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FA-82416 / Chess move legality / Open access

Horizontal pin through two pawns missed · case 01

With king and enemy rook on the fifth rank and only the two pawns between them, en passant is allowed.

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

ROOT CAUSE

The captured pawn is left on the board during the safety test, so it still shields the king.

VERIFIED REPAIR

Remove both the capturing pawn's origin and the captured pawn before testing king safety.

Unsuccessful approach: Removing only the victim leaves the capturer on its origin square as a shield.

Case contract

Input: {"board", "move": [from, to], "ep": target square or null}. Return true when the move is a legal en passant capture: a pawn on its fifth rank moves diagonally forward onto the ep square, an enemy pawn stands beside it on the origin rank, and after removing both pawns from their squares and placing the capturer, no enemy rook, bishop or queen attacks the mover's king along an open line.

Why this case matters

Move validators gate every move in engines, servers and arbiters; accepting an illegal move or rejecting a legal one corrupts the game record.

1 / The failure

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

N = 1
observations = []
def solve(x):
    b=dict(x['board']); fr,to=x['move']; p=b.get(fr)
    if p not in ('P','p') or to!=x['ep']: return False
    white=p=='P'
    d=1 if white else -1
    f0,r0=ord(fr[0])-97,int(fr[1])-1
    f1,r1=ord(to[0])-97,int(to[1])-1
    if r1-r0!=d or abs(f1-f0)!=1: return False
    if r0!=(4 if white else 3): return False
    victim=to[0]+fr[1]
    if b.get(victim)!=('p' if white else 'P'): return False
    del b[fr]; b[to]=p
    ks=[s for s,v in b.items() if v==('K' if white else 'k')][0]
    kf,kr=ord(ks[0])-97,int(ks[1])-1
    for df,dr in ((1,0),(-1,0),(0,1),(0,-1),(1,1),(1,-1),(-1,1),(-1,-1)):
        cf,cr=kf+df,kr+dr
        while 0<=cf<8 and 0<=cr<8:
            v=b.get(chr(97+cf)+str(cr+1))
            if v:
                if v.isupper()!=white and v.lower() in (('r','q') if df==0 or dr==0 else ('b','q')): return False
                break
            cf+=df; cr+=dr
    return True
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('plain ep',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd6', 'move': ['e5', 'd6']},
   True),
  ('black ep',
   {'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'e3', 'move': ['d4', 'e3']},
   True),
  ('rank pin both pawns',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'r'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   False),
  ('rank pin black side',
   {'board': {'a4': 'Q', 'd4': 'p', 'e1': 'K', 'e4': 'P', 'h4': 'k'},
    'ep': 'e3',
    'move': ['d4', 'e3']},
   False),
  ('bishop diagonal after ep',
   {'board': {'c3': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p', 'f6': 'n', 'h8': 'b'},
    'ep': 'f6',
    'move': ['e5', 'f6']},
   True),
  ('no ep square',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': None, 'move': ['e5', 'd6']},
   False),
  ('wrong target',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p'},
    'ep': 'd6',
    'move': ['e5', 'f6']},
   False),
  ('wrong rank for white',
   {'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'd5', 'move': ['e4', 'd5']},
   False)],
 [('rank pin both pawns',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'r'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   False),
  ('rank pin black side',
   {'board': {'a4': 'Q', 'd4': 'p', 'e1': 'K', 'e4': 'P', 'h4': 'k'},
    'ep': 'e3',
    'move': ['d4', 'e3']},
   False),
  ('no ep square',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': None, 'move': ['e5', 'd6']},
   False),
  ('wrong target',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p'},
    'ep': 'd6',
    'move': ['e5', 'f6']},
   False),
  ('wrong rank for white',
   {'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'd5', 'move': ['e4', 'd5']},
   False),
