FAILURE MAP
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FA-82776 / Chess move legality / Open access

Every position without king moves called checkmate · case 01

A cornered black king that is not in check is declared mated.

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

ROOT CAUSE

The final classification ignores whether the king is currently attacked.

THE FAILURE

The final classification ignores whether the king is currently attacked.

Unsuccessful approach: Deciding check by queen adjacency calls long-range mates stalemate.

Case contract

Input: {"wk", "wq", "bk"} squares, black to move. Return "checkmate" if black has no legal king move and is attacked by the queen, "stalemate" if it has none and is not attacked, else "ongoing". The queen attacks along ranks, files and diagonals and is blocked only by the white king; the black king may capture the queen when it is not adjacent to the white king.

Why this case matters

Game-end detection must distinguish mate from stalemate exactly.

1 / The failure

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

N = 1
observations = []
def solve(x):
    def c(q): return ord(q[0])-97,int(q[1])-1
    wk,wq,bk=c(x['wk']),c(x['wq']),c(x['bk'])
    def queen_hits(t, occupied):
        if t==wq: return False
        df,dr=t[0]-wq[0],t[1]-wq[1]
        if not (df==0 or dr==0 or abs(df)==abs(dr)): return False
        sf=(df>0)-(df<0); sr=(dr>0)-(dr<0)
        f,r=wq[0]+sf,wq[1]+sr
        while (f,r)!=t:
            if (f,r) in occupied: return False
            f+=sf; r+=sr
        return True
    def near(a,b2): return max(abs(a[0]-b2[0]),abs(a[1]-b2[1]))<=1
    in_check=queen_hits(bk,{wk})
    moves=0
    for df in (-1,0,1):
        for dr in (-1,0,1):
            if not (df or dr): continue
            t=(bk[0]+df,bk[1]+dr)
            if not (0<=t[0]<8 and 0<=t[1]<8): continue
            if near(t,wk): continue
            if queen_hits(t,{wk}): continue
            moves+=1
    if moves: return 'ongoing'
    return 'checkmate'
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('corner stalemate', {'bk': 'a8', 'wk': 'c6', 'wq': 'b6'}, 'stalemate'),
  ('queen kiss mate', {'bk': 'a8', 'wk': 'b6', 'wq': 'b7'}, 'checkmate'),
  ('back rank long mate', {'bk': 'h8', 'wk': 'g6', 'wq': 'a8'}, 'checkmate'),
  ('capture free queen', {'bk': 'a8', 'wk': 'h1', 'wq': 'b7'}, 'ongoing'),
  ('queen check escape', {'bk': 'e5', 'wk': 'a1', 'wq': 'e1'}, 'ongoing'),
  ('shielded by king', {'bk': 'e5', 'wk': 'e3', 'wq': 'e1'}, 'ongoing'),
  ('stalemate h-file', {'bk': 'h8', 'wk': 'f7', 'wq': 'g6'}, 'stalemate'),
  ('mate with rank retreat', {'bk': 'b1', 'wk': 'a3', 'wq': 'd1'}, 'checkmate')],
 [('corner stalemate', {'bk': 'a8', 'wk': 'c6', 'wq': 'b6'}, 'stalemate'),
