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FA-82531 / Chess move legality / Open access

Knight check treated as blockable · case 01

A queen move to a square between king and a checking knight is accepted.

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

ROOT CAUSE

Only pawn checks are marked unblockable, so a knight check falls through to the line walk.

THE FAILURE

Only pawn checks are marked unblockable, so a knight check falls through to the line walk.

Unsuccessful approach: Comparing without lower-casing only recognizes black checkers.

Case contract

Input: {"board", "move": [from, to]} for a non-king piece whose king may be in check (pins are ignored). Return true if the move is allowed by the check rules: no check means true; double check means false; a single check is answered by capturing the checker or, for a rook, bishop or queen checker, by moving onto a square strictly between it and the king.

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=x['board']; fr,to=x['move']; p=b[fr]; white=p.isupper()
    ks=[s for s,v in b.items() if v==('K' if white else 'k')][0]
    def at(f,r): return b.get(chr(97+f)+str(r+1)) if 0<=f<8 and 0<=r<8 else None
    def sq(f,r): return chr(97+f)+str(r+1)
    def attackers(s, by_white):
        f,r=ord(s[0])-97,int(s[1])-1
        P,N,B,R,Q=('PNBRQ' if by_white else 'pnbrq')
        out=[]
        pr=r-1 if by_white else r+1
        for df in (-1,1):
            if at(f+df,pr)==P: out.append(sq(f+df,pr))
        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: out.append(sq(f+df,r+dr))
        for df,dr in ((1,0),(-1,0),(0,1),(0,-1),(1,1),(1,-1),(-1,1),(-1,-1)):
            kinds=(R,Q) if df==0 or dr==0 else (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: out.append(sq(ff,rr))
                    break
                ff+=df; rr+=dr
        return out
    checkers=attackers(ks, not white)
    if len(checkers)>=2: return False
    if not checkers: return True
    c=checkers[0]
    if to==c: return True
    if b[c].lower()=='p': return False
    cf,cr=ord(c[0])-97,int(c[1])-1
    kf,kr=ord(ks[0])-97,int(ks[1])-1
    sf=(cf>kf)-(cf<kf); sr=(cr>kr)-(cr<kr)
    f,r=kf+sf,kr+sr
    between=[]
    while (f,r)!=(cf,cr) and 0<=f<8 and 0<=r<8:
        between.append(sq(f,r)); f+=sf; r+=sr
    return to in between
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('no check', {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k'}, 'move': ['b1', 'c3']}, True),
  ('block rook check',