  ('black wrong rank',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'e4', 'move': ['d5', 'e4']},
   False),
  ('file rook behind pawn',
   {'board': {'d1': 'K', 'd5': 'p', 'd8': 'r', 'e5': 'P', 'e8': 'k'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   True),
  ('same file ep target',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd3', 'move': ['e5', 'd6']},
   False)],
 [('rank pin both pawns',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'r'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   False),
  ('rank pin black side',
   {'board': {'a4': 'Q', 'd4': 'p', 'e1': 'K', 'e4': 'P', 'h4': 'k'},
    'ep': 'e3',
    'move': ['d4', 'e3']},
   False),
  ('black wrong rank',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'e4', 'move': ['d5', 'e4']},
   False),
  ('file rook behind pawn',
   {'board': {'d1': 'K', 'd5': 'p', 'd8': 'r', 'e5': 'P', 'e8': 'k'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   True),
  ('same file ep target',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd3', 'move': ['e5', 'd6']},
   False),
  ('pin by own rook harmless',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'R'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   True),
  ('bishop on the rank',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'b'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   True),
  ('rook on the diagonal',
   {'board': {'c3': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'g7': 'r'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   True)],
 [('plain ep',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd6', 'move': ['e5', 'd6']},
   True),
  ('black ep',
   {'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'e3', 'move': ['d4', 'e3']},
   True),
  ('rank pin both pawns',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'r'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   False),
  ('rank pin black side',
   {'board': {'a4': 'Q', 'd4': 'p', 'e1': 'K', 'e4': 'P', 'h4': 'k'},
    'ep': 'e3',
    'move': ['d4', 'e3']},
   False),
  ('pin by own rook harmless',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'R'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   True),
  ('bishop on the rank',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'b'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   True),
  ('rook on the diagonal',
   {'board': {'c3': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'g7': 'r'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   True),
  ('rook on file of ep square',
   {'board': {'a1': 'K', 'd1': 'r', 'd5': 'p', 'e5': 'P', 'e8': 'k'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   False)],
 [('plain ep',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd6', 'move': ['e5', 'd6']},
   True),
  ('black ep',
   {'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'e3', 'move': ['d4', 'e3']},
   True),
  ('rank pin both pawns',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'r'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   False),
  ('rank pin black side',
   {'board': {'a4': 'Q', 'd4': 'p', 'e1': 'K', 'e4': 'P', 'h4': 'k'},
    'ep': 'e3',
    'move': ['d4', 'e3']},
   False),
  ('bishop diagonal after ep',
   {'board': {'c3': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p', 'f6': 'n', 'h8': 'b'},
    'ep': 'f6',
    'move': ['e5', 'f6']},
   True),
  ('no ep square',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': None, 'move': ['e5', 'd6']},
   False),
  ('wrong target',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p'},
    'ep': 'd6',
    'move': ['e5', 'f6']},
   False),
  ('rook on file of ep square',
   {'board': {'a1': 'K', 'd1': 'r', 'd5': 'p', 'e5': 'P', 'e8': 'k'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   False)]]
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
plain epTrueTruePassed
black epTrueTruePassed
rank pin both pawnsTrueFalseFailed
rank pin black sideTrueFalseFailed
bishop diagonal after epTrueTruePassed
no ep squareFalseFalsePassed
wrong targetFalseFalsePassed
wrong rank for whiteFalseFalsePassed

