FAILURE MAP
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FA-3931 / Encodings / Open access

An optional BOM is appended instead of prepended · case 01

An optional BOM is appended instead of prepended.

Verified by executionVariant 1 · 4 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The implementation applies an operation whose text or grammar semantics violate this contract: Encode text as UTF-8 with exactly one generated leading BOM; preserve any BOM in the input text.

VERIFIED REPAIR

Implement the complete stated contract, including the boundary fixtures: Encode text as UTF-8 with exactly one generated leading BOM; preserve any BOM in the input text.

Unsuccessful approach: The attempted repair handles the primary example but still violates a separate boundary of the same contract.

Case contract

Encode text as UTF-8 with exactly one generated leading BOM; preserve any BOM in the input text.

Why this case matters

A local executable model for consumers of structured text; oracle values are authored literals, not outputs copied from the repaired implementation.

1 / The failure

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json
import re, unicodedata, json, csv, io, html, base64, binascii, codecs, struct
from urllib.parse import quote, unquote, unquote_to_bytes, urlencode, parse_qsl, urlsplit, urlunsplit
from email.header import decode_header, make_header
from email.utils import getaddresses
from xml.etree import ElementTree as ET
import shlex, string, textwrap

N = 1
observations = []
def solve(x):
    try:
        return list(x.encode('utf-8'))
    except Exception as exc:
        return {"error": type(exc).__name__}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('boundary 1', solve('a'), [239, 187, 191, 97])
check('boundary 2', solve(''), [239, 187, 191])
check('boundary 3', solve('\ufeff'), [239, 187, 191, 239, 187, 191])
check('boundary 4', solve('é'), [239, 187, 191, 195, 169])
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)
Boundary fixtureActualExpectedOutcome
boundary 1[97][239, 187, 191, 97]Failed
boundary 2[][239, 187, 191]Failed
boundary 3[239, 187, 191][239, 187, 191, 239, 187, 191]Failed
boundary 4[195, 169][239, 187, 191, 195, 169]Failed

SHA-256 / 91e25cb7b5d77bdb7dbb22c02959bcb1439f6274633a865b9291953f9d994441

2 / The unsuccessful fix

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json
import re, unicodedata, json, csv, io, html, base64, binascii, codecs, struct
from urllib.parse import quote, unquote, unquote_to_bytes, urlencode, parse_qsl, urlsplit, urlunsplit
from email.header import decode_header, make_header
from email.utils import getaddresses
from xml.etree import ElementTree as ET
import shlex, string, textwrap

N = 1
observations = []
def solve(x):
    try:
        return list(x.encode('utf-8')+bytes.fromhex('efbbbf'))
    except Exception as exc:
        return {"error": type(exc).__name__}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('boundary 1', solve('a'), [239, 187, 191, 97])
check('boundary 2', solve(''), [239, 187, 191])
check('boundary 3', solve('\ufeff'), [239, 187, 191, 239, 187, 191])
check('boundary 4', solve('é'), [239, 187, 191, 195, 169])
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)
Boundary fixtureActualExpectedOutcome
boundary 1[97, 239, 187, 191][239, 187, 191, 97]Failed
boundary 2[239, 187, 191][239, 187, 191]Passed
boundary 3[239, 187, 191, 239, 187, 191][239, 187, 191, 239, 187, 191]Passed
boundary 4[195, 169, 239, 187, 191][239, 187, 191, 195, 169]Failed

SHA-256 / b767ce18b008b6b5bf0a09e1dddb3e59a8cd98ef06b1e266c0441ac8aa101a7a

3 / The verified repair

Exit 0
"""Failure Map reference implementation. Python standard library only."""
import json
import re, unicodedata, json, csv, io, html, base64, binascii, codecs, struct
from urllib.parse import quote, unquote, unquote_to_bytes, urlencode, parse_qsl, urlsplit, urlunsplit
from email.header import decode_header, make_header
from email.utils import getaddresses
from xml.etree import ElementTree as ET
import shlex, string, textwrap

N = 1
observations = []
def solve(x):
    try:
        return list(x.encode('utf-8-sig'))
    except Exception as exc:
        return {"error": type(exc).__name__}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('boundary 1', solve('a'), [239, 187, 191, 97])
check('boundary 2', solve(''), [239, 187, 191])
check('boundary 3', solve('\ufeff'), [239, 187, 191, 239, 187, 191])
check('boundary 4', solve('é'), [239, 187, 191, 195, 169])
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)
Boundary fixtureActualExpectedOutcome
boundary 1[239, 187, 191, 97][239, 187, 191, 97]Passed
boundary 2[239, 187, 191][239, 187, 191]Passed
boundary 3[239, 187, 191, 239, 187, 191][239, 187, 191, 239, 187, 191]Passed
boundary 4[239, 187, 191, 195, 169][239, 187, 191, 195, 169]Passed

SHA-256 / 6b8d6ef61ab2c0c7fd160ff54b1adfcb148a8aca1b1efc4c0716c2e3a9bbe7a5

Verification & scope

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.

Observations recorded using Python 3.12.14 at 2026-09-29T14:37:33.216376+00:00.

Case digest / 80367087e2ebf9aef6109340345fb1aa2ed569bdd5bd8463b4c89c1762a7a737