FAILURE MAP
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FA-82556 / Chess move legality / Open access

En passant square on the wrong side accepted · case 01

With white to move, an ep square on rank 3 is accepted.

Verified by executionVariant 1 · 8 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

Either rank 3 or 6 is accepted regardless of who is to move.

THE FAILURE

Either rank 3 or 6 is accepted regardless of who is to move.

Unsuccessful approach: Swapping the ranks rejects every genuine ep square.

Case contract

Input: a FEN string. Return {"pieces": {square: piece}, "turn", "castling", "ep", "halfmove", "fullmove"} or "invalid". Placement lists rank 8 first; digits skip empty squares; each rank must total 8; exactly one king per side. Turn is w or b; castling is "-" or letters from KQkq (returned as "" when "-"); ep is "-" (null) or a square on rank 6 when white is to move, rank 3 when black is; halfmove is a non-negative integer and fullmove at least 1.

Why this case matters

Every position-based legality check starts from a parsed FEN; a misparse silently changes which moves are legal.

1 / The failure

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(x):
    parts=x.split()
    if len(parts)!=6: return 'invalid'
    rows=parts[0].split('/')
    if len(rows)!=8: return 'invalid'
    board={}
    for i,row in enumerate(rows):
        rank=8-i; f=0
        for ch in row:
            if ch.isdigit(): f+=int(ch)
            elif ch in 'pnbrqkPNBRQK':
                if f>7: return 'invalid'
                board[chr(97+f)+str(rank)]=ch; f+=1
            else: return 'invalid'
        if f!=8: return 'invalid'
    vals=list(board.values())
    if vals.count('K')!=1 or vals.count('k')!=1: return 'invalid'
    turn=parts[1]
    if turn not in ('w','b'): return 'invalid'
    cast='' if parts[2]=='-' else parts[2]
    if any(c not in 'KQkq' for c in cast): return 'invalid'
    ep=None if parts[3]=='-' else parts[3]
    if ep is not None and ep[1:] not in ('3','6'): return 'invalid'
    if not (parts[4].isdigit() and parts[5].isdigit()) or int(parts[5])<1: return 'invalid'
    return {'pieces':board,'turn':turn,'castling':cast,'ep':ep,'halfmove':int(parts[4]),'fullmove':int(parts[5])}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('kings and rook', '4k3/8/8/8/8/8/8/R3K3 w Q - 0 1',
   {'castling': 'Q',
    'ep': None,
    'fullmove': 1,
    'halfmove': 0,
    'pieces': {'a1': 'R', 'e1': 'K', 'e8': 'k'},
    'turn': 'w'}),
