About RookSac
RookSac is a free tool that reviews many of your chess games at once with the Stockfish engine, finds the mistakes you keep making, and turns them into a concrete study plan. Everything runs in your browser.
Last updated: September 30, 2026
Why it exists
Single-game analysis is easy to find and not very useful on its own. One blunder in one game tells you almost nothing about what to work on. The signal is in the pattern: the opening line you always misplay, the endgame you never convert, the fact that most of your rating points leak away in the first ten minutes of rapid games.
That pattern only shows up when you look at dozens or hundreds of games together. RookSac exists to make that bulk view a single click instead of an afternoon of manual work.
We built it because we couldn't find a free tool that did three things together: analyze all of a player's games, show the mistakes they keep repeating, and turn those mistakes into puzzles and training. Reviewing one game shows you what happened; reviewing hundreds shows you what you actually do. RookSac is for people who are serious about getting better at chess.
It has already shown us things we would not have guessed. When one of us ran their own games through it, two patterns stood out: the mistakes came mostly when time was running low, probably from rushing, and the openings were almost always the same few. Neither is easy to see in a single game, and both are the kind of pattern the report is built to surface.
What it does
- Bulk analysis. Pull your whole game history (or any slice of it: time control, colour, result, rating range, date range) and run every game through Stockfish.
- A cross-game report. Move-quality breakdown, accuracy, how you compare with the opponents you actually faced, accuracy by phase, rating progression, recurring pawn structures, and more.
- An improvement plan. Prioritised, specific actions — the exact opening lines, tactical motifs and endgame theory to study — each linked to the position in your own games where it went wrong.
- Training built from your mistakes. The positions you misplayed become interactive puzzles with staged hints and full explanations.
Not sure what a report looks like? The sample report page walks through one built from 300 real games, and the articles explain how to use each part of it.
How it works, briefly
When you enter a username, your browser fetches that player's public games directly from the Chess.com or Lichess API. Each game is analysed locally by Stockfish compiled to WebAssembly, running across a pool of Web Workers. The report and the training course are assembled in the page. Nothing is uploaded, and there is no account. For the full technical picture, see how the analysis works.
Free and private
RookSac is free to use with no sign-up. Because analysis happens on your own device, your game history is never sent to us or stored on a server. The site shows ads through Google AdSense to cover its costs; what that means for your data is described in the privacy policy.
RookSac builds on Stockfish (GPLv3), chess.js (BSD-2-Clause), the Lichess chess-openings dataset (CC0-1.0), and the Cburnett piece set, each used under its own licence. The engine runs in your browser as the Stockfish.js 17.1 WebAssembly build; its complete source code, like Stockfish's, is available at those links under the GNU General Public License v3.
Who makes it
RookSac is built by Terako, a small technology collective based in Montréal. Questions, bug reports and feature requests are welcome — see the contact page.