Chess Analyzer (userscript)

One script, three features, two sites (chess.com and lichess.org):

1. Analyzer — draws the top 3 engine move suggestions (plus a dashed ponder arrow) directly on the board, with a live score badge.
2. Full-strength engine — Stockfish 18 with the full NNUE network (~113MB, downloaded once and cached by the browser), the same engine the Chess Analyzer web app uses. On lichess the script prefers the instant lichess cloud evaluation (depth 60+) and only falls back to the local engine for positions the cloud doesn't know.
3. Relay — mirrors the game you are playing to the web app's Live tab on any other device via a session ID.

Installation

1. Install a userscript manager extension in your browser (e.g., Violentmonkey, Tampermonkey, or similar).
2. Open your userscript manager and select Create a new script.
3. Copy all the code from the userscript.js file in this repository.
4. Paste the code into the script editor and click Save.
5. Refresh your chess game page, and the analyzer is ready to use!

Hotkeys

• A — Show or hide the move suggestions (starts/stops the analyzer).
• Insert — Hide or show the Analyze button. Useful for keeping the interface clean; the 'A' hotkey still works while the button is hidden.
• Ctrl+Shift+C — Copy your Relay ID to the clipboard (a small toast confirms and shows the ID). There is no persistent relay UI.

Engine

• On chess.com the script fetches the full Stockfish 18 NNUE build from chess.0xpnj.dev and runs it in a worker. First start shows "Loading engine… (one-time ~113MB)"; after that it starts instantly from the browser cache. If the download fails it falls back to chess.com's built-in engine.
• On lichess suggestions come from the lichess cloud evaluation API (instant, very deep, White/Black handled automatically). Positions the cloud doesn't know (most middlegames of live games) fall back to the local engine: the script downloads it through Tampermonkey's GM_xmlhttpRequest (declared in the header — approve the chess.0xpnj.dev permission if your userscript manager asks) and caches the 113MB in IndexedDB, so it downloads once ever, not once per page.

Live relay (watch from another device)

1. On the game page press Ctrl+Shift+C — your Relay ID is copied to the clipboard (a small toast shows it). The ID stays the same on this browser.
2. In the Chess Analyzer web app: Live tab → Session ID → paste the ID → Connect Session.
3. Every move appears on the web app's board within ~2 seconds with engine suggestions; when the game ends, a full Game Review starts automatically.

How it works / privacy

Positions are sent as small JSON messages through the public ntfy.sh pub/sub relay under the topic chessweb-<id>. Messages contain only the position (FEN) and the player usernames shown on the page. Anyone who knows the random ID could read them, so treat the ID like a private link.

Relay on lichess: lichess's page security policy blocks plain fetches, so the script sends through Tampermonkey's GM_xmlhttpRequest (approve the ntfy.sh permission if asked). The extension version works out of the box (its isolated world is exempt). If neither channel is available the relay goes quiet — you can always use the web app's Live → lichess username watch, which needs no relay at all.

Troubleshooting

• Analyzer is stuck or arrows don't appear: press 'A' twice (off and on again). This resets the engine and resyncs it with the current board.
• No arrows on some lichess positions (score shows "—"): that position is not in the lichess cloud and the local engine could not be loaded — usually because the userscript manager denied the chess.0xpnj.dev permission. Grant it (or use the extension version) and the engine takes over for those positions.

Extension version

The same script ships as a Chrome extension in the sibling chess-analyzer folder (scripts/main.js is generated from userscript.js).