  ('black to move ep', '4k3/8/8/8/3Pp3/8/8/4K3 b - d3 0 12',
   {'castling': '',
    'ep': 'd3',
    'fullmove': 12,
    'halfmove': 0,
    'pieces': {'d4': 'P', 'e1': 'K', 'e4': 'p', 'e8': 'k'},
    'turn': 'b'}),
  ('white ep target', '4k3/8/8/3pP3/8/8/8/4K3 w - d6 0 9',
   {'castling': '',
    'ep': 'd6',
    'fullmove': 9,
    'halfmove': 0,
    'pieces': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'},
    'turn': 'w'}),
  ('ep wrong side', '4k3/8/8/3pP3/8/8/8/4K3 w - d3 0 9', 'invalid'),
  ('no castling dash', 'r3k2r/8/8/8/8/8/8/R3K2R b - - 3 20',
   {'castling': '',
    'ep': None,
    'fullmove': 20,
    'halfmove': 3,
    'pieces': {'a1': 'R', 'a8': 'r', 'e1': 'K', 'e8': 'k', 'h1': 'R', 'h8': 'r'},
    'turn': 'b'}),
  ('black rights only', 'r3k2r/8/8/8/8/8/8/R3K2R w kq - 0 5',
   {'castling': 'kq',
    'ep': None,
    'fullmove': 5,
    'halfmove': 0,
    'pieces': {'a1': 'R', 'a8': 'r', 'e1': 'K', 'e8': 'k', 'h1': 'R', 'h8': 'r'},
    'turn': 'w'}),
  ('two white kings', '4k3/8/8/8/8/8/8/K3K3 w - - 0 1', 'invalid'),
  ('two white kings no black', '8/8/8/8/8/8/8/K3K3 w - - 0 1', 'invalid')],
 [('white ep target', '4k3/8/8/3pP3/8/8/8/4K3 w - d6 0 9',
   {'castling': '',
    'ep': 'd6',
    'fullmove': 9,
    'halfmove': 0,
    'pieces': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'},
    'turn': 'w'}),
  ('ep wrong side', '4k3/8/8/3pP3/8/8/8/4K3 w - d3 0 9', 'invalid'),
  ('two white kings', '4k3/8/8/8/8/8/8/K3K3 w - - 0 1', 'invalid'),
  ('two white kings no black', '8/8/8/8/8/8/8/K3K3 w - - 0 1', 'invalid'),
  ('missing black king', '8/8/8/8/8/8/8/4K3 w - - 0 1', 'invalid'),
  ('short rank', '4k3/8/8/8/8/8/8/4K2 w - - 0 1', 'invalid'),
  ('fullmove zero', '4k3/8/8/8/8/8/8/4K3 w - - 0 0', 'invalid'),
  ('halfmove dash', '4k3/8/8/8/8/8/8/4K3 w - - - 1', 'invalid')],
 [('black to move ep', '4k3/8/8/8/3Pp3/8/8/4K3 b - d3 0 12',
   {'castling': '',
    'ep': 'd3',
    'fullmove': 12,
    'halfmove': 0,
    'pieces': {'d4': 'P', 'e1': 'K', 'e4': 'p', 'e8': 'k'},
    'turn': 'b'}),
  ('ep wrong side', '4k3/8/8/3pP3/8/8/8/4K3 w - d3 0 9', 'invalid'),
  ('short rank', '4k3/8/8/8/8/8/8/4K2 w - - 0 1', 'invalid'),
  ('fullmove zero', '4k3/8/8/8/8/8/8/4K3 w - - 0 0', 'invalid'),
  ('halfmove dash', '4k3/8/8/8/8/8/8/4K3 w - - - 1', 'invalid'),
  ('pawns and knights', 'rn2k3/pp6/8/8/8/8/6PP/4K1NR w K - 7 30',
   {'castling': 'K',
    'ep': None,
    'fullmove': 30,
    'halfmove': 7,
    'pieces': {'a7': 'p',
