Hidden Solutions
Version 43.1 sudoku Debian GNU/Linux 12 (bookworm) gnome version 43.6
when you fill a clear field wrongly based on the number being that should be in another clear field (that you haven't filled yet) it will show you the solution by marking it red. for example you have two fields you could put the 5 in now you can go ahead and just try if no other field is marked red the solution is right and if another clear field shows a red mark you know you choose the wrong field even though the clear field hasnt been filled yet since the game knows which number is supposed to be there.