iirc there are multi syntaxes for writing down chess moves (some with very slight variations)

I finally managed to figure it out by trying out all combinations I could think of. Seems to me like this thing is too dumb to recognize all valid moves, as don’t see why xc5 should be illegal.

Anyhow, this thing accepts knight on F4 as the correct answer, written as “Nf4+”

Of course it changes when you reload the darn page 🫠