               'a8': 'r',
               'b7': 'p',
               'b8': 'n',
               'e1': 'K',
               'e8': 'k',
               'g1': 'N',
               'g2': 'P',
               'h1': 'R',
               'h2': 'P'},
    'turn': 'w'}),
  ('bad castling letter', '4k3/8/8/8/8/8/8/4K3 w X - 0 1', 'invalid'),
  ('bad turn', '4k3/8/8/8/8/8/8/4K3 x - - 0 1', 'invalid')],
 [('kings and rook', '4k3/8/8/8/8/8/8/R3K3 w Q - 0 1',
   {'castling': 'Q',
    'ep': None,
    'fullmove': 1,
    'halfmove': 0,
    'pieces': {'a1': 'R', 'e1': 'K', 'e8': 'k'},
    'turn': 'w'}),
  ('black to move ep', '4k3/8/8/8/3Pp3/8/8/4K3 b - d3 0 12',
   {'castling': '',
    'ep': 'd3',
    'fullmove': 12,
    'halfmove': 0,
    'pieces': {'d4': 'P', 'e1': 'K', 'e4': 'p', 'e8': 'k'},
    'turn': 'b'}),
  ('white ep target', '4k3/8/8/3pP3/8/8/8/4K3 w - d6 0 9',
   {'castling': '',
    'ep': 'd6',
    'fullmove': 9,
    'halfmove': 0,
    'pieces': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'},
    'turn': 'w'}),
  ('ep wrong side', '4k3/8/8/3pP3/8/8/8/4K3 w - d3 0 9', 'invalid'),
  ('pawns and knights', 'rn2k3/pp6/8/8/8/8/6PP/4K1NR w K - 7 30',
   {'castling': 'K',
    'ep': None,
    'fullmove': 30,
    'halfmove': 7,
    'pieces': {'a7': 'p',
               'a8': 'r',
               'b7': 'p',
               'b8': 'n',
               'e1': 'K',
               'e8': 'k',
               'g1': 'N',
               'g2': 'P',
               'h1': 'R',
               'h2': 'P'},
    'turn': 'w'}),
  ('bad castling letter', '4k3/8/8/8/8/8/8/4K3 w X - 0 1', 'invalid'),
  ('bad turn', '4k3/8/8/8/8/8/8/4K3 x - - 0 1', 'invalid'),
  ('fullmove dash', '4k3/8/8/8/8/8/8/4K3 w - - 0 -', 'invalid')],
 [('kings and rook', '4k3/8/8/8/8/8/8/R3K3 w Q - 0 1',
   {'castling': 'Q',
    'ep': None,
    'fullmove': 1,
    'halfmove': 0,
    'pieces': {'a1': 'R', 'e1': 'K', 'e8': 'k'},
    'turn': 'w'}),
  ('black to move ep', '4k3/8/8/8/3Pp3/8/8/4K3 b - d3 0 12',
   {'castling': '',
    'ep': 'd3',
    'fullmove': 12,
    'halfmove': 0,
    'pieces': {'d4': 'P', 'e1': 'K', 'e4': 'p', 'e8': 'k'},
    'turn': 'b'}),
  ('white ep target', '4k3/8/8/3pP3/8/8/8/4K3 w - d6 0 9',
   {'castling': '',
    'ep': 'd6',
    'fullmove': 9,
    'halfmove': 0,
    'pieces': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'},
    'turn': 'w'}),
  ('ep wrong side', '4k3/8/8/3pP3/8/8/8/4K3 w - d3 0 9', 'invalid'),
  ('no castling dash', 'r3k2r/8/8/8/8/8/8/R3K2R b - - 3 20',
   {'castling': '',