SHA-256 / 5e5adb91453dba1a3064c0b21bad638ab01e53f3b24ac7e5efb9096cb67a8c84

2 / The unsuccessful fix

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

N = 1
observations = []
def solve(x):
    b=dict(x['board']); fr,to=x['move']; p=b.get(fr)
    if p not in ('P','p') or to!=x['ep']: return False
    white=p=='P'
    d=1 if white else -1
    f0,r0=ord(fr[0])-97,int(fr[1])-1
    f1,r1=ord(to[0])-97,int(to[1])-1
    if r1-r0!=d or abs(f1-f0)!=1: return False
    if r0!=(4 if white else 3): return False
    victim=to[0]+fr[1]
    if b.get(victim)!=('p' if white else 'P'): return False
    del b[victim]; b[to]=p
    ks=[s for s,v in b.items() if v==('K' if white else 'k')][0]
    kf,kr=ord(ks[0])-97,int(ks[1])-1
    for df,dr in ((1,0),(-1,0),(0,1),(0,-1),(1,1),(1,-1),(-1,1),(-1,-1)):
        cf,cr=kf+df,kr+dr
        while 0<=cf<8 and 0<=cr<8:
            v=b.get(chr(97+cf)+str(cr+1))
            if v:
                if v.isupper()!=white and v.lower() in (('r','q') if df==0 or dr==0 else ('b','q')): return False
                break
            cf+=df; cr+=dr
    return True
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('plain ep',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd6', 'move': ['e5', 'd6']},
   True),
  ('black ep',
   {'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'e3', 'move': ['d4', 'e3']},
   True),
  ('rank pin both pawns',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'r'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   False),
  ('rank pin black side',
   {'board': {'a4': 'Q', 'd4': 'p', 'e1': 'K', 'e4': 'P', 'h4': 'k'},
    'ep': 'e3',
    'move': ['d4', 'e3']},
   False),
  ('bishop diagonal after ep',
   {'board': {'c3': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p', 'f6': 'n', 'h8': 'b'},
    'ep': 'f6',
    'move': ['e5', 'f6']},
   True),
  ('no ep square',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': None, 'move': ['e5', 'd6']},
   False),
  ('wrong target',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p'},
    'ep': 'd6',
    'move': ['e5', 'f6']},
   False),
  ('wrong rank for white',
   {'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'd5', 'move': ['e4', 'd5']},
   False)],
 [('rank pin both pawns',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'r'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   False),
  ('rank pin black side',
   {'board': {'a4': 'Q', 'd4': 'p', 'e1': 'K', 'e4': 'P', 'h4': 'k'},
    'ep': 'e3',
    'move': ['d4', 'e3']},
   False),
  ('no ep square',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': None, 'move': ['e5', 'd6']},
   False),
  ('wrong target',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p'},
    'ep': 'd6',
    'move': ['e5', 'f6']},
   False),
  ('wrong rank for white',
   {'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'd5', 'move': ['e4', 'd5']},
   False),
  ('black wrong rank',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'e4', 'move': ['d5', 'e4']},
   False),
  ('file rook behind pawn',
   {'board': {'d1': 'K', 'd5': 'p', 'd8': 'r', 'e5': 'P', 'e8': 'k'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   True),
  ('same file ep target',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd3', 'move': ['e5', 'd6']},
   False)],
 [('rank pin both pawns',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'r'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   False),
  ('rank pin black side',
   {'board': {'a4': 'Q', 'd4': 'p', 'e1': 'K', 'e4': 'P', 'h4': 'k'},
    'ep': 'e3',
    'move': ['d4', 'e3']},
   False),
  ('black wrong rank',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'e4', 'move': ['d5', 'e4']},
   False),
  ('file rook behind pawn',
   {'board': {'d1': 'K', 'd5': 'p', 'd8': 'r', 'e5': 'P', 'e8': 'k'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   True),
  ('same file ep target',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd3', 'move': ['e5', 'd6']},
   False),
  ('pin by own rook harmless',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'R'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   True),
  ('bishop on the rank',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'b'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   True),
  ('rook on the diagonal',
   {'board': {'c3': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'g7': 'r'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   True)],
 [('plain ep',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd6', 'move': ['e5', 'd6']},
   True),
  ('black ep',
   {'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'e3', 'move': ['d4', 'e3']},
   True),
  ('rank pin both pawns',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'r'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   False),
  ('rank pin black side',
   {'board': {'a4': 'Q', 'd4': 'p', 'e1': 'K', 'e4': 'P', 'h4': 'k'},
    'ep': 'e3',
    'move': ['d4', 'e3']},
   False),
  ('pin by own rook harmless',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'R'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   True),
  ('bishop on the rank',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'b'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   True),
  ('rook on the diagonal',
   {'board': {'c3': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'g7': 'r'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   True),
  ('rook on file of ep square',
   {'board': {'a1': 'K', 'd1': 'r', 'd5': 'p', 'e5': 'P', 'e8': 'k'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   False)],
 [('plain ep',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd6', 'move': ['e5', 'd6']},
   True),
  ('black ep',
   {'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'e3', 'move': ['d4', 'e3']},
   True),
  ('rank pin both pawns',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'r'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   False),
  ('rank pin black side',
   {'board': {'a4': 'Q', 'd4': 'p', 'e1': 'K', 'e4': 'P', 'h4': 'k'},
    'ep': 'e3',
    'move': ['d4', 'e3']},
   False),
  ('bishop diagonal after ep',
   {'board': {'c3': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p', 'f6': 'n', 'h8': 'b'},
    'ep': 'f6',
    'move': ['e5', 'f6']},
   True),
  ('no ep square',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': None, 'move': ['e5', 'd6']},
   False),
  ('wrong target',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p'},
    'ep': 'd6',
    'move': ['e5', 'f6']},
   False),
  ('rook on file of ep square',
   {'board': {'a1': 'K', 'd1': 'r', 'd5': 'p', 'e5': 'P', 'e8': 'k'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   False)]]
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
plain epTrueTruePassed
black epTrueTruePassed
rank pin both pawnsTrueFalseFailed
rank pin black sideTrueFalseFailed
bishop diagonal after epTrueTruePassed
no ep squareFalseFalsePassed
wrong targetFalseFalsePassed
wrong rank for whiteFalseFalsePassed

