{"abstract":"Customers whose payment was already refunded receive an unused-time credit on top of the refund.","category":"Subscription proration billing","checks":8,"contract":"Input {list, paid, status paid|partial|open|void|refunded, left, period}. Open, void and refunded invoices earn no unused-time credit. Paid and partially paid invoices earn paid*left/period half-up (the amount actually collected, not the list price). Return cents.","evaluation_group":"w2-subscription-proration-unused-time-credit-basis","failed_approach":"The attempt excludes only invoices with zero recorded payment, but refunded invoices keep their original paid amount.","family":"w2-subscription-proration-unused-time-credit-basis-refunded-invoice-exclusion","id":"FA-59836","implementations":{"attempt":{"sha256":"f656857f8a73c8cc8e1b180ab9b22fb3afc8553e8f3a0fe5bab1e008ae55c7bd","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(x):\n    if x['status'] in ('open', 'void') or x['paid'] == 0:\n        return 0\n    return (x['paid'] * x['left'] * 2 + x['period']) // (2 * x['period'])\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\nfixtures = [[('regression', {'list': 1000, 'paid': 461, 'status': 'refunded', 'left': 10, 'period': 31}, 0), ('regression', {'list': 4900, 'paid': 2450, 'status': 'refunded', 'left': 20, 'period': 31}, 0), ('partial-repair probe', {'list': 9900, 'paid': 4739, 'status': 'refunded', 'left': 24, 'period': 31}, 0), ('partial-repair probe', {'list': 4900, 'paid': 1142, 'status': 'refunded', 'left': 8, 'period': 31}, 0), ('normal control', {'list': 9900, 'paid': 4950, 'status': 'void', 'left': 9, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 0, 'status': 'void', 'left': 9, 'period': 31}, 0), ('normal control', {'list': 9900, 'paid': 0, 'status': 'open', 'left': 4, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 2450, 'status': 'open', 'left': 29, 'period': 31}, 0)], [('regression', {'list': 4900, 'paid': 2450, 'status': 'refunded', 'left': 20, 'period': 31}, 0), ('regression', {'list': 9900, 'paid': 4739, 'status': 'refunded', 'left': 24, 'period': 31}, 0), ('partial-repair probe', {'list': 4900, 'paid': 1142, 'status': 'refunded', 'left': 8, 'period': 31}, 0), ('partial-repair probe', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 7, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 2450, 'status': 'partial', 'left': 24, 'period': 31}, 1897), ('normal control', {'list': 4900, 'paid': 4900, 'status': 'paid', 'left': 19, 'period': 31}, 3003), ('normal control', {'list': 1000, 'paid': 500, 'status': 'open', 'left': 15, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 2450, 'status': 'paid', 'left': 6, 'period': 31}, 474)], [('regression', {'list': 9900, 'paid': 4739, 'status': 'refunded', 'left': 24, 'period': 31}, 0), ('regression', {'list': 4900, 'paid': 1142, 'status': 'refunded', 'left': 8, 'period': 31}, 0), ('partial-repair probe', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 7, 'period': 31}, 0), ('partial-repair probe', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 25, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 0, 'status': 'refunded', 'left': 1, 'period': 31}, 0), ('normal control', {'list': 1000, 'paid': 0, 'status': 'partial', 'left': 13, 'period': 31}, 0), ('normal control', {'list': 1000, 'paid': 500, 'status': 'open', 'left': 9, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 2450, 'status': 'open', 'left': 20, 'period': 31}, 0)], [('regression', {'list': 4900, 'paid': 1142, 'status': 'refunded', 'left': 8, 'period': 31}, 0), ('regression', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 7, 'period': 31}, 0), ('partial-repair probe', {'list': 4900, 'paid': 2450, 'status': 'refunded', 'left': 10, 'period': 31}, 0), ('partial-repair probe', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 26, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 4900, 'status': 'void', 'left': 3, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 4900, 'status': 'open', 'left': 25, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 935, 'status': 'partial', 'left': 23, 'period': 31}, 694), ('normal control', {'list': 4900, 'paid': 0, 'status': 'void', 'left': 29, 'period': 31}, 0)], [('regression', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 7, 'period': 31}, 0), ('regression', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 25, 'period': 31}, 0), ('partial-repair probe', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 22, 'period': 31}, 0), ('partial-repair probe', {'list': 1000, 'paid': 14, 'status': 'refunded', 'left': 31, 'period': 31}, 0), ('normal control', {'list': 9900, 'paid': 2540, 'status': 'open', 'left': 27, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 0, 'status': 'refunded', 'left': 6, 'period': 31}, 0), ('normal control', {'list': 9900, 'paid': 4950, 'status': 'paid', 'left': 25, 'period': 31}, 3992), ('normal control', {'list': 4900, 'paid': 3688, 'status': 'partial', 'left': 5, 'period': 31}, 595)]]\nfor i, (label, args, expected) in enumerate(fixtures[N-1]):\n    check(\"%s %d\" % (label, i), solve(args), expected)\nprint(json.dumps({\"observations\": observations, \"passed\": all(x[\"passed\"] for x in observations)}, ensure_ascii=False))\nraise SystemExit(0 if all(x[\"passed\"] for x in observations) else 1)\n"},"broken":{"sha256":"7de310d026b3466fa367c0b08e1a4f7f8c9ee334226687f9bdde216d0e6a647c","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(x):\n    if x['status'] in ('open', 'void'):\n        return 0\n    return (x['paid'] * x['left'] * 2 + x['period']) // (2 * x['period'])\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\nfixtures = [[('regression', {'list': 1000, 'paid': 461, 'status': 'refunded', 'left': 10, 'period': 31}, 0), ('regression', {'list': 4900, 'paid': 2450, 'status': 'refunded', 'left': 20, 'period': 31}, 0), ('partial-repair probe', {'list': 9900, 'paid': 4739, 'status': 'refunded', 'left': 24, 'period': 31}, 0), ('partial-repair probe', {'list': 4900, 'paid': 1142, 'status': 'refunded', 'left': 8, 'period': 31}, 0), ('normal control', {'list': 9900, 'paid': 4950, 'status': 'void', 'left': 9, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 0, 'status': 'void', 'left': 9, 'period': 31}, 0), ('normal control', {'list': 9900, 'paid': 0, 'status': 'open', 'left': 4, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 2450, 'status': 'open', 'left': 29, 'period': 31}, 0)], [('regression', {'list': 4900, 'paid': 2450, 'status': 'refunded', 'left': 20, 'period': 31}, 0), ('regression', {'list': 9900, 'paid': 4739, 'status': 'refunded', 'left': 24, 'period': 31}, 0), ('partial-repair probe', {'list': 4900, 'paid': 1142, 'status': 'refunded', 'left': 8, 'period': 31}, 0), ('partial-repair probe', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 7, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 2450, 'status': 'partial', 'left': 24, 'period': 31}, 1897), ('normal control', {'list': 4900, 'paid': 4900, 'status': 'paid', 'left': 19, 'period': 31}, 3003), ('normal control', {'list': 1000, 'paid': 500, 'status': 'open', 'left': 15, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 2450, 'status': 'paid', 'left': 6, 'period': 31}, 474)], [('regression', {'list': 9900, 'paid': 4739, 'status': 'refunded', 'left': 24, 'period': 31}, 0), ('regression', {'list': 4900, 'paid': 1142, 'status': 'refunded', 'left': 8, 'period': 31}, 0), ('partial-repair probe', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 7, 'period': 31}, 0), ('partial-repair probe', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 25, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 0, 'status': 'refunded', 'left': 1, 'period': 31}, 0), ('normal control', {'list': 1000, 'paid': 0, 'status': 'partial', 'left': 13, 'period': 31}, 0), ('normal control', {'list': 1000, 'paid': 500, 'status': 'open', 'left': 9, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 2450, 'status': 'open', 'left': 20, 'period': 31}, 0)], [('regression', {'list': 4900, 'paid': 1142, 'status': 'refunded', 'left': 8, 'period': 31}, 0), ('regression', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 7, 'period': 31}, 0), ('partial-repair probe', {'list': 4900, 'paid': 2450, 'status': 'refunded', 'left': 10, 'period': 31}, 0), ('partial-repair probe', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 26, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 4900, 'status': 'void', 'left': 3, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 4900, 'status': 'open', 'left': 25, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 935, 'status': 'partial', 'left': 23, 'period': 31}, 694), ('normal control', {'list': 4900, 'paid': 0, 'status': 'void', 'left': 29, 'period': 31}, 0)], [('regression', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 7, 'period': 31}, 0), ('regression', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 25, 'period': 31}, 0), ('partial-repair probe', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 22, 'period': 31}, 0), ('partial-repair probe', {'list': 1000, 'paid': 14, 'status': 'refunded', 'left': 31, 'period': 31}, 0), ('normal control', {'list': 9900, 'paid': 2540, 'status': 'open', 'left': 27, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 0, 'status': 'refunded', 'left': 6, 'period': 31}, 0), ('normal control', {'list': 9900, 'paid': 4950, 'status': 'paid', 'left': 25, 'period': 31}, 3992), ('normal control', {'list': 4900, 'paid': 3688, 'status': 'partial', 'left': 5, 'period': 31}, 595)]]\nfor i, (label, args, expected) in enumerate(fixtures[N-1]):\n    check(\"%s %d\" % (label, i), solve(args), expected)\nprint(json.dumps({\"observations\": observations, \"passed\": all(x[\"passed\"] for x in observations)}, ensure_ascii=False))\nraise SystemExit(0 if all(x[\"passed\"] for x in observations) else 1)\n"},"fixed":{"sha256":"c101e45805668082f9f34ac924327912d8031069965e79ff6a760cad79fa19f1","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(x):\n    if x['status'] in ('open', 'void', 'refunded'):\n        return 0\n    return (x['paid'] * x['left'] * 2 + x['period']) // (2 * x['period'])\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\nfixtures = [[('regression', {'list': 1000, 'paid': 461, 'status': 'refunded', 'left': 10, 'period': 31}, 0), ('regression', {'list': 4900, 'paid': 2450, 'status': 'refunded', 'left': 20, 'period': 31}, 0), ('partial-repair probe', {'list': 9900, 'paid': 4739, 'status': 'refunded', 'left': 24, 'period': 31}, 0), ('partial-repair probe', {'list': 4900, 'paid': 1142, 'status': 'refunded', 'left': 8, 'period': 31}, 0), ('normal control', {'list': 9900, 'paid': 4950, 'status': 'void', 'left': 9, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 0, 'status': 'void', 'left': 9, 'period': 31}, 0), ('normal control', {'list': 9900, 'paid': 0, 'status': 'open', 'left': 4, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 2450, 'status': 'open', 'left': 29, 'period': 31}, 0)], [('regression', {'list': 4900, 'paid': 2450, 'status': 'refunded', 'left': 20, 'period': 31}, 0), ('regression', {'list': 9900, 'paid': 4739, 'status': 'refunded', 'left': 24, 'period': 31}, 0), ('partial-repair probe', {'list': 4900, 'paid': 1142, 'status': 'refunded', 'left': 8, 'period': 31}, 0), ('partial-repair probe', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 7, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 2450, 'status': 'partial', 'left': 24, 'period': 31}, 1897), ('normal control', {'list': 4900, 'paid': 4900, 'status': 'paid', 'left': 19, 'period': 31}, 3003), ('normal control', {'list': 1000, 'paid': 500, 'status': 'open', 'left': 15, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 2450, 'status': 'paid', 'left': 6, 'period': 31}, 474)], [('regression', {'list': 9900, 'paid': 4739, 'status': 'refunded', 'left': 