{"abstract":"Vary header union is case insensitive and stable.","category":"Content negotiation","checks":4,"contract":"Bounded offline decision model: vary header union is case insensitive and stable. Inputs are already validated protocol fields; the explicit fixtures define the supported policy, not a complete protocol implementation.","evaluation_group":"xh-vary-header-union-is-case-insensitive-and-stable","failed_approach":"The attempted repair uses `sorted(set(old+new))` and still violates a separate fixture.","family":"xh-vary-header-union-is-case-insensitive-and-stable","id":"FA-6911","implementations":{"attempt":{"sha256":"0a1b54c39f68c6ed0602d251c7288c472dab6571d6ba6c1b344afb6a47d18329","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\nimport ipaddress\nimport re\nfrom urllib.parse import urlsplit, urljoin\nN = 1\nobservations = []\ndef solve(old, new):\n    return sorted(set(old+new))\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\n_fixtures = [\"check('fixture 1', solve(['Accept'], ['accept', 'Encoding']), ['accept', 'encoding'])\", \"check('fixture 2', solve([], []), [])\", \"check('fixture 3', solve(['z'], ['a']), ['z', 'a'])\", \"check('fixture 4', solve(['A'], []), ['a'])\"]\nfor _line in _fixtures[N % len(_fixtures):] + _fixtures[:N % len(_fixtures)]:\n    exec(_line)\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":"0103758b3773ea1b2369dc282fee147b1ccf97bb47c190a01b03bfd5b556f480","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\nimport ipaddress\nimport re\nfrom urllib.parse import urlsplit, urljoin\nN = 1\nobservations = []\ndef solve(old, new):\n    return old+new\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\n_fixtures = [\"check('fixture 1', solve(['Accept'], ['accept', 'Encoding']), ['accept', 'encoding'])\", \"check('fixture 2', solve([], []), [])\", \"check('fixture 3', solve(['z'], ['a']), ['z', 'a'])\", \"check('fixture 4', solve(['A'], []), ['a'])\"]\nfor _line in _fixtures[N % len(_fixtures):] + _fixtures[:N % len(_fixtures)]:\n    exec(_line)\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":"c09160cc9c8e51e47095219cf1b2cd48908017eae7ec47cfafe412a734d2e2ea","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\nimport ipaddress\nimport re\nfrom urllib.parse import urlsplit, urljoin\nN = 1\nobservations = []\ndef solve(old, new):\n    return list(dict.fromkeys(x.lower() for x in old+new))\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\n_fixtures = [\"check('fixture 1', solve(['Accept'], ['accept', 'Encoding']), ['accept', 'encoding'])\", \"check('fixture 2', solve([], []), [])\", \"check('fixture 3', solve(['z'], ['a']), ['z', 'a'])\", \"check('fixture 4', solve(['A'], []), ['a'])\"]\nfor _line in _fixtures[N % len(_fixtures):] + _fixtures[:N % len(_fixtures)]:\n    exec(_line)\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":"Bounded deterministic policy model over validated inputs; not a complete protocol stack or an interoperability claim. The five variants rotate the same explicit fixture set. 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":"xh-vary-header-union-is-case-insensitive-and-stable","generated_at":"2026-09-29T14:38:06.441251+00:00","license":"CC0-1.0","python":"3.12.14","seed":1,"split":"open-access"},"relevance":"This deterministic content negotiation model isolates the vary-header-union-is-case-insensitive-and-stable policy at a network component boundary.","repair":"Apply the explicit vary-header-union-is-case-insensitive-and-stable contract, including the tested boundary and negative cases.","root_cause":"The faulty implementation uses `old+new` for the vary-header-union-is-case-insensitive-and-stable decision.","sha256":"65aa5b45b4d51290ff9bef2ee166a6442b63d72ed0709fd17dca2cd2aa3bf740","title":"Vary header union is case insensitive and stable · case 01","variant":1,"variant_policy":"Five numbered records share a model and may reuse boundary fixtures.","verification":{"attempt":{"elapsed_ms":37.03,"exit_code":1,"observations":[{"actual":[],"check":"fixture 2","expected":[],"passed":true},{"actual":["a","z"],"check":"fixture 3","expected":["z","a"],"passed":false},{"actual":["A"],"check":"fixture 4","expected":["a"],"passed":false},{"actual":["Accept","Encoding","accept"],"check":"fixture 1","expected":["accept","encoding"],"passed":false}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"fixture 2\", \"actual\": [], \"expected\": [], \"passed\": true}, {\"check\": \"fixture 3\", \"actual\": [\"a\", \"z\"], \"expected\": [\"z\", \"a\"], \"passed\": false}, {\"check\": \"fixture 4\", \"actual\": [\"A\"], \"expected\": [\"a\"], \"passed\": false}, {\"check\": \"fixture 1\", \"actual\": [\"Accept\", \"Encoding\", \"accept\"], \"expected\": [\"accept\", \"encoding\"], \"passed\": false}], \"passed\": false}\n"},"broken":{"elapsed_ms":41.333,"exit_code":1,"observations":[{"actual":[],"check":"fixture 2","expected":[],"passed":true},{"actual":["z","a"],"check":"fixture 3","expected":["z","a"],"passed":true},{"actual":["A"],"check":"fixture 4","expected":["a"],"passed":false},{"actual":["Accept","accept","Encoding"],"check":"fixture 1","expected":["accept","encoding"],"passed":false}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"fixture 2\", \"actual\": [], \"expected\": [], \"passed\": true}, {\"check\": \"fixture 3\", \"actual\": [\"z\", \"a\"], \"expected\": [\"z\", \"a\"], \"passed\": true}, {\"check\": \"fixture 4\", \"actual\": [\"A\"], \"expected\": [\"a\"], \"passed\": false}, {\"check\": \"fixture 1\", \"actual\": [\"Accept\", \"accept\", \"Encoding\"], \"expected\": [\"accept\", \"encoding\"], \"passed\": false}], \"passed\": false}\n"},"fixed":{"elapsed_ms":39.681,"exit_code":0,"observations":[{"actual":[],"check":"fixture 2","expected":[],"passed":true},{"actual":["z","a"],"check":"fixture 3","expected":["z","a"],"passed":true},{"actual":["a"],"check":"fixture 4","expected":["a"],"passed":true},{"actual":["accept","encoding"],"check":"fixture 1","expected":["accept","encoding"],"passed":true}],"passed":true,"stderr":"","stdout":"{\"observations\": [{\"check\": \"fixture 2\", \"actual\": [], \"expected\": [], \"passed\": true}, {\"check\": \"fixture 3\", \"actual\": [\"z\", \"a\"], \"expected\": [\"z\", \"a\"], \"passed\": true}, {\"check\": \"fixture 4\", \"actual\": [\"a\"], \"expected\": [\"a\"], \"passed\": true}, {\"check\": \"fixture 1\", \"actual\": [\"accept\", \"encoding\"], \"expected\": [\"accept\", \"encoding\"], \"passed\": true}], \"passed\": true}\n"}},"verified":true,"visibility":"public"}