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FA-82476 / Chess move legality / Open access

King refuses captures and lands on friends · case 01

The king may move onto its own pawn but not capture an undefended enemy pawn.

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

ROOT CAUSE

The occupancy colour test is inverted.

THE FAILURE

The occupancy colour test is inverted.

Unsuccessful approach: Forbidding only king-occupied squares lets the king land on friendly pieces.

Case contract

Input: {"board", "from": king square, "to": square}. Return true when the king move is legal: exactly one square in any direction, not onto a friendly piece, and the destination is not attacked by the opponent (including the enemy king) once the king has left its origin square. Captures of undefended enemy pieces are allowed.

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):
    def attacked(bd, sq, by_white):
        f,r=ord(sq[0])-97,int(sq[1])-1
        def at(ff,rr):
            return bd.get(chr(97+ff)+str(rr+1)) if 0<=ff<8 and 0<=rr<8 else None
        P,N,B,R,Q,K=('PNBRQK' if by_white else 'pnbrqk')
        pr=r-1 if by_white else r+1
        if at(f-1,pr)==P or at(f+1,pr)==P: return True
        for df,dr in ((1,2),(2,1),(-1,2),(-2,1),(1,-2),(2,-1),(-1,-2),(-2,-1)):
            if at(f+df,r+dr)==N: return True
        for df in (-1,0,1):
            for dr in (-1,0,1):
                if (df or dr) and at(f+df,r+dr)==K: return True
        for df,dr,kinds in ((1,0,(R,Q)),(-1,0,(R,Q)),(0,1,(R,Q)),(0,-1,(R,Q)),(1,1,(B,Q)),(1,-1,(B,Q)),(-1,1,(B,Q)),(-1,-1,(B,Q))):
            ff,rr=f+df,r+dr
            while 0<=ff<8 and 0<=rr<8:
                v=at(ff,rr)
                if v:
                    if v in kinds: return True
                    break
                ff+=df; rr+=dr
        return False
    
