A Chrome extension that reads the board on a live Chess.com game and overlays the Stockfish-recommended move as an arrow, updating automatically as the game progresses.

Before you use this

Using engine assistance during a live/rated game violates Chess.com's Fair Play rules and can get your account flagged or banned. This tool is meant for post-game analysis, practice against friends, or study — not for getting moves during rated play. Use responsibly.

Features

• Reads the live board directly from the page — no manual FEN entry
• Calls a hosted Stockfish 17 engine for the best move
• Draws an arrow overlay directly on the board
• Shows evaluation (centipawns or mate-in-N) in a status badge
• On/off toggle and adjustable engine depth from the extension popup
• Polls the board every 700ms, so it keeps up during fast/bot games

Installation

Since this isn't published on the Chrome Web Store, you'll install it as an unpacked extension:

1. Download this repository:
   • Click Code → Download ZIP, or
   • git clone https://github.com/YOUR_USERNAME/YOUR_REPO.git
2. Unzip it if needed.
3. Open Chrome and go to chrome://extensions.
4. Turn on Developer mode (top-right toggle).
5. Click Load unpacked and select the project folder (the one containing manifest.json).
6. Pin the extension, open a Chess.com game, click the icon, and turn Enabled on.

How it works

• fen-builder.js scrapes the .piece elements on the board (Chess.com encodes each piece's square in its CSS class) and reconstructs a FEN string.
• content.js polls the board, and whenever the position changes, sends the FEN to chess-api.com (a free hosted Stockfish endpoint) and draws an arrow for the returned best move.
• popup.html/popup.js provide the on/off toggle and depth slider, stored in chrome.storage.local.

Known limitations

• En passant isn't detected (rare edge case).
• Castling rights are approximated from king/rook starting squares.
• Whoever's turn it is is inferred from the move-list panel; if Chess.com changes that markup, this can misfire — check the on-board status badge.
• No local/offline engine — requires a network call per move (see "Going fully offline" below to remove this dependency).

Troubleshooting

Chess.com periodically changes their internal markup, which can break board reading. If the extension stops detecting the board:

1. On a live game, right-click a piece → Inspect.
2. Check the piece's class names (e.g. wp, bn) and square class (e.g. square-52).
3. Update PIECE_CLASS_REGEX / SQUARE_CLASS_REGEX in fen-builder.js to match.
4. Reload the extension and the page.

Going fully offline (optional)

To avoid any network calls, swap the engine call in content.js for a local Stockfish WASM worker:

1. npm install stockfish (or download a release).
2. Copy stockfish.js + stockfish.wasm into the project.
3. Add a background/offscreen document that runs Stockfish via UCI commands and messages the result back to the content script.

License

MIT — see LICENSE.

Contributing

Issues and PRs welcome, especially fixes for Chess.com DOM changes — see the Troubleshooting section for what usually needs updating.