    'ep': None,
    'fullmove': 20,
    'halfmove': 3,
    'pieces': {'a1': 'R', 'a8': 'r', 'e1': 'K', 'e8': 'k', 'h1': 'R', 'h8': 'r'},
    'turn': 'b'}),
  ('black rights only', 'r3k2r/8/8/8/8/8/8/R3K2R w kq - 0 5',
   {'castling': 'kq',
    'ep': None,
    'fullmove': 5,
    'halfmove': 0,
    'pieces': {'a1': 'R', 'a8': 'r', 'e1': 'K', 'e8': 'k', 'h1': 'R', 'h8': 'r'},
    'turn': 'w'}),
  ('two white kings', '4k3/8/8/8/8/8/8/K3K3 w - - 0 1', 'invalid'),
  ('fullmove dash', '4k3/8/8/8/8/8/8/4K3 w - - 0 -', 'invalid')]]
for label, args, expected in fixtures[N-1]:
    try:
        actual = solve(args)
    except Exception as exc:
        actual = 'error:' + type(exc).__name__
    check(label, actual, expected)
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
kings and rook{'castling': 'Q', 'ep': None, 'fullmove': 1, 'halfmove': 0, 'pieces': {'a1': 'R', 'e1': 'K', 'e8': 'k'}, 'turn': 'w'}{'castling': 'Q', 'ep': None, 'fullmove': 1, 'halfmove': 0, 'pieces': {'a1': 'R', 'e1': 'K', 'e8': 'k'}, 'turn': 'w'}Passed
black to move ep{'castling': '', 'ep': 'd3', 'fullmove': 12, 'halfmove': 0, 'pieces': {'d4': 'P', 'e1': 'K', 'e4': 'p', 'e8': 'k'}, 'turn': 'b'}{'castling': '', 'ep': 'd3', 'fullmove': 12, 'halfmove': 0, 'pieces': {'d4': 'P', 'e1': 'K', 'e4': 'p', 'e8': 'k'}, 'turn': 'b'}Passed
white ep target{'castling': '', 'ep': 'd6', 'fullmove': 9, 'halfmove': 0, 'pieces': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'turn': 'w'}{'castling': '', 'ep': 'd6', 'fullmove': 9, 'halfmove': 0, 'pieces': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'turn': 'w'}Passed
ep wrong side{'castling': '', 'ep': 'd3', 'fullmove': 9, 'halfmove': 0, 'pieces': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'turn': 'w'}invalidFailed
no castling dash{'castling': '', 'ep': None, 'fullmove': 20, 'halfmove': 3, 'pieces': {'a1': 'R', 'a8': 'r', 'e1': 'K', 'e8': 'k', 'h1': 'R', 'h8': 'r'}, 'turn': 'b'}{'castling': '', 'ep': None, 'fullmove': 20, 'halfmove': 3, 'pieces': {'a1': 'R', 'a8': 'r', 'e1': 'K', 'e8': 'k', 'h1': 'R', 'h8': 'r'}, 'turn': 'b'}Passed
black rights only{'castling': 'kq', 'ep': None, 'fullmove': 5, 'halfmove': 0, 'pieces': {'a1': 'R', 'a8': 'r', 'e1': 'K', 'e8': 'k', 'h1': 'R', 'h8': 'r'}, 'turn': 'w'}{'castling': 'kq', 'ep': None, 'fullmove': 5, 'halfmove': 0, 'pieces': {'a1': 'R', 'a8': 'r', 'e1': 'K', 'e8': 'k', 'h1': 'R', 'h8': 'r'}, 'turn': 'w'}Passed
two white kingsinvalidinvalidPassed
two white kings no blackinvalidinvalidPassed