  ('shielded by king', {'bk': 'e5', 'wk': 'e3', 'wq': 'e1'}, 'ongoing'),
  ('stalemate h-file', {'bk': 'h8', 'wk': 'f7', 'wq': 'g6'}, 'stalemate'),
  ('queen behind white king', {'bk': 'a1', 'wk': 'b3', 'wq': 'b5'}, 'ongoing'),
  ('mate with rank retreat', {'bk': 'b1', 'wk': 'a3', 'wq': 'd1'}, 'checkmate'),
  ('queen kiss far side', {'bk': 'b8', 'wk': 'a6', 'wq': 'd8'}, 'checkmate'),
  ('edge mate', {'bk': 'e8', 'wk': 'e6', 'wq': 'e7'}, 'checkmate'),
  ('orthogonal free capture', {'bk': 'a8', 'wk': 'h1', 'wq': 'a7'}, 'ongoing')],
 [('corner stalemate', {'bk': 'a8', 'wk': 'c6', 'wq': 'b6'}, 'stalemate'),
  ('back rank long mate', {'bk': 'h8', 'wk': 'g6', 'wq': 'a8'}, 'checkmate'),
  ('stalemate h-file', {'bk': 'h8', 'wk': 'f7', 'wq': 'g6'}, 'stalemate'),
  ('mate with rank retreat', {'bk': 'b1', 'wk': 'a3', 'wq': 'd1'}, 'checkmate'),
  ('queen kiss far side', {'bk': 'b8', 'wk': 'a6', 'wq': 'd8'}, 'checkmate'),
  ('orthogonal free capture', {'bk': 'a8', 'wk': 'h1', 'wq': 'a7'}, 'ongoing'),
  ('far queen ongoing', {'bk': 'd5', 'wk': 'a1', 'wq': 'h2'}, 'ongoing'),
  ('king retreat along queen file', {'bk': 'd7', 'wk': 'a1', 'wq': 'd1'}, 'ongoing')],
 [('corner stalemate', {'bk': 'a8', 'wk': 'c6', 'wq': 'b6'}, 'stalemate'),
  ('queen kiss mate', {'bk': 'a8', 'wk': 'b6', 'wq': 'b7'}, 'checkmate'),
  ('back rank long mate', {'bk': 'h8', 'wk': 'g6', 'wq': 'a8'}, 'checkmate'),
  ('capture free queen', {'bk': 'a8', 'wk': 'h1', 'wq': 'b7'}, 'ongoing'),
  ('queen check escape', {'bk': 'e5', 'wk': 'a1', 'wq': 'e1'}, 'ongoing'),
  ('stalemate h-file', {'bk': 'h8', 'wk': 'f7', 'wq': 'g6'}, 'stalemate'),
  ('mate with rank retreat', {'bk': 'b1', 'wk': 'a3', 'wq': 'd1'}, 'checkmate'),
  ('queen kiss far side', {'bk': 'b8', 'wk': 'a6', 'wq': 'd8'}, 'checkmate')],
 [('corner stalemate', {'bk': 'a8', 'wk': 'c6', 'wq': 'b6'}, 'stalemate'),
  ('queen kiss mate', {'bk': 'a8', 'wk': 'b6', 'wq': 'b7'}, 'checkmate'),
  ('capture free queen', {'bk': 'a8', 'wk': 'h1', 'wq': 'b7'}, 'ongoing'),
  ('queen check escape', {'bk': 'e5', 'wk': 'a1', 'wq': 'e1'}, 'ongoing'),
  ('shielded by king', {'bk': 'e5', 'wk': 'e3', 'wq': 'e1'}, 'ongoing'),
  ('stalemate h-file', {'bk': 'h8', 'wk': 'f7', 'wq': 'g6'}, 'stalemate'),
  ('mate with rank retreat', {'bk': 'b1', 'wk': 'a3', 'wq': 'd1'}, 'checkmate'),
  ('queen kiss far side', {'bk': 'b8', 'wk': 'a6', 'wq': 'd8'}, 'checkmate')]]
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
corner stalematecheckmatestalemateFailed
queen kiss matecheckmatecheckmatePassed
back rank long matecheckmatecheckmatePassed
capture free queenongoingongoingPassed
queen check escapeongoingongoingPassed
shielded by kingongoingongoingPassed
stalemate h-filecheckmatestalemateFailed
mate with rank retreatcheckmatecheckmatePassed