    b=x['board']; fr,to=x['from'],x['to']; k=b[fr]; white=k=='K'
    df=ord(to[0])-ord(fr[0]); dr=int(to[1])-int(fr[1])
    if max(abs(df),abs(dr))!=1: return False
    occ=b.get(to)
    if occ and occ.isupper()!=white: return False
    nb=dict(b); del nb[fr]; nb[to]=k
    return not attacked(nb, to, not white)
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('retreat along rook file',
   {'board': {'a1': 'k', 'e4': 'K', 'e8': 'r'}, 'from': 'e4', 'to': 'e3'}, False),
  ('sidestep rook file', {'board': {'a1': 'k', 'e4': 'K', 'e8': 'r'}, 'from': 'e4', 'to': 'd3'},
   True),
  ('diagonal step', {'board': {'a8': 'k', 'e4': 'K'}, 'from': 'e4', 'to': 'f5'}, True),
  ('stand still', {'board': {'a8': 'k', 'e4': 'K'}, 'from': 'e4', 'to': 'e4'}, False),
  ('capture own', {'board': {'a8': 'k', 'e4': 'K', 'e5': 'P'}, 'from': 'e4', 'to': 'e5'}, False),
  ('capture undefended', {'board': {'a8': 'k', 'e4': 'K', 'e5': 'p'}, 'from': 'e4', 'to': 'e5'},
   True),
  ('capture defended',
   {'board': {'a8': 'k', 'd6': 'p', 'e4': 'K', 'e5': 'p'}, 'from': 'e4', 'to': 'e5'}, False),
  ('next to enemy king orthogonal', {'board': {'e4': 'K', 'e6': 'k'}, 'from': 'e4', 'to': 'e5'},
   False)],
 [('stand still', {'board': {'a8': 'k', 'e4': 'K'}, 'from': 'e4', 'to': 'e4'}, False),
  ('capture own', {'board': {'a8': 'k', 'e4': 'K', 'e5': 'P'}, 'from': 'e4', 'to': 'e5'}, False),
  ('capture undefended', {'board': {'a8': 'k', 'e4': 'K', 'e5': 'p'}, 'from': 'e4', 'to': 'e5'},
   True),
  ('capture defended',
   {'board': {'a8': 'k', 'd6': 'p', 'e4': 'K', 'e5': 'p'}, 'from': 'e4', 'to': 'e5'}, False),
  ('next to enemy king orthogonal', {'board': {'e4': 'K', 'e6': 'k'}, 'from': 'e4', 'to': 'e5'},
   False),
  ('next to enemy king diagonal', {'board': {'e4': 'K', 'g6': 'k'}, 'from': 'e4', 'to': 'f5'},
   False),
  ('knight below', {'board': {'a8': 'k', 'e4': 'K', 'f1': 'n'}, 'from': 'e4', 'to': 'e3'}, False),
  ('knight above', {'board': {'a8': 'k', 'd7': 'n', 'e4': 'K'}, 'from': 'e4', 'to': 'e5'}, False)],
 [('capture own', {'board': {'a8': 'k', 'e4': 'K', 'e5': 'P'}, 'from': 'e4', 'to': 'e5'}, False),
  ('capture undefended', {'board': {'a8': 'k', 'e4': 'K', 'e5': 'p'}, 'from': 'e4', 'to': 'e5'},
   True),
  ('next to enemy king diagonal', {'board': {'e4': 'K', 'g6': 'k'}, 'from': 'e4', 'to': 'f5'},
   False),
  ('knight below', {'board': {'a8': 'k', 'e4': 'K', 'f1': 'n'}, 'from': 'e4', 'to': 'e3'}, False),
  ('knight above', {'board': {'a8': 'k', 'd7': 'n', 'e4': 'K'}, 'from': 'e4', 'to': 'e5'}, False),
  ('black king bishop diag retreat',
   {'board': {'b3': 'B', 'd5': 'k', 'h1': 'K'}, 'from': 'd5', 'to': 'e6'}, False),
  ('capture along check line',
   {'board': {'a1': 'k', 'e3': 'p', 'e4': 'K', 'e8': 'r'}, 'from': 'e4', 'to': 'e3'}, False),
  ('capture defended knight',
   {'board': {'a8': 'k', 'e4': 'K', 'e5': 'n', 'e7': 'r'}, 'from': 'e4', 'to': 'e5'}, False)],
 [('retreat along rook file',
   {'board': {'a1': 'k', 'e4': 'K', 'e8': 'r'}, 'from': 'e4', 'to': 'e3'}, False),
  ('capture own', {'board': {'a8': 'k', 'e4': 'K', 'e5': 'P'}, 'from': 'e4', 'to': 'e5'}, False),
  ('capture undefended', {'board': {'a8': 'k', 'e4': 'K', 'e5': 'p'}, 'from': 'e4', 'to': 'e5'},
   True),
  ('black king bishop diag retreat',
   {'board': {'b3': 'B', 'd5': 'k', 'h1': 'K'}, 'from': 'd5', 'to': 'e6'}, False),
  ('capture along check line',
   {'board': {'a1': 'k', 'e3': 'p', 'e4': 'K', 'e8': 'r'}, 'from': 'e4', 'to': 'e3'}, False),
  ('capture defended knight',
   {'board': {'a8': 'k', 'e4': 'K', 'e5': 'n', 'e7': 'r'}, 'from': 'e4', 'to': 'e5'}, False),
  ('two squares', {'board': {'a8': 'k', 'e4': 'K'}, 'from': 'e4', 'to': 'e6'}, False),
  ('knight far below', {'board': {'a8': 'k', 'c4': 'n', 'e4': 'K'}, 'from': 'e4', 'to': 'd3'},
   True)],
 [('retreat along rook file',
   {'board': {'a1': 'k', 'e4': 'K', 'e8': 'r'}, 'from': 'e4', 'to': 'e3'}, False),
  ('sidestep rook file', {'board': {'a1': 'k', 'e4': 'K', 'e8': 'r'}, 'from': 'e4', 'to': 'd3'},
   True),
  ('diagonal step', {'board': {'a8': 'k', 'e4': 'K'}, 'from': 'e4', 'to': 'f5'}, True),
  ('stand still', {'board': {'a8': 'k', 'e4': 'K'}, 'from': 'e4', 'to': 'e4'}, False),
  ('capture own', {'board': {'a8': 'k', 'e4': 'K', 'e5': 'P'}, 'from': 'e4', 'to': 'e5'}, False),
  ('capture undefended', {'board': {'a8': 'k', 'e4': 'K', 'e5': 'p'}, 'from': 'e4', 'to': 'e5'},
   True),
  ('two squares', {'board': {'a8': 'k', 'e4': 'K'}, 'from': 'e4', 'to': 'e6'}, False),
  ('knight far below', {'board': {'a8': 'k', 'c4': 'n', 'e4': 'K'}, 'from': 'e4', 'to': 'd3'},
   True)]]
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
retreat along rook fileFalseFalsePassed
sidestep rook fileTrueTruePassed
diagonal stepTrueTruePassed
stand stillFalseFalsePassed
capture ownTrueFalseFailed
capture undefendedFalseTrueFailed
capture defendedFalseFalsePassed
next to enemy king orthogonalFalseFalsePassed