24, 'period': 31}, 0), ('regression', {'list': 4900, 'paid': 1142, 'status': 'refunded', 'left': 8, 'period': 31}, 0), ('partial-repair probe', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 7, 'period': 31}, 0), ('partial-repair probe', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 25, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 0, 'status': 'refunded', 'left': 1, 'period': 31}, 0), ('normal control', {'list': 1000, 'paid': 0, 'status': 'partial', 'left': 13, 'period': 31}, 0), ('normal control', {'list': 1000, 'paid': 500, 'status': 'open', 'left': 9, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 2450, 'status': 'open', 'left': 20, 'period': 31}, 0)], [('regression', {'list': 4900, 'paid': 1142, 'status': 'refunded', 'left': 8, 'period': 31}, 0), ('regression', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 7, 'period': 31}, 0), ('partial-repair probe', {'list': 4900, 'paid': 2450, 'status': 'refunded', 'left': 10, 'period': 31}, 0), ('partial-repair probe', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 26, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 4900, 'status': 'void', 'left': 3, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 4900, 'status': 'open', 'left': 25, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 935, 'status': 'partial', 'left': 23, 'period': 31}, 694), ('normal control', {'list': 4900, 'paid': 0, 'status': 'void', 'left': 29, 'period': 31}, 0)], [('regression', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 7, 'period': 31}, 0), ('regression', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 25, 'period': 31}, 0), ('partial-repair probe', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 22, 'period': 31}, 0), ('partial-repair probe', {'list': 1000, 'paid': 14, 'status': 'refunded', 'left': 31, 'period': 31}, 0), ('normal control', {'list': 9900, 'paid': 2540, 'status': 'open', 'left': 27, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 0, 'status': 'refunded', 'left': 6, 'period': 31}, 0), ('normal control', {'list': 9900, 'paid': 4950, 'status': 'paid', 'left': 25, 'period': 31}, 3992), ('normal control', {'list': 4900, 'paid': 3688, 'status': 'partial', 'left': 5, 'period': 31}, 595)]]\nfor i, (label, args, expected) in enumerate(fixtures[N-1]):\n    check(\"%s %d\" % (label, i), solve(args), expected)\nprint(json.dumps({\"observations\": observations, \"passed\": all(x[\"passed\"] for x in observations)}, ensure_ascii=False))\nraise SystemExit(0 if all(x[\"passed\"] for x in observations) else 1)\n"}},"limitations":"A deterministic teaching model of a stipulated billing rule. It makes no claim to reproduce any billing provider's exact behaviour and is not billing 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.","method":"Deterministic executable model with adversarial boundary fixtures.","provenance":{"created_by":"Failure Map","dependencies":"Python standard library","family":"w2-subscription-proration-unused-time-credit-basis-refunded-invoice-exclusion","generated_at":"2026-09-29T14:46:40.093210+00:00","license":"CC0-1.0","python":"3.12.14","seed":1,"split":"open-access"},"relevance":"Credits for unused time must not exceed what the customer actually paid for the period.","repair":"Restore the contract rule at the refunded invoice exclusion step: use `if x['status'] in ('open', 'void', 'refunded'):`.","root_cause":"Refunded invoices are not excluded from unused-time credits.","sha256":"2b63d1881fa42ad86ccb1d5966207e1511740120fe97999a57a752b0fdb2b988","title":"Unused-time credit based on amount paid: refunded invoice exclusion · case 01","variant":1,"variant_policy":"Five numbered records share a model and may reuse boundary fixtures.","verification":{"attempt":{"elapsed_ms":40.126,"exit_code":1,"observations":[{"actual":149,"check":"regression 0","expected":0,"passed":false},{"actual":1581,"check":"regression 1","expected":0,"passed":false},{"actual":3669,"check":"partial-repair