How do the chess engines on Chess.com work?

There are a lot of technical details that affect different pages of the site in different ways:

Chess engines that Chess.com runs server side are always modern engines (at the moment Stockfish 18) with the full NNUE. This includes Game Review and Cloud Analysis in Events (which determines the move classifications and accuracy scores you see on the events page as well as Chess.com broadcasts).

The Cloud Analysis used during broadcasts on events balances quality analysis with readily available feedback for move classifications and evaluation changes. Our research has found diminishing returns in engine quality as depth gets past the low 20’s. We want to get the best of both worlds (speed and quality together) for our broadcasts, and are beginning to pursue a multi-core analysis setup for this purpose.

Chess engines that run on a user’s computer (local engines) can be a bit trickier. If you go to chess.com/analysis, even as a basic user on most web browsers, you can access Stockfish 18. The default is a 'Lite' version that only uses the handcrafted evaluation (HCE) and no NNUE. This is because the full NNUE is 108 MB, which we cannot force-download onto every user’s machine for performance reasons.

The events page has a more intricate technical limitation. Websites cannot reliably load third-party content (such as Twitch stream embeds) on the same page as multi-threaded engines (anything after Stockfish 11). This is a limitation placed on us by internet browsers and is out of our control (if you go to any other chess site's event with a stream embed on the same page, you will similarly get an older engine).

We have a workaround for our broadcast where they get Stockfish 18 with the full NNUE (at the expense of features like the stream embed). This assures that the evals you see on broadcast are accurate for all the analysis being performed.

We are in the process of finalizing updates to local engines on our site that should improve user experience.