CHA-Solver

CHA-Solver<br>The tool to determine whether a player can be checkmated by any means, not necessarily by force, in a given chess position.<br>Required for fair timeout adjudication under FIDE Article 6.9.<br>Start the TutorialTry the Analyzer<br>Peer-reviewed (FUN 2022)Open source · MITcrates.iov3.0.0

White ran out of time. Draw!<br>Real Lichess game hPiwD75i, incorrectly adjudicated as a loss.

Why does it matter?<br>When a player runs out of time, the game is almost always ruled a loss, but it is exceptionally declared a draw if their opponent cannot possibly checkmate them. Indeed, Article 6.9 of the FIDE Laws of Chess states:<br>“[...] the game is drawn if the position is such that the opponent cannot checkmate the player’s king by any possible series of legal moves.”<br>FIDE Laws of Chess, Article 6.9<br>Chess servers have historically ignored this rule, treating the problem as computationally intractable, leading to many wrongly decided games.<br>CHA-Solver is an efficient algorithm that decides with mathematical certainty whether a position is unwinnable.<br>How it works

Questions & Answers<br>About<br>What does CHA mean?

How do chess servers adjudicate timeouts?

Can I see examples of unfairly adjudicated games?

Can I try CHA-Solver?

How<br>How does CHA-Solver work?

How reliable are CHA-Solver's verdicts?

How do I interpret "Unwinnable"?

Next<br>Is CHA-Solver open source?

Will Lichess adopt/integrate CHA-Solver?

We found 199,186 wrongly decided games on Lichess<br>Every rated game where a timeout was misjudged, scanned across the entire Lichess Open Database. Browse them all, or see how the numbers break down.

Browse the games<br>Browse every misjudged game, or search by username to check your own.<br>See the statistics<br>How the numbers break down: by reason, rating, time control and over time.