probe 2","expected":0,"passed":false},{"actual":295,"check":"partial-repair probe 3","expected":0,"passed":false},{"actual":0,"check":"normal control 4","expected":0,"passed":true},{"actual":0,"check":"normal control 5","expected":0,"passed":true},{"actual":0,"check":"normal control 6","expected":0,"passed":true},{"actual":0,"check":"normal control 7","expected":0,"passed":true}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"regression 0\", \"actual\": 149, \"expected\": 0, \"passed\": false}, {\"check\": \"regression 1\", \"actual\": 1581, \"expected\": 0, \"passed\": false}, {\"check\": \"partial-repair probe 2\", \"actual\": 3669, \"expected\": 0, \"passed\": false}, {\"check\": \"partial-repair probe 3\", \"actual\": 295, \"expected\": 0, \"passed\": false}, {\"check\": \"normal control 4\", \"actual\": 0, \"expected\": 0, \"passed\": true}, {\"check\": \"normal control 5\", \"actual\": 0, \"expected\": 0, \"passed\": true}, {\"check\": \"normal control 6\", \"actual\": 0, \"expected\": 0, \"passed\": true}, {\"check\": \"normal control 7\", \"actual\": 0, \"expected\": 0, \"passed\": true}], \"passed\": false}\n"},"broken":{"elapsed_ms":41.69,"exit_code":1,"observations":[{"actual":149,"check":"regression 0","expected":0,"passed":false},{"actual":1581,"check":"regression 1","expected":0,"passed":false},{"actual":3669,"check":"partial-repair probe 2","expected":0,"passed":false},{"actual":295,"check":"partial-repair probe 3","expected":0,"passed":false},{"actual":0,"check":"normal control 4","expected":0,"passed":true},{"actual":0,"check":"normal control 5","expected":0,"passed":true},{"actual":0,"check":"normal control 6","expected":0,"passed":true},{"actual":0,"check":"normal control 7","expected":0,"passed":true}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"regression 0\", \"actual\": 149, \"expected\": 0, \"passed\": false}, {\"check\": \"regression 1\", \"actual\": 1581, \"expected\": 0, \"passed\": false}, {\"check\": \"partial-repair probe 2\", \"actual\": 3669, \"expected\": 0, \"passed\": false}, {\"check\": \"partial-repair probe 3\", \"actual\": 295, \"expected\": 0, \"passed\": false}, {\"check\": \"normal control 4\", \"actual\": 0, \"expected\": 0, \"passed\": true}, {\"check\": \"normal control 5\", \"actual\": 0, \"expected\": 0, \"passed\": true}, {\"check\": \"normal control 6\", \"actual\": 0, \"expected\": 0, \"passed\": true}, {\"check\": \"normal control 7\", \"actual\": 0, \"expected\": 0, \"passed\": true}], \"passed\": false}\n"},"fixed":{"elapsed_ms":40.878,"exit_code":0,"observations":[{"actual":0,"check":"regression 0","expected":0,"passed":true},{"actual":0,"check":"regression 1","expected":0,"passed":true},{"actual":0,"check":"partial-repair probe 2","expected":0,"passed":true},{"actual":0,"check":"partial-repair probe 3","expected":0,"passed":true},{"actual":0,"check":"normal control 4","expected":0,"passed":true},{"actual":0,"check":"normal control 5","expected":0,"passed":true},{"actual":0,"check":"normal control 6","expected":0,"passed":true},{"actual":0,"check":"normal control 7","expected":0,"passed":true}],"passed":true,"stderr":"","stdout":"{\"observations\": [{\"check\": \"regression 0\", \"actual\": 0, \"expected\": 0, \"passed\": true}, {\"check\": \"regression 1\", \"actual\": 0, \"expected\": 0, \"passed\": true}, {\"check\": \"partial-repair probe 2\", \"actual\": 0, \"expected\": 0, \"passed\": true}, {\"check\": \"partial-repair probe 3\", \"actual\": 0, \"expected\": 0, \"passed\": true}, {\"check\": \"normal control 4\", \"actual\": 0, \"expected\": 0, \"passed\": true}, {\"check\": \"normal control 5\", \"actual\": 0, \"expected\": 0, \"passed\": true}, {\"check\": \"normal control 6\", \"actual\": 0, \"expected\": 0, \"passed\": true}, {\"check\": \"normal control 7\", \"actual\": 0, \"expected\": 0, \"passed\": true}], \"passed\": true}\n"}},"verified":true,"visibility":"public"}