   {'board': {'a8': 'k', 'b1': 'N', 'e1': 'K', 'e8': 'r'}, 'move': ['b1', 'c3']}, False),
  ('interpose on file',
   {'board': {'a8': 'k', 'd1': 'Q', 'e1': 'K', 'e8': 'r'}, 'move': ['d1', 'e2']}, True),
  ('capture rook checker',
   {'board': {'a8': 'k', 'e1': 'K', 'e8': 'r', 'h8': 'R'}, 'move': ['h8', 'e8']}, True),
  ('knight check sidestep piece',
   {'board': {'a8': 'k', 'b2': 'B', 'd3': 'n', 'e1': 'K'}, 'move': ['b2', 'c3']}, False),
  ('bishop takes knight checker',
   {'board': {'a8': 'k', 'e1': 'K', 'f3': 'n', 'g2': 'B'}, 'move': ['g2', 'f3']}, True),
  ('cannot block knight',
   {'board': {'a8': 'k', 'd1': 'Q', 'e1': 'K', 'f3': 'n'}, 'move': ['d1', 'e2']}, False),
  ('black cannot block white knight',
   {'board': {'a1': 'K', 'c8': 'b', 'd6': 'N', 'e8': 'k'}, 'move': ['c8', 'd7']}, False)],
 [('capture rook checker',
   {'board': {'a8': 'k', 'e1': 'K', 'e8': 'r', 'h8': 'R'}, 'move': ['h8', 'e8']}, True),
  ('knight check sidestep piece',
   {'board': {'a8': 'k', 'b2': 'B', 'd3': 'n', 'e1': 'K'}, 'move': ['b2', 'c3']}, False),
  ('bishop takes knight checker',
   {'board': {'a8': 'k', 'e1': 'K', 'f3': 'n', 'g2': 'B'}, 'move': ['g2', 'f3']}, True),
  ('cannot block knight',
   {'board': {'a8': 'k', 'd1': 'Q', 'e1': 'K', 'f3': 'n'}, 'move': ['d1', 'e2']}, False),
  ('double check block',
   {'board': {'a8': 'k', 'd1': 'Q', 'e1': 'K', 'e8': 'r', 'f3': 'n'}, 'move': ['d1', 'e2']},
   False),
  ('double check capture one',
   {'board': {'a8': 'k', 'e1': 'K', 'e8': 'r', 'f3': 'n', 'g2': 'B'}, 'move': ['g2', 'f3']},
   False),
  ('diagonal interpose',
   {'board': {'a5': 'b', 'c1': 'N', 'e1': 'K', 'h8': 'k'}, 'move': ['c1', 'b3']}, False),
  ('black cannot block white knight',
   {'board': {'a1': 'K', 'c8': 'b', 'd6': 'N', 'e8': 'k'}, 'move': ['c8', 'd7']}, False)],
 [('knight check sidestep piece',
   {'board': {'a8': 'k', 'b2': 'B', 'd3': 'n', 'e1': 'K'}, 'move': ['b2', 'c3']}, False),
  ('double check block',
   {'board': {'a8': 'k', 'd1': 'Q', 'e1': 'K', 'e8': 'r', 'f3': 'n'}, 'move': ['d1', 'e2']},
   False),
  ('double check capture one',
   {'board': {'a8': 'k', 'e1': 'K', 'e8': 'r', 'f3': 'n', 'g2': 'B'}, 'move': ['g2', 'f3']},
   False),
  ('diagonal interpose',
   {'board': {'a5': 'b', 'c1': 'N', 'e1': 'K', 'h8': 'k'}, 'move': ['c1', 'b3']}, False),
  ('diagonal interpose d2',
   {'board': {'a5': 'b', 'e1': 'K', 'f3': 'N', 'h8': 'k'}, 'move': ['f3', 'd2']}, True),
  ('black blocks white queen',
   {'board': {'a1': 'K', 'c6': 'n', 'e1': 'Q', 'e8': 'k'}, 'move': ['c6', 'e7']}, True),
