FA-82493 / Chess move legality / Member archive

Knights below the king are invisible · case 03

The king may step next to a knight that attacks the square from below.

Member previewVariant 3 · 3 implementations · 8 checks per implementation

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.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
knight below	`true`	`false`	Failed

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