SHA-256 / a612c39df2fbef55d2901b4e65dd8e7fe38a9afeb728c234d53750775cafd44c

3 / The verified repair

Exit 0
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(x):
    b=dict(x['board']); fr,to=x['move']; p=b.get(fr)
    if p not in ('P','p') or to!=x['ep']: return False
    white=p=='P'
    d=1 if white else -1
    f0,r0=ord(fr[0])-97,int(fr[1])-1
    f1,r1=ord(to[0])-97,int(to[1])-1
    if r1-r0!=d or abs(f1-f0)!=1: return False
    if r0!=(4 if white else 3): return False
    victim=to[0]+fr[1]
    if b.get(victim)!=('p' if white else 'P'): return False
    del b[fr]; del b[victim]; b[to]=p
    ks=[s for s,v in b.items() if v==('K' if white else 'k')][0]
    kf,kr=ord(ks[0])-97,int(ks[1])-1
    for df,dr in ((1,0),(-1,0),(0,1),(0,-1),(1,1),(1,-1),(-1,1),(-1,-1)):
        cf,cr=kf+df,kr+dr
        while 0<=cf<8 and 0<=cr<8:
            v=b.get(chr(97+cf)+str(cr+1))
            if v:
                if v.isupper()!=white and v.lower() in (('r','q') if df==0 or dr==0 else ('b','q')): return False
                break
            cf+=df; cr+=dr
    return True
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('plain ep',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd6', 'move': ['e5', 'd6']},
   True),
  ('black ep',
   {'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'e3', 'move': ['d4', 'e3']},
   True),
  ('rank pin both pawns',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'r'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   False),
  ('rank pin black side',
   {'board': {'a4': 'Q', 'd4': 'p', 'e1': 'K', 'e4': 'P', 'h4': 'k'},
    'ep': 'e3',
    'move': ['d4', 'e3']},
   False),
  ('bishop diagonal after ep',
   {'board': {'c3': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p', 'f6': 'n', 'h8': 'b'},
    'ep': 'f6',
    'move': ['e5', 'f6']},
   True),
  ('no ep square',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': None, 'move': ['e5', 'd6']},
   False),
  ('wrong target',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p'},
    'ep': 'd6',
    'move': ['e5', 'f6']},
   False),
  ('wrong rank for white',
   {'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'd5', 'move': ['e4', 'd5']},
   False)],
 [('rank pin both pawns',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'r'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   False),
  ('rank pin black side',
   {'board': {'a4': 'Q', 'd4': 'p', 'e1': 'K', 'e4': 'P', 'h4': 'k'},
    'ep': 'e3',
    'move': ['d4', 'e3']},
   False),
  ('no ep square',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': None, 'move': ['e5', 'd6']},
   False),
  ('wrong target',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p'},
    'ep': 'd6',
    'move': ['e5', 'f6']},
   False),
  ('wrong rank for white',
   {'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'd5', 'move': ['e4', 'd5']},
   False),
  ('black wrong rank',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'e4', 'move': ['d5', 'e4']},
   False),
  ('file rook behind pawn',
   {'board': {'d1': 'K', 'd5': 'p', 'd8': 'r', 'e5': 'P', 'e8': 'k'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   True),
  ('same file ep target',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd3', 'move': ['e5', 'd6']},
   False)],
 [('rank pin both pawns',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'r'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   False),
  ('rank pin black side',
   {'board': {'a4': 'Q', 'd4': 'p', 'e1': 'K', 'e4': 'P', 'h4': 'k'},
    'ep': 'e3',
    'move': ['d4', 'e3']},
   False),
  ('black wrong rank',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'e4', 'move': ['d5', 'e4']},