  ('black cannot block white knight',
   {'board': {'a1': 'K', 'c8': 'b', 'd6': 'N', 'e8': 'k'}, 'move': ['c8', 'd7']}, False),
  ('irrelevant move in check',
   {'board': {'a2': 'P', 'a8': 'k', 'e1': 'K', 'e8': 'r'}, 'move': ['a2', 'a3']}, False)],
 [('no check', {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k'}, 'move': ['b1', 'c3']}, True),
  ('block rook check',
   {'board': {'a8': 'k', 'b1': 'N', 'e1': 'K', 'e8': 'r'}, 'move': ['b1', 'c3']}, False),
  ('knight check sidestep piece',
   {'board': {'a8': 'k', 'b2': 'B', 'd3': 'n', 'e1': 'K'}, 'move': ['b2', 'c3']}, False),
  ('diagonal interpose d2',
   {'board': {'a5': 'b', 'e1': 'K', 'f3': 'N', 'h8': 'k'}, 'move': ['f3', 'd2']}, True),
  ('black blocks white queen',
   {'board': {'a1': 'K', 'c6': 'n', 'e1': 'Q', 'e8': 'k'}, 'move': ['c6', 'e7']}, True),
  ('black cannot block white knight',
   {'board': {'a1': 'K', 'c8': 'b', 'd6': 'N', 'e8': 'k'}, 'move': ['c8', 'd7']}, False),
  ('irrelevant move in check',
   {'board': {'a2': 'P', 'a8': 'k', 'e1': 'K', 'e8': 'r'}, 'move': ['a2', 'a3']}, False),
  ('rank check interpose',
   {'board': {'a1': 'K', 'a8': 'k', 'b3': 'N', 'h1': 'q'}, 'move': ['b3', 'c1']}, True)],
 [('no check', {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k'}, 'move': ['b1', 'c3']}, True),
  ('block rook check',
   {'board': {'a8': 'k', 'b1': 'N', 'e1': 'K', 'e8': 'r'}, 'move': ['b1', 'c3']}, False),
  ('interpose on file',
   {'board': {'a8': 'k', 'd1': 'Q', 'e1': 'K', 'e8': 'r'}, 'move': ['d1', 'e2']}, True),
  ('capture rook checker',
   {'board': {'a8': 'k', 'e1': 'K', 'e8': 'r', 'h8': 'R'}, 'move': ['h8', 'e8']}, True),
  ('knight check sidestep piece',
   {'board': {'a8': 'k', 'b2': 'B', 'd3': 'n', 'e1': 'K'}, 'move': ['b2', 'c3']}, False),
  ('bishop takes knight checker',
   {'board': {'a8': 'k', 'e1': 'K', 'f3': 'n', 'g2': 'B'}, 'move': ['g2', 'f3']}, True),
  ('black cannot block white knight',
   {'board': {'a1': 'K', 'c8': 'b', 'd6': 'N', 'e8': 'k'}, 'move': ['c8', 'd7']}, False),
  ('rank check interpose',
   {'board': {'a1': 'K', 'a8': 'k', 'b3': 'N', 'h1': 'q'}, 'move': ['b3', 'c1']}, 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
no checkTrueTruePassed
block rook checkFalseFalsePassed
interpose on fileTrueTruePassed
capture rook checkerTrueTruePassed
knight check sidestep pieceTrueFalseFailed
bishop takes knight checkerTrueTruePassed
cannot block knightFalseFalsePassed
black cannot block white knightTrueFalseFailed

