{"abstract":"Bidirectional block attention is accidentally causal.","category":"Attention mask construction","checks":6,"contract":"Square matrix where each query sees every key with the same block ID.","contract_signature":"blocks","evaluation_group":"block_bidirectional_v1","failed_approach":"A one-token radius omits valid distant keys in the same bidirectional block.","family":"w3-attention-mask-construction-bidirectional-block","id":"FA-98166","implementations":{"attempt":{"sha256":"7c318ff42a739cdb985d6bd867b712c2f629f086c0cf23a6448789d21b8123f3","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(blocks):\n    return [[blocks[i]==blocks[j] and abs(i-j)<=1 for j in range(len(blocks))] for i in range(len(blocks))]\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\nb=[0]*N+[1,1]\ncheck('future within block',solve(b),[[b[i]==b[j] for j in range(len(b))] for i in range(len(b))])\ncheck('two-way pair',solve([8,8]),[[True,True],[True,True]])\ncheck('isolated blocks',solve([1,2]),[[True,False],[False,True]])\ncheck('empty',solve([]),[])\ncheck('singleton',solve([3]),[[True]])\ncheck('repeated separated ID',solve([1,2,1]),[[True,False,True],[False,True,False],[True,False,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":"3dd12e9abc7f25bc84fdd54750feaf646c180166f1a8a33b686ae708bd581846","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(blocks):\n    return [[blocks[i]==blocks[j] and j<=i for j in range(len(blocks))] for i in range(len(blocks))]\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\nb=[0]*N+[1,1]\ncheck('future within block',solve(b),[[b[i]==b[j] for j in range(len(b))] for i in range(len(b))])\ncheck('two-way pair',solve([8,8]),[[True,True],[True,True]])\ncheck('isolated blocks',solve([1,2]),[[True,False],[False,True]])\ncheck('empty',solve([]),[])\ncheck('singleton',solve([3]),[[True]])\ncheck('repeated separated ID',solve([1,2,1]),[[True,False,True],[False,True,False],[True,False,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-bidirectional-block","generated_at":"2026-09-30T16:48:10.192888+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 block-local mask retains causal ordering when each block is fully bidirectional.","sha256":"cdba072670077abdc5d14f053d97d2ec6855fbd0076c78ddd7a162be65b7b836","title":"Bidirectional block attention is accidentally causal · 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":27.357,"exit_code":1,"observations":[{"actual":[[true,false,false],[false,true,true],[false,true,true]],"check":"future within block","expected":[[true,false,false],[false,true,true],[false,true,true]],"passed":true},{"actual":[[true,true],[true,true]],"check":"two-way pair","expected":[[true,true],[true,true]],"passed":true},{"actual":[[true,false],[false,true]],"check":"isolated blocks","expected":[[true,false],[false,true]],"passed":true},{"actual":[],"check":"empty","expected":[],"passed":true},{"actual":[[true]],"check":"singleton","expected":[[true]],"passed":true},{"actual":[[true,false,false],[false,true,false],[false,false,true]],"check":"repeated separated ID","expected":[[true,false,true],[false,true,false],[true,false,true]],"passed":false}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"future within block\", \"actual\": [[true, false, false], [false, true, true], [false, true, true]], \"expected\": [[true, false, false], [false, true, true], [false, true, true]], \"passed\": true}, {\"check\": \"two-way pair\", \"actual\": [[true, true], [true, true]], \"expected\": [[true, true], [true, true]], \"passed\": true}, {\"check\": \"isolated blocks\", \"actual\": [[true, false], [false, true]], \"expected\": [[true, false], [false, true]], \"passed\": true}, {\"check\": \"empty\", \"actual\": [], \"expected\": [], \"passed\": true}, {\"check\": \"singleton\", \"actual\": [[true]], \"expected\": [[true]], \"passed\": true}, {\"check\": \"repeated separated ID\", \"actual\": [[true, false, false], [false, true, false], [false, false, true]], \"expected\": [[true, false, true], [false, true, false], [true, false, true]], \"passed\": false}], \"passed\": false}\n"},"broken":{"elapsed_ms":28.093,"exit_code":1,"observations":[{"actual":[[true,false,false],[false,true,false],[false,true,true]],"check":"future within block","expected":[[true,false,false],[false,true,true],[false,true,true]],"passed":false},{"actual":[[true,false],[true,true]],"check":"two-way pair","expected":[[true,true],[true,true]],"passed":false},{"actual":[[true,false],[false,true]],"check":"isolated blocks","expected":[[true,false],[false,true]],"passed":true},{"actual":[],"check":"empty","expected":[],"passed":true},{"actual":[[true]],"check":"singleton","expected":[[true]],"passed":true},{"actual":[[true,false,false],[false,true,false],[true,false,true]],"check":"repeated separated ID","expected":[[true,false,true],[false,true,false],[true,false,true]],"passed":false}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"future within block\", \"actual\": [[true, false, false], [false, true, false], [false, true, true]], \"expected\": [[true, false, false], [false, true, true], [false, true, true]], \"passed\": false}, {\"check\": \"two-way pair\", \"actual\": [[true, false], [true, true]], \"expected\": [[true, true], [true, true]], \"passed\": false}, {\"check\": \"isolated blocks\", \"actual\": [[true, false], [false, true]], \"expected\": [[true, false], [false, true]], \"passed\": true}, {\"check\": \"empty\", \"actual\": [], \"expected\": [], \"passed\": true}, {\"check\": \"singleton\", \"actual\": [[true]], \"expected\": [[true]], \"passed\": true}, {\"check\": \"repeated separated ID\", \"actual\": [[true, false, false], [false, true, false], [true, false, true]], \"expected\": [[true, false, true], [false, true, false], [true, false, 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."}}