   False),
  ('file rook behind pawn',
   {'board': {'d1': 'K', 'd5': 'p', 'd8': 'r', 'e5': 'P', 'e8': 'k'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   True),
  ('same file ep target',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd3', 'move': ['e5', 'd6']},
   False),
  ('pin by own rook harmless',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'R'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   True),
  ('bishop on the rank',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'b'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   True),
  ('rook on the diagonal',
   {'board': {'c3': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'g7': 'r'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   True)],
 [('plain ep',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd6', 'move': ['e5', 'd6']},
   True),
  ('black ep',
   {'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'e3', 'move': ['d4', 'e3']},
   True),
  ('rank pin both pawns',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'r'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   False),
  ('rank pin black side',
   {'board': {'a4': 'Q', 'd4': 'p', 'e1': 'K', 'e4': 'P', 'h4': 'k'},
    'ep': 'e3',
    'move': ['d4', 'e3']},
   False),
  ('pin by own rook harmless',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'R'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   True),
  ('bishop on the rank',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'b'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   True),
  ('rook on the diagonal',
   {'board': {'c3': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'g7': 'r'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   True),
  ('rook on file of ep square',
   {'board': {'a1': 'K', 'd1': 'r', 'd5': 'p', 'e5': 'P', 'e8': 'k'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   False)],
 [('plain ep',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd6', 'move': ['e5', 'd6']},
   True),
  ('black ep',
   {'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'e3', 'move': ['d4', 'e3']},
   True),
  ('rank pin both pawns',
   {'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'r'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   False),
  ('rank pin black side',
   {'board': {'a4': 'Q', 'd4': 'p', 'e1': 'K', 'e4': 'P', 'h4': 'k'},
    'ep': 'e3',
    'move': ['d4', 'e3']},
   False),
  ('bishop diagonal after ep',
   {'board': {'c3': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p', 'f6': 'n', 'h8': 'b'},
    'ep': 'f6',
    'move': ['e5', 'f6']},
   True),
  ('no ep square',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': None, 'move': ['e5', 'd6']},
   False),
  ('wrong target',
   {'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p'},
    'ep': 'd6',
    'move': ['e5', 'f6']},
   False),
  ('rook on file of ep square',
   {'board': {'a1': 'K', 'd1': 'r', 'd5': 'p', 'e5': 'P', 'e8': 'k'},
    'ep': 'd6',
    'move': ['e5', 'd6']},
   False)]]
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
plain epTrueTruePassed
black epTrueTruePassed
rank pin both pawnsFalseFalsePassed
rank pin black sideFalseFalsePassed
bishop diagonal after epTrueTruePassed
no ep squareFalseFalsePassed
wrong targetFalseFalsePassed
wrong rank for whiteFalseFalsePassed

SHA-256 / cf293895bd33136e5c8e1fa57c4adc9a0a518206e453424fef04036246e8c119

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 2026-09-29T14:50:12.053374+00:00.

Case digest / 0e1397ebbf2ca4e941835b39d2efec1ebd7b9b2466b093a235ff4a78c24090dd