Exact Rulings Around Illegal Moves

Hi all,

I'm working to create my own chess website (personal only for now) as a fun project for myself, and I got to thinking around illegal moves. The first thing you do when building a chess board in software is build the board, and then make the pieces movable. In a sense this is actually the closest you get to OTB chess software-wise, since on the board you can move a piece wherever you want in theory, there's no legal move checker preventing moves (e.g. not responding to check).

I've decided it would be cool to have a mode where you can play all manner of illegal moves, and follow the same rulings as FIDE. These being that if one illegal move is raised, the position is reset and the opponent gets 2 minutes added to their clock. If a second illegal move is raised from the same player, that player automatically forfeits the game. I think it'd be fun to build a mode where this is the case, where the software takes note of an illegal move, but doesn't prevent it, and follows these rulings if a player decides to claim an illegal move.

However in that thinking we get to my actual question. Imagine a game OTB where 2 illegal moves have been played, but only 1 or neither were spotted during the game. It's only after the game when the player is analysing their game they realise an illegal move was played by their opponent.
How would this be ruled if the player brought it to the arbiter after the conclusion of the game/tournament? Would the game be reset, would an auto-win be awarded to the slighted player, or would something else happen? I'm not sure if this is something that's really been encountered often, but I was curious about it following my thoughts on this game mode.

If a player lost a game in that game mode, then in analysis realised an illegal move was played by their opponent, could they claim after the conclusion and essentially get the win flipped to them? Or would they need to claim an illegal move during the game, before a result ruling has been made? Any help would be appreciated!

If a game is completed then that is the result. Noting one (or more) illegal moves after the game finished is too late to change results; the player or arbiter must claim an illegal move after it is made.

One thing to note: in FIDE, arbiters intervene if they witness an illegal move. So you would need to determine how you want to answer this philosophical question: do you want your software to act as a perfect FIDE arbiter?

@1

"there's no legal move checker" ++ So even more mouse slips...

"Imagine a game OTB where 2 illegal moves have been played, but only 1 or neither were spotted during the game. It's only after the game when the player is analysing their game they realise an illegal move was played by their opponent."

'7.5.1 An illegal move is completed once the player has pressed his/her clock. If during a game it is found that an illegal move has been completed, the position immediately before the irregularity shall be reinstated. If the position immediately before the irregularity cannot be determined, the game shall continue from the last identifiable position prior to the irregularity. Articles 4.3 and 4.7 apply to the move replacing the illegal move. The game shall then continue from this reinstated position.'

"How would this be ruled if the player brought it to the arbiter after the conclusion of the game/tournament?"
'8.7 At the conclusion of the game both players shall indicate the result of the game by signing both scoresheets or approve the result on their electronic scoresheets. Even if incorrect, this result shall stand, unless the arbiter decides otherwise.'

"Would the game be reset, would an auto-win be awarded to the slighted player, or would something else happen?" ++ The game result stands.

"could they claim after the conclusion" ++ No.

"would they need to claim an illegal move during the game" ++ Yes.

Yeah theoretically this was something I considered, but I feel like the spirit of the idea would require no software intervention otherwise it's not that much different from existing software. I'm sure it could be customisable too, for example what if there was a version where pieces and pawns can only make legal moves with respect to placement, but pins and forks aren't required allowing someone to "hang" their king. At that point it's basically just a variant I suppose though!

Thanks for this, I found the stuff about the general claiming of illegal moves, but didn't see the specification of results standing, an oversight on my part.
As for the mouse slips, my thinking was it would of course be optional, so at least you know what you're getting into. But also probably limit it to slower time controls (maybe allow blitz but more likely Rapid+).

Probably best not to over-estimate my abilities, with the idea I could make a good enough chess site capable of generating money for me.

This rule is for slow games. It's not used in blitz for obvious reasons. In blitz if you make an illegal move, you lose.

@8

That is no longer true.

A.2 Players do not need to record the moves, but do not lose their rights to claims normally based on a scoresheet. The player can, at any time, ask the arbiter to provide him/her with a scoresheet, in order to write the moves.

A.3 The penalties mentioned in Articles 7 and 9 of the Competitive Rules of Play shall be one minute instead of two minutes.

A.4 The Competitive Rules of Play shall apply if:

A.4.1 one arbiter supervises at most three games and

A.4.2 each game is recorded by the arbiter or his/her assistant and, if possible, by electronic means.

A.4.3 The player may at any time, when it is his/her move, ask the arbiter or his/her assistant to show him/her the scoresheet. This may be requested a maximum of five times in a game. More requests shall be considered as a distraction of the opponent.

B.2 The Competitive Rules of Play shall apply if:

B.2.1 one arbiter supervises one game and

B.2.2 each game is recorded by the arbiter or his/her assistant and, if possible, by electronic means.

B.2.3 The player may at any time, when it is his/her move, ask the arbiter or his/her assistant to show him/her the scoresheet. This may be requested a maximum of five times in a game. More requests shall be considered as a distraction of the opponent.

B.3 Otherwise, play shall be governed by the Rapid chess Laws as in Article A.2, A.3 and A.5.

https://handbook.fide.com/chapter/E012023

hey man! this seems really cool. keep working, and release it soon!

Are you still working on it or you completely forgot about that? It's really a fabulous idea

I'm working on it in the background but life gets in the way! Would be a fun thing to launch someday if I get the chance.