SHA-256 / bae54173dce47a2eeb6ec17357dce2edb50cbbbbc3e5bb06b09ed338d0f364d4

2 / The unsuccessful fix

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

N = 1
observations = []
def solve(x):
    def attacked(bd, sq, by_white):
        f,r=ord(sq[0])-97,int(sq[1])-1
        def at(ff,rr):
            return bd.get(chr(97+ff)+str(rr+1)) if 0<=ff<8 and 0<=rr<8 else None
        P,N,B,R,Q,K=('PNBRQK' if by_white else 'pnbrqk')
        pr=r-1 if by_white else r+1
        if at(f-1,pr)==P or at(f+1,pr)==P: return True
        for df,dr in ((1,2),(2,1),(-1,2),(-2,1),(1,-2),(2,-1),(-1,-2),(-2,-1)):
            if at(f+df,r+dr)==N: return True
        for df in (-1,0,1):
            for dr in (-1,0,1):
                if (df or dr) and at(f+df,r+dr)==K: return True
        for df,dr,kinds in ((1,0,(R,Q)),(-1,0,(R,Q)),(0,1,(R,Q)),(0,-1,(R,Q)),(1,1,(B,Q)),(1,-1,(B,Q)),(-1,1,(B,Q)),(-1,-1,(B,Q))):
            ff,rr=f+df,r+dr
            while 0<=ff<8 and 0<=rr<8:
                v=at(ff,rr)
                if v:
                    if v in kinds: return True
                    break
                ff+=df; rr+=dr
        return False
    