SHA-256 / 94eca61355b9547312970b508349c49edab4cba41b3403ab664f2b5d6f3e9e28

2 / The unsuccessful fix

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

N = 1
observations = []
def solve(x):
    b=x['board']; fr,to=x['move']; p=b[fr]; white=p.isupper()
    ks=[s for s,v in b.items() if v==('K' if white else 'k')][0]
    def at(f,r): return b.get(chr(97+f)+str(r+1)) if 0<=f<8 and 0<=r<8 else None
    def sq(f,r): return chr(97+f)+str(r+1)
    def attackers(s, by_white):
        f,r=ord(s[0])-97,int(s[1])-1
        P,N,B,R,Q=('PNBRQ' if by_white else 'pnbrq')
        out=[]
        pr=r-1 if by_white else r+1
        for df in (-1,1):
            if at(f+df,pr)==P: out.append(sq(f+df,pr))
        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: out.append(sq(f+df,r+dr))
        for df,dr in ((1,0),(-1,0),(0,1),(0,-1),(1,1),(1,-1),(-1,1),(-1,-1)):
            kinds=(R,Q) if df==0 or dr==0 else (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: out.append(sq(ff,rr))
                    break
                ff+=df; rr+=dr
        return out
    checkers=attackers(ks, not white)
    if len(checkers)>=2: return False
    if not checkers: return True
    c=checkers[0]
    if to==c: return True
    if b[c] in ('n','p'): return False
    cf,cr=ord(c[0])-97,int(c[1])-1
    kf,kr=ord(ks[0])-97,int(ks[1])-1
    sf=(cf>kf)-(cf<kf); sr=(cr>kr)-(cr<kr)
    f,r=kf+sf,kr+sr
    between=[]
    while (f,r)!=(cf,cr) and 0<=f<8 and 0<=r<8:
        between.append(sq(f,r)); f+=sf; r+=sr
    return to in between
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('no check', {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k'}, 'move': ['b1', 'c3']}, True),
  ('block rook check',
   {'board': {'a8': 'k', 'b1': 'N', 'e1': 'K', 'e8': 'r'}, 'move': ['b1', 'c3']}, False),
  ('interpose on file',
   {'board': {'a8': 'k', 'd1': 'Q', 'e1': 'K', 'e8': 'r'}, 'move': ['d1', 'e2']}, True),
  ('capture rook checker',
   {'board': {'a8': 'k', 'e1': 'K', 'e8': 'r', 'h8': 'R'}, 'move': ['h8', 'e8']}, True),
  ('knight check sidestep piece',
   {'board': {'a8': 'k', 'b2': 'B', 'd3': 'n', 'e1': 'K'}, 'move': ['b2', 'c3']}, False),
  ('bishop takes knight checker',
   {'board': {'a8': 'k', 'e1': 'K', 'f3': 'n', 'g2': 'B'}, 'move': ['g2', 'f3']}, True),
  ('cannot block knight',
   {'board': {'a8': 'k', 'd1': 'Q', 'e1': 'K', 'f3': 'n'}, 'move': ['d1', 'e2']}, False),
  ('black cannot block white knight',
   {'board': {'a1': 'K', 'c8': 'b', 'd6': 'N', 'e8': 'k'}, 'move': ['c8', 'd7']}, False)],
 [('capture rook checker',
   {'board': {'a8': 'k', 'e1': 'K', 'e8': 'r', 'h8': 'R'}, 'move': ['h8', 'e8']}, True),
  ('knight check sidestep piece',
   {'board': {'a8': 'k', 'b2': 'B', 'd3': 'n', 'e1': 'K'}, 'move': ['b2', 'c3']}, False),
  ('bishop takes knight checker',
   {'board': {'a8': 'k', 'e1': 'K', 'f3': 'n', 'g2': 'B'}, 'move': ['g2', 'f3']}, True),
  ('cannot block knight',
   {'board': {'a8': 'k', 'd1': 'Q', 'e1': 'K', 'f3': 'n'}, 'move': ['d1', 'e2']}, False),
  ('double check block',
   {'board': {'a8': 'k', 'd1': 'Q', 'e1': 'K', 'e8': 'r', 'f3': 'n'}, 'move': ['d1', 'e2']},
   False),
  ('double check capture one',
   {'board': {'a8': 'k', 'e1': 'K', 'e8': 'r', 'f3': 'n', 'g2': 'B'}, 'move': ['g2', 'f3']},
   False),
  ('diagonal interpose',
   {'board': {'a5': 'b', 'c1': 'N', 'e1': 'K', 'h8': 'k'}, 'move': ['c1', 'b3']}, False),
  ('black cannot block white knight',
   {'board': {'a1': 'K', 'c8': 'b', 'd6': 'N', 'e8': 'k'}, 'move': ['c8', 'd7']}, False)],
