365, 'interest': 305}], ['control 2', {'principal': 12345, 'rate_bp': 365, 'convention': 'act/360', 'start': [2024, 4, 30], 'first_due': [2024, 5, 1]}, {'days': 1, 'basis': 360, 'interest': 1}], ['control 3', {'principal': 36500, 'rate_bp': 1200, 'convention': 'act/365', 'start': [2023, 4, 30], 'first_due': [2023, 4, 30]}, {'error': 'due_not_after_start'}], ['control 4', {'principal': 36500, 'rate_bp': 600, 'convention': 'act/365', 'start': [2023, 12, 31], 'first_due': [2024, 1, 28]}, {'days': 28, 'basis': 365, 'interest': 168}], ['control 5', {'principal': 36500, 'rate_bp': 1200, 'convention': 'act/365', 'start': [2024, 11, 1], 'first_due': [2025, 11, 1]}, {'days': 365, 'basis': 365, 'interest': 4380}]], [['regression: interest rounding stage', {'principal': 1000000, 'rate_bp': 600, 'convention': 'act/360', 'start': [2024, 6, 28], 'first_due': [2024, 7, 30]}, {'days': 32, 'basis': 360, 'interest': 5333}], ['regression: interest rounding stage, partial-repair probe', {'principal': 1000000, 'rate_bp': 500, 'convention': 'act/360', 'start': [2024, 1, 29], 'first_due': [2024, 1, 30]}, {'days': 1, 'basis': 360, 'interest': 139}], ['control 1', {'principal': 36500, 'rate_bp': 500, 'convention': 'act/365', 'start': [2024, 2, 29], 'first_due': [2024, 3, 29]}, {'days': 29, 'basis': 365, 'interest': 145}], ['control 2', {'principal': 100000, 'rate_bp': 365, 'convention': 'act/365', 'start': [2024, 6, 28], 'first_due': [2025, 5, 15]}, {'days': 321, 'basis': 365, 'interest': 3210}], ['control 3', {'principal': 12345, 'rate_bp': 365, 'convention': 'act/360', 'start': [2024, 7, 30], 'first_due': [2024, 7, 30]}, {'error': 'due_not_after_start'}], ['control 4', {'principal': 100000, 'rate_bp': 365, 'convention': 'act/365', 'start': [2024, 9, 28], 'first_due': [2025, 9, 30]}, {'days': 367, 'basis': 365, 'interest': 3670}], ['control 5', {'principal': 36500, 'rate_bp': 365, 'convention': '30/360', 'start': [2024, 6, 30], 'first_due': [2024, 6, 15]}, {'error': 'due_not_after_start'}]]]
for label, args, expected in fixtures[N-1]:
    try:
        actual = solve(args)
    except Exception as exc:
        actual = 'raised ' + type(exc).__name__
    check(label, actual, 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: interest rounding stage	`{'basis': 360, 'days': 352, 'interest': 441}`	`{'basis': 360, 'days': 352, 'interest': 441}`	Passed
control 1	`{'basis': 360, 'days': 1, 'interest': 10}`	`{'basis': 360, 'days': 1, 'interest': 10}`	Passed
control 2	`{'basis': 360, 'days': 2, 'interest': 24}`	`{'basis': 360, 'days': 2, 'interest': 24}`	Passed
control 3	`{'error': 'due_not_after_start'}`	`{'error': 'due_not_after_start'}`	Passed
control 4	`{'error': 'due_not_after_start'}`	`{'error': 'due_not_after_start'}`	Passed
control 5	`{'error': 'due_not_after_start'}`	`{'error': 'due_not_after_start'}`	Passed
control 6	`{'basis': 365, 'days': 30, 'interest': 4110}`	`{'basis': 365, 'days': 30, 'interest': 4110}`	Passed

SHA-256 / 03195d691f18b93b3e66b7e7267f80e7faf6aa175ce9b6ce79ee3b57fea9e8e8

Verification & scope

A deterministic bounded teaching model with stipulated toy lending rules stated in the contract; money is integer cents and rates are basis points; it makes no claim of conformance to any regulation, servicing standard or product. 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 2026-09-29T14:46:26.978081+00:00.

Case digest / 743cd74da70433038b04e6545c6ee47e58725a0b5ff9472aa9e3115e68448211