SHA-256 / 19d2cf27209379b6678ce858a6539b9f9b2ab115db81681a928590c0be51f51f

2 / The unsuccessful fix

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

N = 1
observations = []
def solve(x):
    def c(q): return ord(q[0])-97,int(q[1])-1
    wk,wq,bk=c(x['wk']),c(x['wq']),c(x['bk'])
    def queen_hits(t, occupied):
        if t==wq: return False
        df,dr=t[0]-wq[0],t[1]-wq[1]
        if not (df==0 or dr==0 or abs(df)==abs(dr)): return False
        sf=(df>0)-(df<0); sr=(dr>0)-(dr<0)
        f,r=wq[0]+sf,wq[1]+sr
        while (f,r)!=t:
            if (f,r) in occupied: return False
            f+=sf; r+=sr
        return True
    def near(a,b2): return max(abs(a[0]-b2[0]),abs(a[1]-b2[1]))<=1
    in_check=queen_hits(bk,{wk})
    moves=0
    for df in (-1,0,1):
        for dr in (-1,0,1):
            if not (df or dr): continue
            t=(bk[0]+df,bk[1]+dr)
            if not (0<=t[0]<8 and 0<=t[1]<8): continue
            if near(t,wk): continue
            if queen_hits(t,{wk}): continue
            moves+=1
    if moves: return 'ongoing'
    return 'checkmate' if near(bk,wq) else 'stalemate'
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('corner stalemate', {'bk': 'a8', 'wk': 'c6', 'wq': 'b6'}, 'stalemate'),
  ('queen kiss mate', {'bk': 'a8', 'wk': 'b6', 'wq': 'b7'}, 'checkmate'),
  ('back rank long mate', {'bk': 'h8', 'wk': 'g6', 'wq': 'a8'}, 'checkmate'),
  ('capture free queen', {'bk': 'a8', 'wk': 'h1', 'wq': 'b7'}, 'ongoing'),
  ('queen check escape', {'bk': 'e5', 'wk': 'a1', 'wq': 'e1'}, 'ongoing'),
  ('shielded by king', {'bk': 'e5', 'wk': 'e3', 'wq': 'e1'}, 'ongoing'),
  ('stalemate h-file', {'bk': 'h8', 'wk': 'f7', 'wq': 'g6'}, 'stalemate'),
  ('mate with rank retreat', {'bk': 'b1', 'wk': 'a3', 'wq': 'd1'}, 'checkmate')],
 [('corner stalemate', {'bk': 'a8', 'wk': 'c6', 'wq': 'b6'}, 'stalemate'),
  ('shielded by king', {'bk': 'e5', 'wk': 'e3', 'wq': 'e1'}, 'ongoing'),
  ('stalemate h-file', {'bk': 'h8', 'wk': 'f7', 'wq': 'g6'}, 'stalemate'),
  ('queen behind white king', {'bk': 'a1', 'wk': 'b3', 'wq': 'b5'}, 'ongoing'),
  ('mate with rank retreat', {'bk': 'b1', 'wk': 'a3', 'wq': 'd1'}, 'checkmate'),
  ('queen kiss far side', {'bk': 'b8', 'wk': 'a6', 'wq': 'd8'}, 'checkmate'),
  ('edge mate', {'bk': 'e8', 'wk': 'e6', 'wq': 'e7'}, 'checkmate'),
  ('orthogonal free capture', {'bk': 'a8', 'wk': 'h1', 'wq': 'a7'}, 'ongoing')],
 [('corner stalemate', {'bk': 'a8', 'wk': 'c6', 'wq': 'b6'}, 'stalemate'),
  ('back rank long mate', {'bk': 'h8', 'wk': 'g6', 'wq': 'a8'}, 'checkmate'),
  ('stalemate h-file', {'bk': 'h8', 'wk': 'f7', 'wq': 'g6'}, 'stalemate'),
  ('mate with rank retreat', {'bk': 'b1', 'wk': 'a3', 'wq': 'd1'}, 'checkmate'),
  ('queen kiss far side', {'bk': 'b8', 'wk': 'a6', 'wq': 'd8'}, 'checkmate'),
  ('orthogonal free capture', {'bk': 'a8', 'wk': 'h1', 'wq': 'a7'}, 'ongoing'),
  ('far queen ongoing', {'bk': 'd5', 'wk': 'a1', 'wq': 'h2'}, 'ongoing'),
  ('king retreat along queen file', {'bk': 'd7', 'wk': 'a1', 'wq': 'd1'}, 'ongoing')],
 [('corner stalemate', {'bk': 'a8', 'wk': 'c6', 'wq': 'b6'}, 'stalemate'),
  ('queen kiss mate', {'bk': 'a8', 'wk': 'b6', 'wq': 'b7'}, 'checkmate'),
  ('back rank long mate', {'bk': 'h8', 'wk': 'g6', 'wq': 'a8'}, 'checkmate'),
  ('capture free queen', {'bk': 'a8', 'wk': 'h1', 'wq': 'b7'}, 'ongoing'),
  ('queen check escape', {'bk': 'e5', 'wk': 'a1', 'wq': 'e1'}, 'ongoing'),
  ('stalemate h-file', {'bk': 'h8', 'wk': 'f7', 'wq': 'g6'}, 'stalemate'),
  ('mate with rank retreat', {'bk': 'b1', 'wk': 'a3', 'wq': 'd1'}, 'checkmate'),
  ('queen kiss far side', {'bk': 'b8', 'wk': 'a6', 'wq': 'd8'}, 'checkmate')],
 [('corner stalemate', {'bk': 'a8', 'wk': 'c6', 'wq': 'b6'}, 'stalemate'),
  ('queen kiss mate', {'bk': 'a8', 'wk': 'b6', 'wq': 'b7'}, 'checkmate'),
  ('capture free queen', {'bk': 'a8', 'wk': 'h1', 'wq': 'b7'}, 'ongoing'),
  ('queen check escape', {'bk': 'e5', 'wk': 'a1', 'wq': 'e1'}, 'ongoing'),
  ('shielded by king', {'bk': 'e5', 'wk': 'e3', 'wq': 'e1'}, 'ongoing'),
  ('stalemate h-file', {'bk': 'h8', 'wk': 'f7', 'wq': 'g6'}, 'stalemate'),
  ('mate with rank retreat', {'bk': 'b1', 'wk': 'a3', 'wq': 'd1'}, 'checkmate'),
  ('queen kiss far side', {'bk': 'b8', 'wk': 'a6', 'wq': 'd8'}, 'checkmate')]]
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
corner stalematestalematestalematePassed
queen kiss matecheckmatecheckmatePassed
back rank long matestalematecheckmateFailed
capture free queenongoingongoingPassed
queen check escapeongoingongoingPassed
shielded by kingongoingongoingPassed
stalemate h-filestalematestalematePassed
mate with rank retreatstalematecheckmateFailed

SHA-256 / 4cfe0a6be0bc50fb7e55246090eb819e5471feab31ab9c6b3783df96109a9fd1

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 2026-09-29T14:50:15.292871+00:00.

Case digest / a511ca0572dcb4f34315863b8b8ef97de2c17cc9a2fb46d68fb096ad442844bc