Normally the length (sequence of moves) of what stockfish info prints out as 'pv' matches the search depth, but sometimes it doesn't. When determines its length?
1 Answer
I'm not an expert on this, but my understanding is that every now and then as Stockfish searches there will be a so-called "fail high" or "fail low" which forces a re-search. Under these situations Stockfish might not know what the best PV is, but it does know the best move it has found is. That is when it outputs a 1-move PV.
See also aspiration windows on the Chessprogramming Wiki.
