modules/official-bots/python/src/train.py

@@ -53,6 +53,11 @@ class NNUEDataset(Dataset):
         eval_val = self.evals[idx]
         features = fen_to_features(fen)
 
+        # Board is flipped for Black-to-move in fen_to_features; negate eval
+        # so the label still means "good for the side shown as White after flip"
+        if ' b ' in fen:
+            eval_val = -eval_val
+
         # Use evaluation as-is if normalized, otherwise apply sigmoid scaling
         if self.is_normalized:
             target = torch.tensor(eval_val, dtype=torch.float32)
@@ -75,10 +80,18 @@ _PIECE_TO_IDX = {
 
 
 def fen_to_features(fen):
-    """Convert FEN to 98304-dim king-relative (HalfKP) feature vector."""
+    """Convert FEN to 98304-dim king-relative (HalfKP) feature vector.
+
+    For Black-to-move positions the board is mirrored (ranks flipped, colours
+    swapped) so the network always sees the position from the side-to-move's
+    perspective.  The caller is responsible for negating the eval label to match.
+    """
     features = torch.zeros(INPUT_SIZE, dtype=torch.float32)
     try:
         board = chess.Board(fen)
+        # Perspective flip: present all positions as if White is to move
+        if board.turn == chess.BLACK:
+            board = board.mirror()
         wk = board.king(chess.WHITE)
         bk = board.king(chess.BLACK)
         if wk is None or bk is None:

modules/official-bots/src/main/scala/de/nowchess/bot/bots/nnue/NNUE.scala

@@ -35,6 +35,9 @@ class NNUE(model: NbaiModel):
 
   private def squareNum(sq: Square): Int = sq.rank.ordinal * 8 + sq.file.ordinal
 
+  // Mirror square vertically (rank 0 ↔ rank 7) for the perspective flip
+  private def flipSqNum(sqNum: Int): Int = (7 - sqNum / 8) * 8 + sqNum % 8
+
   private def pieceIdx(piece: Piece): Int =
     if piece.color == Color.White then 6 + piece.pieceType.ordinal else piece.pieceType.ordinal
 
@@ -225,11 +228,23 @@ class NNUE(model: NbaiModel):
   private val legacyL1 = new Array[Float](accSize)
 
   def evaluate(context: GameContext): Int =
-    val wkSq = wkSqOf(context.board)
+    // Match training: for Black-to-move positions, mirror the board (ranks flipped,
-    val bkSq = bkSqOf(context.board)
+    // colours swapped) so the model always sees from the side-to-move's perspective.
+    // The scoreFromOutput negation then converts back to White's absolute perspective.
+    val (wkSq, bkSq, pieces, turn) =
+      if context.turn == Color.Black then
+        val wk = flipSqNum(bkSqOf(context.board)) // flipped Black king → new "White" king
+        val bk = flipSqNum(wkSqOf(context.board)) // flipped White king → new "Black" king
+        val flipped = context.board.pieces.map { case (sq, p) =>
+          (sq, Piece(p.color.opposite, p.pieceType))
+        }
+        (wk, bk, flipped, Color.Black) // pass Black so scoreFromOutput negates the result
+      else (wkSqOf(context.board), bkSqOf(context.board), context.board.pieces, context.turn)
     System.arraycopy(model.weights(0).bias, 0, legacyL1, 0, accSize)
-    for (sq, piece) <- context.board.pieces do addPiece(legacyL1, piece, squareNum(sq), wkSq, bkSq)
+    for (sq, piece) <- pieces do
-    runL2toOutput(legacyL1, context.turn)
+      val sqNum = if turn == Color.Black then flipSqNum(squareNum(sq)) else squareNum(sq)
+      addPiece(legacyL1, piece, sqNum, wkSq, bkSq)
+    runL2toOutput(legacyL1, turn)
 
   def benchmark(): Unit =
     val context    = GameContext.initial