SHA-256 / d4f033d07660d9afe065f503016342f32ed42b7a5d0a2ada367dce3ea0b87a80

2 / The unsuccessful fix

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(x):
    parts=x.split()
    if len(parts)!=6: return 'invalid'
    rows=parts[0].split('/')
    if len(rows)!=8: return 'invalid'
    board={}
    for i,row in enumerate(rows):
        rank=8-i; f=0
        for ch in row:
            if ch.isdigit(): f+=int(ch)
            elif ch in 'pnbrqkPNBRQK':
                if f>7: return 'invalid'
                board[chr(97+f)+str(rank)]=ch; f+=1
            else: return 'invalid'
        if f!=8: return 'invalid'
    vals=list(board.values())
    if vals.count('K')!=1 or vals.count('k')!=1: return 'invalid'
    turn=parts[1]
    if turn not in ('w','b'): return 'invalid'
    cast='' if parts[2]=='-' else parts[2]
    if any(c not in 'KQkq' for c in cast): return 'invalid'
    ep=None if parts[3]=='-' else parts[3]
    if ep is not None and ep[1:]!=('3' if turn=='w' else '6'): return 'invalid'
    if not (parts[4].isdigit() and parts[5].isdigit()) or int(parts[5])<1: return 'invalid'
    return {'pieces':board,'turn':turn,'castling':cast,'ep':ep,'halfmove':int(parts[4]),'fullmove':int(parts[5])}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('kings and rook', '4k3/8/8/8/8/8/8/R3K3 w Q - 0 1',
   {'castling': 'Q',
    'ep': None,
    'fullmove': 1,
    'halfmove': 0,
    'pieces': {'a1': 'R', 'e1': 'K', 'e8': 'k'},
    'turn': 'w'}),
  ('black to move ep', '4k3/8/8/8/3Pp3/8/8/4K3 b - d3 0 12',
   {'castling': '',
    'ep': 'd3',
    'fullmove': 12,
    'halfmove': 0,
    'pieces': {'d4': 'P', 'e1': 'K', 'e4': 'p', 'e8': 'k'},
    'turn': 'b'}),
  ('white ep target', '4k3/8/8/3pP3/8/8/8/4K3 w - d6 0 9',
   {'castling': '',
    'ep': 'd6',
    'fullmove': 9,
    'halfmove': 0,
    'pieces': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'},
    'turn': 'w'}),
  ('ep wrong side', '4k3/8/8/3pP3/8/8/8/4K3 w - d3 0 9', 'invalid'),
  ('no castling dash', 'r3k2r/8/8/8/8/8/8/R3K2R b - - 3 20',
   {'castling': '',
    'ep': None,
    'fullmove': 20,
    'halfmove': 3,
    'pieces': {'a1': 'R', 'a8': 'r', 'e1': 'K', 'e8': 'k', 'h1': 'R', 'h8': 'r'},
    'turn': 'b'}),
  ('black rights only', 'r3k2r/8/8/8/8/8/8/R3K2R w kq - 0 5',
   {'castling': 'kq',
    'ep': None,
    'fullmove': 5,
    'halfmove': 0,
    'pieces': {'a1': 'R', 'a8': 'r', 'e1': 'K', 'e8': 'k', 'h1': 'R', 'h8': 'r'},
    'turn': 'w'}),
  ('two white kings', '4k3/8/8/8/8/8/8/K3K3 w - - 0 1', 'invalid'),
  ('two white kings no black', '8/8/8/8/8/8/8/K3K3 w - - 0 1', 'invalid')],
 [('white ep target', '4k3/8/8/3pP3/8/8/8/4K3 w - d6 0 9',
   {'castling': '',
    'ep': 'd6',
    'fullmove': 9,
    'halfmove': 0,
    'pieces': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'},
    'turn': 'w'}),
  ('ep wrong side', '4k3/8/8/3pP3/8/8/8/4K3 w - d3 0 9', 'invalid'),
  ('two white kings', '4k3/8/8/8/8/8/8/K3K3 w - - 0 1', 'invalid'),
  ('two white kings no black', '8/8/8/8/8/8/8/K3K3 w - - 0 1', 'invalid'),
  ('missing black king', '8/8/8/8/8/8/8/4K3 w - - 0 1', 'invalid'),
  ('short rank', '4k3/8/8/8/8/8/8/4K2 w - - 0 1', 'invalid'),
  ('fullmove zero', '4k3/8/8/8/8/8/8/4K3 w - - 0 0', 'invalid'),
