Why Stockfish says black is very better in this position

Stockfish is not as good at evaluating illegal positions

Oh yes I see 5 rooks and 8 pawns for black is impossible

That's not the reason.

The reason is engines can't detect fortresses.

Yes with 2 rooks it's the same. The computer counts material but is unable to sacrifice a rook, because white will not take.

Keep in mind, Stockfish is designed primarily to make the best move. It provides an evaluation as a side benefit, but in this position, Stockfish has no trouble playing optimal moves.

@Cedur216 said in #4:
> That's not the reason.

Is Stockfish good at evaluating all illegal positions? :-)

But yeah, in general Stockfish isn't designed for detecting fortresses, although multiple groups of developers made versions of Stockfish for this very purpose, for example:
www.reddit.com/r/chess/comments/rornau/crystal_40_has_just_been_released_crystal_is_a/

But also, this is hardly a new topic, and Stockfish isn't the best tool for fortress detection either:
lichess.org/forum/lichess-feedback/dead-draw-detector?page=3#23

Next time I go into a car dealership I'll complain that cars don't float in water, or fly through the air.

Stockfish is optimized to play, not to solve studies.
If you want to solve studies, use Crystal 4.0 or ShashChess 29

@Vinvin said in #8:
> Stockfish is optimized to play, not to solve studies.
> If you want to solve studies, use Crystal 4.0 or ShashChess 29
Crystal still evaluates this position as winning for Black up to d20 (around a quarter billion nodes). Huntsman is by far the best at positions that are specifically like this - it evaluates the position as even immediately and never varies.

But human is the best in illegal positions