FA-82471 / Chess move legality / Open access

King denied diagonal steps · case 01

A king cannot step from e4 to f5.

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

ROOT CAUSE

The step test uses Manhattan distance 1, which excludes diagonal moves.

VERIFIED REPAIR

A king step has Chebyshev distance exactly 1.

Unsuccessful approach: Accepting a Manhattan distance of 1 or 2 also admits two-square straight moves such as e4-e6.

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 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),
  ('two squares', {'board': {'a8': 'k', 'e4': 'K'}, 'from': 'e4', 'to': 'e6'}, False)],
 [('diagonal step', {'board': {'a8': 'k', 'e4': 'K'}, 'from': 'e4', 'to': 'f5'}, True),
  ('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),
  ('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)],
 [('sidestep rook file', {'board': {'a1': 'k', 'e4': 'K', 'e8': 'r'}, 'from': 'e4', 'to': 'd3'},
   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),
  ('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),
  ('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 fixture	Actual	Expected	Outcome
retreat along rook file	`False`	`False`	Passed
sidestep rook file	`False`	`True`	Failed
diagonal step	`False`	`True`	Failed
stand still	`False`	`False`	Passed
capture own	`False`	`False`	Passed
capture undefended	`True`	`True`	Passed
capture defended	`False`	`False`	Passed
two squares	`False`	`False`	Passed

SHA-256 / 89672ce17fbdcbf28c927627f94b4b538966f609114dc4d304ec072650972661

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 abs(df)+abs(dr) not in (1,2): 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),
  ('two squares', {'board': {'a8': 'k', 'e4': 'K'}, 'from': 'e4', 'to': 'e6'}, False)],
 [('diagonal step', {'board': {'a8': 'k', 'e4': 'K'}, 'from': 'e4', 'to': 'f5'}, True),
  ('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),
  ('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)],
 [('sidestep rook file', {'board': {'a1': 'k', 'e4': 'K', 'e8': 'r'}, 'from': 'e4', 'to': 'd3'},
   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),
  ('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),
  ('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 fixture	Actual	Expected	Outcome
retreat along rook file	`False`	`False`	Passed
sidestep rook file	`True`	`True`	Passed
diagonal step	`True`	`True`	Passed
stand still	`False`	`False`	Passed
capture own	`False`	`False`	Passed
capture undefended	`True`	`True`	Passed
capture defended	`False`	`False`	Passed
two squares	`True`	`False`	Failed

SHA-256 / b9952108bfa5f64ce37c0b246324d49f98070a9447cb8055d91dd334d7f58edf

3 / The verified repair

Exit 0

"""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),
  ('two squares', {'board': {'a8': 'k', 'e4': 'K'}, 'from': 'e4', 'to': 'e6'}, False)],
 [('diagonal step', {'board': {'a8': 'k', 'e4': 'K'}, 'from': 'e4', 'to': 'f5'}, True),
  ('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),
  ('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)],
 [('sidestep rook file', {'board': {'a1': 'k', 'e4': 'K', 'e8': 'r'}, 'from': 'e4', 'to': 'd3'},
   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),
  ('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),
  ('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 fixture	Actual	Expected	Outcome
retreat along rook file	`False`	`False`	Passed
sidestep rook file	`True`	`True`	Passed
diagonal step	`True`	`True`	Passed
stand still	`False`	`False`	Passed
capture own	`False`	`False`	Passed
capture undefended	`True`	`True`	Passed
capture defended	`False`	`False`	Passed
two squares	`False`	`False`	Passed

SHA-256 / a8a18c33f01aaf0f7bc1b3a6128015d1e8821b914b64f32ccd01df98182c17dd

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 2026-09-29T14:50:12.502761+00:00.

Case digest / e15fec605dddfc46dd3d1440e0944e336612f97e203f89fb712483078fc2d73a