  ('halfmove dash', '4k3/8/8/8/8/8/8/4K3 w - - - 1', 'invalid')],
 [('black to move ep', '4k3/8/8/8/3Pp3/8/8/4K3 b - d3 0 12',
   {'castling': '',
    'ep': 'd3',
    'fullmove': 12,
    'halfmove': 0,
    'pieces': {'d4': 'P', 'e1': 'K', 'e4': 'p', 'e8': 'k'},
    'turn': 'b'}),
  ('ep wrong side', '4k3/8/8/3pP3/8/8/8/4K3 w - d3 0 9', 'invalid'),
  ('short rank', '4k3/8/8/8/8/8/8/4K2 w - - 0 1', 'invalid'),
  ('fullmove zero', '4k3/8/8/8/8/8/8/4K3 w - - 0 0', 'invalid'),
  ('halfmove dash', '4k3/8/8/8/8/8/8/4K3 w - - - 1', 'invalid'),
  ('pawns and knights', 'rn2k3/pp6/8/8/8/8/6PP/4K1NR w K - 7 30',
   {'castling': 'K',
    'ep': None,
    'fullmove': 30,
    'halfmove': 7,
    'pieces': {'a7': 'p',
               'a8': 'r',
               'b7': 'p',
               'b8': 'n',
               'e1': 'K',
               'e8': 'k',
               'g1': 'N',
               'g2': 'P',
               'h1': 'R',
               'h2': 'P'},
    'turn': 'w'}),
  ('bad castling letter', '4k3/8/8/8/8/8/8/4K3 w X - 0 1', 'invalid'),
  ('bad turn', '4k3/8/8/8/8/8/8/4K3 x - - 0 1', 'invalid')],
 [('kings and rook', '4k3/8/8/8/8/8/8/R3K3 w Q - 0 1',
   {'castling': 'Q',
    'ep': None,
    'fullmove': 1,
    'halfmove': 0,
    'pieces': {'a1': 'R', 'e1': 'K', 'e8': 'k'},
    'turn': 'w'}),
  ('black to move ep', '4k3/8/8/8/3Pp3/8/8/4K3 b - d3 0 12',
   {'castling': '',
    'ep': 'd3',
    'fullmove': 12,
    'halfmove': 0,
    'pieces': {'d4': 'P', 'e1': 'K', 'e4': 'p', 'e8': 'k'},
    'turn': 'b'}),
  ('white ep target', '4k3/8/8/3pP3/8/8/8/4K3 w - d6 0 9',
   {'castling': '',
    'ep': 'd6',
    'fullmove': 9,
    'halfmove': 0,
    'pieces': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'},
    'turn': 'w'}),
  ('ep wrong side', '4k3/8/8/3pP3/8/8/8/4K3 w - d3 0 9', 'invalid'),
  ('pawns and knights', 'rn2k3/pp6/8/8/8/8/6PP/4K1NR w K - 7 30',
   {'castling': 'K',
    'ep': None,
    'fullmove': 30,
    'halfmove': 7,
    'pieces': {'a7': 'p',
               'a8': 'r',
               'b7': 'p',
               'b8': 'n',
               'e1': 'K',
               'e8': 'k',
               'g1': 'N',
               'g2': 'P',
               'h1': 'R',
               'h2': 'P'},
    'turn': 'w'}),
  ('bad castling letter', '4k3/8/8/8/8/8/8/4K3 w X - 0 1', 'invalid'),
  ('bad turn', '4k3/8/8/8/8/8/8/4K3 x - - 0 1', 'invalid'),
  ('fullmove dash', '4k3/8/8/8/8/8/8/4K3 w - - 0 -', 'invalid')],
 [('kings and rook', '4k3/8/8/8/8/8/8/R3K3 w Q - 0 1',
   {'castling': 'Q',
    'ep': None,
    'fullmove': 1,
    'halfmove': 0,
    'pieces': {'a1': 'R', 'e1': 'K', 'e8': 'k'},
    'turn': 'w'}),
  ('black to move ep', '4k3/8/8/8/3Pp3/8/8/4K3 b - d3 0 12',
   {'castling': '',
    'ep': 'd3',
    'fullmove': 12,
    'halfmove': 0,
    'pieces': {'d4': 'P', 'e1': 'K', 'e4': 'p', 'e8': 'k'},
    'turn': 'b'}),
  ('white ep target', '4k3/8/8/3pP3/8/8/8/4K3 w - d6 0 9',
   {'castling': '',
    'ep': 'd6',
    'fullmove': 9,
    'halfmove': 0,
    'pieces': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'},
    'turn': 'w'}),
  ('ep wrong side', '4k3/8/8/3pP3/8/8/8/4K3 w - d3 0 9', 'invalid'),
  ('no castling dash', 'r3k2r/8/8/8/8/8/8/R3K2R b - - 3 20',