    b=x['board']; fr,to=x['from'],x['to']; k=b[fr]; white=k=='K'
    df=ord(to[0])-ord(fr[0]); dr=int(to[1])-int(fr[1])
    if max(abs(df),abs(dr))!=1: return False
    occ=b.get(to)
    if occ and occ.lower()=='k': return False
    nb=dict(b); del nb[fr]; nb[to]=k
    return not attacked(nb, to, not white)
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('retreat along rook file',
   {'board': {'a1': 'k', 'e4': 'K', 'e8': 'r'}, 'from': 'e4', 'to': 'e3'}, False),
  ('sidestep rook file', {'board': {'a1': 'k', 'e4': 'K', 'e8': 'r'}, 'from': 'e4', 'to': 'd3'},
   True),
  ('diagonal step', {'board': {'a8': 'k', 'e4': 'K'}, 'from': 'e4', 'to': 'f5'}, True),
  ('stand still', {'board': {'a8': 'k', 'e4': 'K'}, 'from': 'e4', 'to': 'e4'}, False),
  ('capture own', {'board': {'a8': 'k', 'e4': 'K', 'e5': 'P'}, 'from': 'e4', 'to': 'e5'}, False),
  ('capture undefended', {'board': {'a8': 'k', 'e4': 'K', 'e5': 'p'}, 'from': 'e4', 'to': 'e5'},
   True),
  ('capture defended',
   {'board': {'a8': 'k', 'd6': 'p', 'e4': 'K', 'e5': 'p'}, 'from': 'e4', 'to': 'e5'}, False),
  ('next to enemy king orthogonal', {'board': {'e4': 'K', 'e6': 'k'}, 'from': 'e4', 'to': 'e5'},
   False)],
 [('stand still', {'board': {'a8': 'k', 'e4': 'K'}, 'from': 'e4', 'to': 'e4'}, False),
  ('capture own', {'board': {'a8': 'k', 'e4': 'K', 'e5': 'P'}, 'from': 'e4', 'to': 'e5'}, False),
  ('capture undefended', {'board': {'a8': 'k', 'e4': 'K', 'e5': 'p'}, 'from': 'e4', 'to': 'e5'},
   True),
  ('capture defended',
   {'board': {'a8': 'k', 'd6': 'p', 'e4': 'K', 'e5': 'p'}, 'from': 'e4', 'to': 'e5'}, False),
  ('next to enemy king orthogonal', {'board': {'e4': 'K', 'e6': 'k'}, 'from': 'e4', 'to': 'e5'},
   False),
  ('next to enemy king diagonal', {'board': {'e4': 'K', 'g6': 'k'}, 'from': 'e4', 'to': 'f5'},
   False),
  ('knight below', {'board': {'a8': 'k', 'e4': 'K', 'f1': 'n'}, 'from': 'e4', 'to': 'e3'}, False),
  ('knight above', {'board': {'a8': 'k', 'd7': 'n', 'e4': 'K'}, 'from': 'e4', 'to': 'e5'}, False)],
 [('capture own', {'board': {'a8': 'k', 'e4': 'K', 'e5': 'P'}, 'from': 'e4', 'to': 'e5'}, False),
  ('capture undefended', {'board': {'a8': 'k', 'e4': 'K', 'e5': 'p'}, 'from': 'e4', 'to': 'e5'},
   True),
  ('next to enemy king diagonal', {'board': {'e4': 'K', 'g6': 'k'}, 'from': 'e4', 'to': 'f5'},
   False),
  ('knight below', {'board': {'a8': 'k', 'e4': 'K', 'f1': 'n'}, 'from': 'e4', 'to': 'e3'}, False),
  ('knight above', {'board': {'a8': 'k', 'd7': 'n', 'e4': 'K'}, 'from': 'e4', 'to': 'e5'}, False),
  ('black king bishop diag retreat',
   {'board': {'b3': 'B', 'd5': 'k', 'h1': 'K'}, 'from': 'd5', 'to': 'e6'}, False),
  ('capture along check line',
   {'board': {'a1': 'k', 'e3': 'p', 'e4': 'K', 'e8': 'r'}, 'from': 'e4', 'to': 'e3'}, False),
  ('capture defended knight',
   {'board': {'a8': 'k', 'e4': 'K', 'e5': 'n', 'e7': 'r'}, 'from': 'e4', 'to': 'e5'}, False)],
 [('retreat along rook file',
   {'board': {'a1': 'k', 'e4': 'K', 'e8': 'r'}, 'from': 'e4', 'to': 'e3'}, False),
  ('capture own', {'board': {'a8': 'k', 'e4': 'K', 'e5': 'P'}, 'from': 'e4', 'to': 'e5'}, False),
  ('capture undefended', {'board': {'a8': 'k', 'e4': 'K', 'e5': 'p'}, 'from': 'e4', 'to': 'e5'},
   True),
  ('black king bishop diag retreat',
   {'board': {'b3': 'B', 'd5': 'k', 'h1': 'K'}, 'from': 'd5', 'to': 'e6'}, False),
  ('capture along check line',
   {'board': {'a1': 'k', 'e3': 'p', 'e4': 'K', 'e8': 'r'}, 'from': 'e4', 'to': 'e3'}, False),
  ('capture defended knight',
   {'board': {'a8': 'k', 'e4': 'K', 'e5': 'n', 'e7': 'r'}, 'from': 'e4', 'to': 'e5'}, False),
  ('two squares', {'board': {'a8': 'k', 'e4': 'K'}, 'from': 'e4', 'to': 'e6'}, False),
  ('knight far below', {'board': {'a8': 'k', 'c4': 'n', 'e4': 'K'}, 'from': 'e4', 'to': 'd3'},
   True)],
 [('retreat along rook file',
   {'board': {'a1': 'k', 'e4': 'K', 'e8': 'r'}, 'from': 'e4', 'to': 'e3'}, False),
  ('sidestep rook file', {'board': {'a1': 'k', 'e4': 'K', 'e8': 'r'}, 'from': 'e4', 'to': 'd3'},
   True),
  ('diagonal step', {'board': {'a8': 'k', 'e4': 'K'}, 'from': 'e4', 'to': 'f5'}, True),
  ('stand still', {'board': {'a8': 'k', 'e4': 'K'}, 'from': 'e4', 'to': 'e4'}, False),
  ('capture own', {'board': {'a8': 'k', 'e4': 'K', 'e5': 'P'}, 'from': 'e4', 'to': 'e5'}, False),
  ('capture undefended', {'board': {'a8': 'k', 'e4': 'K', 'e5': 'p'}, 'from': 'e4', 'to': 'e5'},
   True),
  ('two squares', {'board': {'a8': 'k', 'e4': 'K'}, 'from': 'e4', 'to': 'e6'}, False),
  ('knight far below', {'board': {'a8': 'k', 'c4': 'n', 'e4': 'K'}, 'from': 'e4', 'to': 'd3'},
   True)]]
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
retreat along rook fileFalseFalsePassed
sidestep rook fileTrueTruePassed
diagonal stepTrueTruePassed
stand stillFalseFalsePassed
capture ownTrueFalseFailed
capture undefendedTrueTruePassed
capture defendedFalseFalsePassed
next to enemy king orthogonalFalseFalsePassed

SHA-256 / 9f63193f7ec9a5fea788943be7c8b623b1d2440f185d7f92748f186240c08b8a

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.

Member access is invitation-based. Sign in with your invited account to inspect the repair.

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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 2026-09-29T14:50:12.567320+00:00.

Case digest / 69b29258362c3c086ad499a48eda72f074b9835259146bb4a9474177a366b86b