notlichess.org

A free mistakes trainer built on your own lichess or chess.com games.

It reads your rated games, runs Stockfish in your browser to find the moves that cost you, names each mistake (a hanging piece, a fork you walked into, a mate you missed) and trains you on those exact positions, spaced out, until they stop happening. After each position you see why: what your move allowed, the better move played out, and what really happened in your game. Insights rank your mistakes by the games they decided.

One static HTML file plus the engine. No server, no account, no ads. Your games and progress are saved in this browser, not on our servers. notlichess uses Microsoft Clarity to see how the site is used: it records clicks and screens, with all text hidden, and the site says so on the landing page, in the footer and in Settings. The page never calls Clarity's consent API: Clarity applies its own consent mode to visitors from the EEA, the UK and Switzerland (no cookies until a consent signal).

Run it

Serve this folder with any static file server (for example python3 -m http.server) and open it. The engine runs in a Web Worker, which browsers do not allow from a file:// page.

Layout

• index.html: the whole app (HTML, CSS and one script), built from src/
• sw.js: the service worker, so the app opens offline (it keeps the page, the manifest, the icons, the font and Stockfish)
• fonts/: Nunito, the page's font, served from this site (SIL Open Font License, fonts/OFL.txt)
• src/: the source. page.html is the page, app.css the styles, and js/ the script in parts, joined in the order in js/ORDER
• build.py: joins src/ into index.html (and refuses any em dash)
• test/: checks for the chess core, with sample data (see test/README.md)
• sf/: Stockfish 17.1 lite, compiled to WebAssembly (see sf/README.md)
• tools/: scripts that built the peer baselines (and the retired opening book) from the Lichess open database

Making a change

Edit src/, then:

python3 build.py
npm install        # once, for the linter
npm test
npm run lint

Commit src/ and the rebuilt index.html together. If you touched the mistake explanations, run npm run explain and review git diff test/explanations.txt before committing.

The earlier version, with the opening trainer, puzzles and Play tab, is kept in git: tag v1-full-platform and branch archive/full-platform.

Deploy

Render serves the repo root as a static site: no build command, the root as the publish directory, index.html as committed. Set these headers in the static site's settings (Headers):

Path	Header	Value
/sw.js	Cache-Control	no-cache
/	Cache-Control	no-cache
/index.html	Cache-Control	no-cache

• /sw.js is served from the root, as JavaScript, with no redirect (no redirect or rewrite rule may catch it): browsers refuse a service worker that comes through a redirect, and a copy held by the CDN would keep every player on an old version.
• / and /index.html are revalidated on every visit (the service worker asks with no-cache, and the ETag makes that a 304), so a deploy shows on the next visit.
• No Vary: * on any file the service worker keeps (the page, manifest.json, icons/, fonts/, sf/): the browser's Cache API refuses such a response.
• .wasm files as application/wasm, the type browsers need to compile Stockfish as it downloads.

After each deploy, two checks:

curl -sI https://notlichess.org/sw.js
curl -sI https://notlichess.org/

The first shows 200, a JavaScript type (application/javascript or text/javascript), cache-control: no-cache and no location. The second shows the new etag (it changes with each deploy of index.html) and cache-control: no-cache.

The service worker's off switch

A normal deploy needs nothing more: the page comes from the network first on every visit, and a change to Stockfish, an icon or the font comes with a new VERSION in sw.js (npm test names the value). The off switch is for an emergency: players stuck on a broken page or cache that a normal deploy does not reach, for example after a bug in sw.js itself. Deploy this as /sw.js in place of the normal one (the same code is test/sw-off.js):

/* notlichess.org: the service worker's off switch. In an emergency, deploy
   this file as /sw.js in place of the normal worker (README.md, Deploy):
   it takes over from the worker in charge at once, deletes every cache
   named notlichess-* (no other) and unregisters itself. It has no fetch
   handler, so every request goes to the network, as if there had never
   been a worker. To return to the normal worker, deploy sw.js from git. */
'use strict';
self.addEventListener('install', function () { self.skipWaiting(); });
self.addEventListener('activate', function (event) {
  event.waitUntil(caches.keys().then(function (names) {
    return Promise.all(names.filter(function (n) { return n.indexOf('notlichess-') === 0; }).map(function (n) { return caches.delete(n); }));
  }).then(function () { return self.registration.unregister(); }));
});

Browsers fetch /sw.js again on the next visit (the page registers it on every visit with a player, and an app left open checks hourly). The off switch takes over at once, deletes every cache named notlichess- and unregisters itself; from then on every request goes to the network, as if there had never been a worker. Nothing of the players' is lost: their games and progress live in localStorage, not in those caches. While it is deployed the app does not open offline.

To return to the normal worker, deploy the repo's sw.js again: the page registers it on the next visit, and the app opens offline again.

All of this was checked on Chromium only (headless, against a local server). Before a release, check by hand on an iPhone (Safari), an Android phone (Chrome) and desktop Firefox and Safari: a first visit, a reload with no network, a new deploy picked up on the next visit, and the two checks above after the deploy.

Credits

• Chess pieces: "cburnett" by Colin M.L. Burnett, CC-BY-SA 3.0, via lichess-org/lila
• Font: Nunito by The Nunito Project Authors, SIL Open Font License 1.1, in fonts/ with its licence (the latin subset as Google Fonts serves it, from the @fontsource-variable/nunito package)
• Analysis: Stockfish 17.1 (GPL v3), via stockfish.js, in sf/ with its license
• Mistake patterns: adapted from the tagger in lichess-org/lichess-puzzler
• Peer data: the Lichess open database (CC0), aggregators in tools/
• Built on the lichess.org API and the chess.com published-data API. Not affiliated with lichess or chess.com.

MIT licensed.