{"abstract":"Prefix tokens cannot see the rest of their prefix.","category":"Attention mask construction","checks":6,"contract":"Positions [0,p) are a bidirectional prefix. Positions >=p attend only to suffix positions from p through the current query.","contract_signature":"length, prefix","evaluation_group":"prefix_lm_v1","failed_approach":"Suffix rows that attend to prefix positions leak prefix context into the separately defined suffix-only contract.","family":"w3-attention-mask-construction-prefix-lm","id":"FA-98171","implementations":{"attempt":{"sha256":"6cc7dfdb94e44f8d4112f7a5c0a871adea0b6ce82ba1725f4b79046e246bfbfd","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(length, prefix):\n    return [[j<prefix or j<=i for j in range(length)] for i in range(length)]\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck('prefix and suffix',solve(N+2,2),[[j<2 if i<2 else 2<=j<=i for j in range(N+2)] for i in range(N+2)])\ncheck('full prefix',solve(3,3),[[True]*3 for _ in range(3)])\ncheck('no prefix',solve(3,0),[[j<=i for j in range(3)] for i in range(3)])\ncheck('empty',solve(0,0),[])\ncheck('singleton prefix',solve(1,1),[[True]])\ncheck('suffix boundary excludes prefix',solve(4,1)[1:],[[False,True,False,False],[False,True,True,False],[False,True,True,True]])\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":"645ed0af29fa72eeb8cb742d8072b6c6e7a1a9ef69c50b5aa35103ca29db40da","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(length, prefix):\n    return [[j<=i for j in range(length)] for i in range(length)]\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck('prefix and suffix',solve(N+2,2),[[j<2 if i<2 else 2<=j<=i for j in range(N+2)] for i in range(N+2)])\ncheck('full prefix',solve(3,3),[[True]*3 for _ in range(3)])\ncheck('no prefix',solve(3,0),[[j<=i for j in range(3)] for i in range(3)])\ncheck('empty',solve(0,0),[])\ncheck('singleton prefix',solve(1,1),[[True]])\ncheck('suffix boundary excludes prefix',solve(4,1)[1:],[[False,True,False,False],[False,True,True,False],[False,True,True,True]])\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":"These are toy Boolean matrix contracts; they do not define a production model architecture or tensor library behavior. 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":"w3-attention-mask-construction-prefix-lm","generated_at":"2026-09-30T16:48:10.193976+00:00","license":"CC0-1.0","python":"3.12.14","seed":1,"split":"open-access"},"relevance":"A small explicit visibility matrix makes sequence-attention boundary behavior reviewable.","root_cause":"A causal mask only permits earlier keys for prefix queries.","sha256":"9529fadcfe246589558e75e0349cb8b31e0d940c9285572d931035fb853bd971","title":"Prefix tokens cannot see the rest of their prefix · case 01","variant":1,"variant_policy":"The fixture code references N, supplied as 1 through 5. Fixed boundary checks may repeat. These records share one mechanism and must remain together in evaluation splits.","verified":true,"visibility":"public","verification":{"attempt":{"elapsed_ms":26.74,"exit_code":1,"observations":[{"actual":[[true,true,false],[true,true,false],[true,true,true]],"check":"prefix and suffix","expected":[[true,true,false],[true,true,false],[false,false,true]],"passed":false},{"actual":[[true,true,true],[true,true,true],[true,true,true]],"check":"full prefix","expected":[[true,true,true],[true,true,true],[true,true,true]],"passed":true},{"actual":[[true,false,false],[true,true,false],[true,true,true]],"check":"no prefix","expected":[[true,false,false],[true,true,false],[true,true,true]],"passed":true},{"actual":[],"check":"empty","expected":[],"passed":true},{"actual":[[true]],"check":"singleton prefix","expected":[[true]],"passed":true},{"actual":[[true,true,false,false],[true,true,true,false],[true,true,true,true]],"check":"suffix boundary excludes prefix","expected":[[false,true,false,false],[false,true,true,false],[false,true,true,true]],"passed":false}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"prefix and suffix\", \"actual\": [[true, true, false], [true, true, false], [true, true, true]], \"expected\": [[true, true, false], [true, true, false], [false, false, true]], \"passed\": false}, {\"check\": \"full prefix\", \"actual\": [[true, true, true], [true, true, true], [true, true, true]], \"expected\": [[true, true, true], [true, true, true], [true, true, true]], \"passed\": true}, {\"check\": \"no prefix\", \"actual\": [[true, false, false], [true, true, false], [true, true, true]], \"expected\": [[true, false, false], [true, true, false], [true, true, true]], \"passed\": true}, {\"check\": \"empty\", \"actual\": [], \"expected\": [], \"passed\": true}, {\"check\": \"singleton prefix\", \"actual\": [[true]], \"expected\": [[true]], \"passed\": true}, {\"check\": \"suffix boundary excludes prefix\", \"actual\": [[true, true, false, false], [true, true, true, false], [true, true, true, true]], \"expected\": [[false, true, false, false], [false, true, true, false], [false, true, true, true]], \"passed\": false}], \"passed\": false}\n"},"broken":{"elapsed_ms":28.297,"exit_code":1,"observations":[{"actual":[[true,false,false],[true,true,false],[true,true,true]],"check":"prefix and suffix","expected":[[true,true,false],[true,true,false],[false,false,true]],"passed":false},{"actual":[[true,false,false],[true,true,false],[true,true,true]],"check":"full prefix","expected":[[true,true,true],[true,true,true],[true,true,true]],"passed":false},{"actual":[[true,false,false],[true,true,false],[true,true,true]],"check":"no prefix","expected":[[true,false,false],[true,true,false],[true,true,true]],"passed":true},{"actual":[],"check":"empty","expected":[],"passed":true},{"actual":[[true]],"check":"singleton prefix","expected":[[true]],"passed":true},{"actual":[[true,true,false,false],[true,true,true,false],[true,true,true,true]],"check":"suffix boundary excludes prefix","expected":[[false,true,false,false],[false,true,true,false],[false,true,true,true]],"passed":false}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"prefix and suffix\", \"actual\": [[true, false, false], [true, true, false], [true, true, true]], \"expected\": [[true, true, false], [true, true, false], [false, false, true]], \"passed\": false}, {\"check\": \"full prefix\", \"actual\": [[true, false, false], [true, true, false], [true, true, true]], \"expected\": [[true, true, true], [true, true, true], [true, true, true]], \"passed\": false}, {\"check\": \"no prefix\", \"actual\": [[true, false, false], [true, true, false], [true, true, true]], \"expected\": [[true, false, false], [true, true, false], [true, true, true]], \"passed\": true}, {\"check\": \"empty\", \"actual\": [], \"expected\": [], \"passed\": true}, {\"check\": \"singleton prefix\", \"actual\": [[true]], \"expected\": [[true]], \"passed\": true}, {\"check\": \"suffix boundary excludes prefix\", \"actual\": [[true, true, false, false], [true, true, true, false], [true, true, true, true]], \"expected\": [[false, true, false, false], [false, true, true, false], [false, true, true, true]], \"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."}}