What engines do Chess.com use? What are all the differences between them? There are a lot of technical details that affect different pages of the site in different ways!
Server-side engines
Chess engines that Chess.com runs server side are always modern engines (at the moment Stockfish 16 or Stockfish 16.1) 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.
Local engines
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 16. The default is a “lite” version that only uses the handcrafted evaluation (HCE) and no NNUE. This is because the full NNUE is 45 MB, which we cannot force-download onto every user’s machine for performance reasons. Members can turn on the full NNUE version of Stockfish 16 in their settings menu and increase the “maximum depth” to 99 (unlimited) to get the highest-quality analysis.
Browser limitations
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 16 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.
Engine updates coming soon
We are moving from Stockfish 16 to Stockfish 16.1, as well as from Torch to Torch 2!
The “lite” versions of these engines will be smaller NNUEs instead of pure HCE (Stockfish officially got rid of their HCE with version 16.1 and Torch never had one). This means that the default engines on our site will be stronger, and the full NNUE versions will be available in your settings as they always have been.
To give you even more control of your computer usage when doing analysis on Chess.com, we will be adding a setting where you can specify the number of threads you want your computer to devote to engine analysis. This will let you balance engine analysis quality (more threads) with overall computer performance (fewer threads).
Our engineering team has made single-threaded versions of both Stockfish 16.1 and Torch 2. This means that you will have access to these engines even on our events pages, although they will be slightly more sluggish than the multi-threaded versions
We are preparing a user-facing advanced setting on the events page where you can opt into getting the multi-threaded versions of the engine, though this will come at the expense of stream embed functionality.