{"abstract":"One-based weekday is subtracted as a zero-based offset.","category":"Calendar arithmetic","checks":4,"contract":"Valid ISO Gregorian dates in years 1 through 9999; generated dates must remain in that range. Month is 1 through 12 and zero-based weekday is Monday 0 through Sunday 6 unless the output convention says otherwise. Monday start of week. Exact operational definition: (date.fromisoformat(text)-timedelta(days=date.fromisoformat(text).weekday())).isoformat()","contract_signature":"text","evaluation_group":"model-83bfb425e9368442","failed_approach":"The week is anchored to Sunday rather than Monday.","family":"xn-monday-start-of-week","id":"FA-6531","implementations":{"attempt":{"sha256":"f7665317b45ac8712d478474d84488b90c2be566cd6ac8fa663a414db121fbea","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\nimport math\nimport calendar\nimport statistics\nimport itertools\nfrom fractions import Fraction\nfrom datetime import date, datetime, timedelta, timezone\nfrom decimal import Decimal, ROUND_HALF_UP, ROUND_DOWN, ROUND_CEILING, ROUND_FLOOR\n\nN = 1\nobservations = []\ndef solve(text):\n    return (date.fromisoformat(text)-timedelta(days=(date.fromisoformat(text).weekday()+1)%7)).isoformat()\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck(\"fixture 1: ('2024-01-01',)\", solve(*('2024-01-01',)), '2024-01-01')\ncheck(\"fixture 2: ('2024-01-07',)\", solve(*('2024-01-07',)), '2024-01-01')\ncheck(\"fixture 3: ('2024-01-10',)\", solve(*('2024-01-10',)), '2024-01-08')\ncheck(\"fixture 4: ('2023-01-01',)\", solve(*('2023-01-01',)), '2022-12-26')\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":"49799bae819297980715db47c37ff2fcfb0d3bcbc4523a55258ef941dad9e2a8","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\nimport math\nimport calendar\nimport statistics\nimport itertools\nfrom fractions import Fraction\nfrom datetime import date, datetime, timedelta, timezone\nfrom decimal import Decimal, ROUND_HALF_UP, ROUND_DOWN, ROUND_CEILING, ROUND_FLOOR\n\nN = 1\nobservations = []\ndef solve(text):\n    return (date.fromisoformat(text)-timedelta(days=date.fromisoformat(text).isoweekday())).isoformat()\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck(\"fixture 1: ('2024-01-01',)\", solve(*('2024-01-01',)), '2024-01-01')\ncheck(\"fixture 2: ('2024-01-07',)\", solve(*('2024-01-07',)), '2024-01-01')\ncheck(\"fixture 3: ('2024-01-10',)\", solve(*('2024-01-10',)), '2024-01-08')\ncheck(\"fixture 4: ('2023-01-01',)\", solve(*('2023-01-01',)), '2022-12-26')\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":" 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":"xn-monday-start-of-week","generated_at":"2026-09-29T14:38:02.988308+00:00","license":"CC0-1.0","python":"3.12.14","seed":1,"split":"open-access"},"relevance":"Small exact fixtures expose this error without platform timing, external services, or probabilistic observations. Calendar conventions results depend on the stated convention.","root_cause":"One-based weekday is subtracted as a zero-based offset.","sha256":"e715092c5a88229725180e0d9fb6fb481d2001f50a03617abfdc9df54b8bc300","title":"Monday start of week · case 01","variant":1,"variant_policy":"Five numbered records share a model and may reuse boundary fixtures.","verified":true,"visibility":"public","verification":{"attempt":{"elapsed_ms":44.96,"exit_code":1,"observations":[{"actual":"2023-12-31","check":"fixture 1: ('2024-01-01',)","expected":"2024-01-01","passed":false},{"actual":"2024-01-07","check":"fixture 2: ('2024-01-07',)","expected":"2024-01-01","passed":false},{"actual":"2024-01-07","check":"fixture 3: ('2024-01-10',)","expected":"2024-01-08","passed":false},{"actual":"2023-01-01","check":"fixture 4: ('2023-01-01',)","expected":"2022-12-26","passed":false}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"fixture 1: ('2024-01-01',)\", \"actual\": \"2023-12-31\", \"expected\": \"2024-01-01\", \"passed\": false}, {\"check\": \"fixture 2: ('2024-01-07',)\", \"actual\": \"2024-01-07\", \"expected\": \"2024-01-01\", \"passed\": false}, {\"check\": \"fixture 3: ('2024-01-10',)\", \"actual\": \"2024-01-07\", \"expected\": \"2024-01-08\", \"passed\": false}, {\"check\": \"fixture 4: ('2023-01-01',)\", \"actual\": \"2023-01-01\", \"expected\": \"2022-12-26\", \"passed\": false}], \"passed\": false}\n"},"broken":{"elapsed_ms":42.005,"exit_code":1,"observations":[{"actual":"2023-12-31","check":"fixture 1: ('2024-01-01',)","expected":"2024-01-01","passed":false},{"actual":"2023-12-31","check":"fixture 2: ('2024-01-07',)","expected":"2024-01-01","passed":false},{"actual":"2024-01-07","check":"fixture 3: ('2024-01-10',)","expected":"2024-01-08","passed":false},{"actual":"2022-12-25","check":"fixture 4: ('2023-01-01',)","expected":"2022-12-26","passed":false}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"fixture 1: ('2024-01-01',)\", \"actual\": \"2023-12-31\", \"expected\": \"2024-01-01\", \"passed\": false}, {\"check\": \"fixture 2: ('2024-01-07',)\", \"actual\": \"2023-12-31\", \"expected\": \"2024-01-01\", \"passed\": false}, {\"check\": \"fixture 3: ('2024-01-10',)\", \"actual\": \"2024-01-07\", \"expected\": \"2024-01-08\", \"passed\": false}, {\"check\": \"fixture 4: ('2023-01-01',)\", \"actual\": \"2022-12-25\", \"expected\": \"2022-12-26\", \"passed\": false}], \"passed\": false}\n"}},"member_only":{"stages":["fixed"],"fields":["implementations.fixed","verification.fixed","harness","repair"],"note":"The verified repair, its recorded checks, the repair description, and the scoring harness are available to members."}}