Move Output

The agent returns exactly one JSON object — no text before or after, no Markdown, only raw JSON.

Schema

{
  "from": "<Square>",
  "to": "<Square>",
  "promotion": "<Symbol>" | null
}

Fields

Field	Type	Required	Description
from	String	Yes	Starting square of the piece, e.g. "e2"
to	String	Yes	Target square of the piece, e.g. "e4"
promotion	String | null	Yes	Promotion piece ("Q", "R", "B", "N") or null

Promotion Rules

• promotion uses uppercase letters regardless of which side is promoting
• Must be set when a pawn reaches the last rank (rank 8 for White, rank 1 for Black)
• Must be null for all non-promotion moves

Special Move Encoding

Castling

Castling is encoded as a king move of two squares:

Move	from	to
White kingside	"e1"	"g1"
White queenside	"e1"	"c1"
Black kingside	"e8"	"g8"
Black queenside	"e8"	"c8"

The rook movement is handled automatically by the system.

En Passant

Encoded as a normal pawn diagonal move. The captured pawn is removed by the system.

{ "from": "e5", "to": "d6", "promotion": null }

This captures the pawn on d5 if en_passant was "d6".

Pawn Promotion

{ "from": "e7", "to": "e8", "promotion": "Q" }

Examples

// Standard pawn move
{ "from": "e2", "to": "e4", "promotion": null }

// Knight move
{ "from": "g1", "to": "f3", "promotion": null }

// Kingside castling
{ "from": "e1", "to": "g1", "promotion": null }

// Queen promotion
{ "from": "d7", "to": "d8", "promotion": "Q" }

// Knight underpromotion
{ "from": "a7", "to": "a8", "promotion": "N" }

Legality Checks

Before outputting a move, the agent must verify:

1. Own piece — The from square contains one of your own pieces
2. No own target — The to square is not occupied by your own piece
3. Correct pattern — The move follows piece movement rules
4. No jumping — No illegal jumps over pieces (except knight)
5. No self-check — After the move, your king is not in check
6. Castling conditions — All conditions from the rules are met
7. En passant validity — Only when en_passant is set and your pawn is positioned correctly
8. Promotion required — Must promote when reaching the last rank
9. No false promotion — promotion must be null when not promoting