Why does Stockfish evaluate this Lichess puzzle position as so advantageous for Black?

Question

[Event "Rated Rapid game"]
[Site "https://lichess.org/tZI4OlZ3"]
[Date "2021.04.20"]
[White "Paulinos"]
[Black "Qedesh76"]
[FEN "r3qrk1/2pbbppp/2nppn2/1B6/1P1P4/P3PN2/1BQN1PPP/2R1K2R b Kq - 0 1"]
[StartFlipped "1"]

1. Nxb4 axb4 2.Bxb5

Link to lichess exercise

I guessed my way through the puzzle, figuring it would be a discovered attack. If the pawn takes back the knight then I capture the bishop for a minor piece trade. I guessed that this was advantageous since Black has an open rook file.

But why does Stockfish think this position is so advantageous, with an evaluation -3.9, for Black? I followed the recommended lines a for a few moves. In certain lines where the White queen doesn't move, then the b4 pawn can be captured by the dark-squared bishop, setting up a powerful bishop pai. But other lines don't seem clearly so advantageous to me.

Answer 1

I daresay we talk about 1...Nxb4 2.axb4 Bxb5. OK, maybe not -3 but...

• White can't castle for some time
• Solid extra pawn (3.Qxc7 Nd5 will end in a desaster)
• Don't object to having the a-file
• Bishop pair (not relevant yet)
• White's Bb2 is bad, and since probably ...d5 soon comes, that won't change
• Finally, want to play something else? c6 hangs and c7 indirectly.

Against myself I'd resign on the spot.

Answer 2

Lichess tactical exercises are generated with code using games on the site and Stockfish.

Sometimes they are not very accurate. The exercise you mention is one of those I downvote because even Stockfish gives an advantage for white, it doesn't earn material enough to consider the game won. If I were analyzing this game with my oponnent I would say as black "I am better. I have the bishop pair and white cannot castle easily while I am castled and developed". But I think the goal of the exercises is to have a winning position or a draw in disadvantage, not a better position that takes long to convert into a win.

For example, this exercise wouldn't pass the chesstempo filter, as said in his FAQ.

I have made a play that I am sure leads to a won ending, but it is marked as wrong. Why is this happening?

Making plays that do not lead to a short-term material advantage, but instead require playing a long and drawn-out finish before the material advantage is realized, can be risky in the tactical problems of Chess Tempo. The program may not have been able to look far enough into the future to see that the ending leads to a position with a material advantage, and therefore your play may score less than it deserves. As frustrating as it is, this seems reasonable, given that tactical problems are all about finding combinations, not long-term victories in an endgame. To avoid losing points in these situations, if you see a solution that you think you win because it leads to a long, won ending, perhaps you should think about looking more carefully for a faster tactical solution.

When exercises are downvoted they are removed from the list so remember to downvote this kind of exercises that don't lead to a clear advantage.

Answer 3

horrible weakness of the light squares is what has not been listed in the answers yet. I'd probably not evaluate it at -3 though (I don't do it in numbers anyway), but it's pretty hopeless for white.