   {'castling': '',
    'ep': None,
    'fullmove': 20,
    'halfmove': 3,
    'pieces': {'a1': 'R', 'a8': 'r', 'e1': 'K', 'e8': 'k', 'h1': 'R', 'h8': 'r'},
    'turn': 'b'}),
  ('black rights only', 'r3k2r/8/8/8/8/8/8/R3K2R w kq - 0 5',
   {'castling': 'kq',
    'ep': None,
    'fullmove': 5,
    'halfmove': 0,
    'pieces': {'a1': 'R', 'a8': 'r', 'e1': 'K', 'e8': 'k', 'h1': 'R', 'h8': 'r'},
    'turn': 'w'}),
  ('two white kings', '4k3/8/8/8/8/8/8/K3K3 w - - 0 1', 'invalid'),
  ('fullmove dash', '4k3/8/8/8/8/8/8/4K3 w - - 0 -', 'invalid')]]
for label, args, expected in fixtures[N-1]:
    try:
        actual = solve(args)
    except Exception as exc:
        actual = 'error:' + type(exc).__name__
    check(label, actual, expected)
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
kings and rook{'castling': 'Q', 'ep': None, 'fullmove': 1, 'halfmove': 0, 'pieces': {'a1': 'R', 'e1': 'K', 'e8': 'k'}, 'turn': 'w'}{'castling': 'Q', 'ep': None, 'fullmove': 1, 'halfmove': 0, 'pieces': {'a1': 'R', 'e1': 'K', 'e8': 'k'}, 'turn': 'w'}Passed
black to move epinvalid{'castling': '', 'ep': 'd3', 'fullmove': 12, 'halfmove': 0, 'pieces': {'d4': 'P', 'e1': 'K', 'e4': 'p', 'e8': 'k'}, 'turn': 'b'}Failed
white ep targetinvalid{'castling': '', 'ep': 'd6', 'fullmove': 9, 'halfmove': 0, 'pieces': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'turn': 'w'}Failed
ep wrong side{'castling': '', 'ep': 'd3', 'fullmove': 9, 'halfmove': 0, 'pieces': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'turn': 'w'}invalidFailed
no castling dash{'castling': '', 'ep': None, 'fullmove': 20, 'halfmove': 3, 'pieces': {'a1': 'R', 'a8': 'r', 'e1': 'K', 'e8': 'k', 'h1': 'R', 'h8': 'r'}, 'turn': 'b'}{'castling': '', 'ep': None, 'fullmove': 20, 'halfmove': 3, 'pieces': {'a1': 'R', 'a8': 'r', 'e1': 'K', 'e8': 'k', 'h1': 'R', 'h8': 'r'}, 'turn': 'b'}Passed
black rights only{'castling': 'kq', 'ep': None, 'fullmove': 5, 'halfmove': 0, 'pieces': {'a1': 'R', 'a8': 'r', 'e1': 'K', 'e8': 'k', 'h1': 'R', 'h8': 'r'}, 'turn': 'w'}{'castling': 'kq', 'ep': None, 'fullmove': 5, 'halfmove': 0, 'pieces': {'a1': 'R', 'a8': 'r', 'e1': 'K', 'e8': 'k', 'h1': 'R', 'h8': 'r'}, 'turn': 'w'}Passed
two white kingsinvalidinvalidPassed
two white kings no blackinvalidinvalidPassed

SHA-256 / a6a2097b1b0c69e2142f7a11d292f7b69757938ad1b8c7006331b6b3e3caf6fa

HELD IN THE MEMBER ARCHIVE

The verified repair and its recorded checks are member-only.

This mechanism has 8 recorded checks per implementation. The open-access tier publishes the failure and the unsuccessful fix; the repaired source that passes every check, and the observations that prove it, are available to members.

Every case sharing this mechanism uses the same contract and the same repair, so this one record is held back for all of them.

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Verification & scope

A deterministic toy model with an explicitly stated contract; boards are {square: piece} with white upper-case and black lower-case pieces; no claim of full FIDE Laws conformance. 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 2026-09-29T14:50:13.351623+00:00.

Case digest / 76dae84fcdaeca970d75b905607b5275cb8bdc99426f51a202cdf2e61c7f10f1