The point where castling is no longer an option is exactly where chess960 starts to look and feel like chess starting from the traditional position (SP518 RNBQKBNR). This is the point where a chess service like Chessify becomes fully viable.
A few weeks before writing that post, I had the opportunity to use Chessify on live chess960 positions. I was traveling for two weeks without any of the software tools I normally use for correspondence chess960. Before leaving home, I loaded six games into Chessify by using their 'Import' function to enter the current FEN position for each of the games. Most of the games had already reached the point where castling was no longer an option, but there were a couple of exceptions. A week into my trip I finally had some time to look at the games.
In the diagram's top position White has lost the castling privilege, while Black can still castle to both sides. I was playing Black. Stockfish NNUE on Chessify told me that 14...g5 was by far the best move, leading to a slight advantage for White. I vaguely remembered from my previous analysis that Black had an advantage in the position and decided that it must have something to do with castling. Since ...O-O-O isn't happening any time soon, the correct line must involve ...O-O. The move 14...O-O is a blunder because of 15.Nd7, which suggests 14...Nb6 to protect d7. Using Stockfish to continue the analysis and manipulating the FEN to simulate ...O-O, I decided that 14...Nb6 must be the right move and played it.
When I returned home, I looked at my previous analysis and discovered that 14...g5 was indeed the right move, leading to a significant advantage for Black. With 14...Nb6, I had spoiled my position and gone from a probable win to a struggle for a draw.
In the bottom position White has just castled, while Black can still castle ...O-O-O. I was again playing Black. Still using Stockfish NNUE, I decided that 15...O-O-O was premature and played 15...f6 instead. Back home I was happy with this move and after 16.f4, continued 16...O-O-O.
A few moves later I started to see weirdness from Chessify's Stockfish. I always calculate multiple variations (MPV in chess jargon) and reached a position where around depth 25, a key move disappeared from White's candidate moves and another move appeared twice. With the help of Chessify support (top notch in my opinion) I discovered that the FEN for the position was wrong. It indicated that both White and Black could still castle to the side where they had already castled. I corrected the FEN and the Stockfish analysis appeared normal. I decided that there was a bug in Stockfish triggered by faulty FEN.
In my four other games nothing unusual happened. While traveling I made a move or two in each game and when I returned home confirmed that my play was correct. My conclusions from this exercise? 1) Playing using only cloud tools is feasible; and 2) Playing chess960 with an engine that doesn't understand chess960 castling is possible, but risky. Always double check the chess960 FEN...