 [('knight check sidestep piece',
   {'board': {'a8': 'k', 'b2': 'B', 'd3': 'n', 'e1': 'K'}, 'move': ['b2', 'c3']}, False),
  ('double check block',
   {'board': {'a8': 'k', 'd1': 'Q', 'e1': 'K', 'e8': 'r', 'f3': 'n'}, 'move': ['d1', 'e2']},
   False),
  ('double check capture one',
   {'board': {'a8': 'k', 'e1': 'K', 'e8': 'r', 'f3': 'n', 'g2': 'B'}, 'move': ['g2', 'f3']},
   False),
  ('diagonal interpose',
   {'board': {'a5': 'b', 'c1': 'N', 'e1': 'K', 'h8': 'k'}, 'move': ['c1', 'b3']}, False),
  ('diagonal interpose d2',
   {'board': {'a5': 'b', 'e1': 'K', 'f3': 'N', 'h8': 'k'}, 'move': ['f3', 'd2']}, True),
  ('black blocks white queen',
   {'board': {'a1': 'K', 'c6': 'n', 'e1': 'Q', 'e8': 'k'}, 'move': ['c6', 'e7']}, True),
  ('black cannot block white knight',
   {'board': {'a1': 'K', 'c8': 'b', 'd6': 'N', 'e8': 'k'}, 'move': ['c8', 'd7']}, False),
  ('irrelevant move in check',
   {'board': {'a2': 'P', 'a8': 'k', 'e1': 'K', 'e8': 'r'}, 'move': ['a2', 'a3']}, False)],
 [('no check', {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k'}, 'move': ['b1', 'c3']}, True),
  ('block rook check',
   {'board': {'a8': 'k', 'b1': 'N', 'e1': 'K', 'e8': 'r'}, 'move': ['b1', 'c3']}, False),
  ('knight check sidestep piece',
   {'board': {'a8': 'k', 'b2': 'B', 'd3': 'n', 'e1': 'K'}, 'move': ['b2', 'c3']}, False),
  ('diagonal interpose d2',
   {'board': {'a5': 'b', 'e1': 'K', 'f3': 'N', 'h8': 'k'}, 'move': ['f3', 'd2']}, True),
  ('black blocks white queen',
   {'board': {'a1': 'K', 'c6': 'n', 'e1': 'Q', 'e8': 'k'}, 'move': ['c6', 'e7']}, True),
  ('black cannot block white knight',
   {'board': {'a1': 'K', 'c8': 'b', 'd6': 'N', 'e8': 'k'}, 'move': ['c8', 'd7']}, False),
  ('irrelevant move in check',
   {'board': {'a2': 'P', 'a8': 'k', 'e1': 'K', 'e8': 'r'}, 'move': ['a2', 'a3']}, False),
  ('rank check interpose',
   {'board': {'a1': 'K', 'a8': 'k', 'b3': 'N', 'h1': 'q'}, 'move': ['b3', 'c1']}, True)],
 [('no check', {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k'}, 'move': ['b1', 'c3']}, True),
  ('block rook check',
   {'board': {'a8': 'k', 'b1': 'N', 'e1': 'K', 'e8': 'r'}, 'move': ['b1', 'c3']}, False),
  ('interpose on file',
   {'board': {'a8': 'k', 'd1': 'Q', 'e1': 'K', 'e8': 'r'}, 'move': ['d1', 'e2']}, True),
  ('capture rook checker',
   {'board': {'a8': 'k', 'e1': 'K', 'e8': 'r', 'h8': 'R'}, 'move': ['h8', 'e8']}, True),
  ('knight check sidestep piece',
   {'board': {'a8': 'k', 'b2': 'B', 'd3': 'n', 'e1': 'K'}, 'move': ['b2', 'c3']}, False),
  ('bishop takes knight checker',
   {'board': {'a8': 'k', 'e1': 'K', 'f3': 'n', 'g2': 'B'}, 'move': ['g2', 'f3']}, True),
  ('black cannot block white knight',
   {'board': {'a1': 'K', 'c8': 'b', 'd6': 'N', 'e8': 'k'}, 'move': ['c8', 'd7']}, False),
  ('rank check interpose',
   {'board': {'a1': 'K', 'a8': 'k', 'b3': 'N', 'h1': 'q'}, 'move': ['b3', 'c1']}, 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
no checkTrueTruePassed
block rook checkFalseFalsePassed
interpose on fileTrueTruePassed
capture rook checkerTrueTruePassed
knight check sidestep pieceFalseFalsePassed
bishop takes knight checkerTrueTruePassed
cannot block knightFalseFalsePassed
black cannot block white knightTrueFalseFailed

SHA-256 / fd95d6afbd4ce73838e6e50d9cabe05fee5a7d1ace8c4f6e6eb6c6568daf0f1d

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 2026-09-29T14:50:12.923074+00:00.

Case digest / 0e389864d73781e89b71843a2e22641238017d44864aa47a3f7d1bc502edc00c