About Opening Checker

Opening Checker is a standalone free tool for players rated under 1200. It reads your finished games, detects the opening you actually played (whether London, Italian, Queen's Gambit, Sicilian, or anything else), checks whether you reached move ten safely, and optionally compares against Opening Tutorial lines when you want repertoire feedback.

What “safe at move ten” means

One definition, stored in one place in the code, so every screen agrees:

  • No piece lost to a tactic before move 15: material down by 2 or more pawns after an engine-verified capture sequence.
  • The engine's evaluation after Black's 10th move is no worse than -1.0 for you.
  • Games that end before move ten count as unsafe, unless you won.

The three labels

You left the line
You played a move the course doesn't. Check shows the course move, its one-sentence why, and the lesson.
They left it (covered)
Your opponent played a reply the lesson names in “the replies you'll meet”, with its share of games at your rating band.
They left it (not covered)
A rare move. Use the three questions: is it a threat, does it break a rule I can punish, and if neither, finish my setup.

The course lines in this build

27 engine-checked lines, 102 positions, built 2026-10-19. Every line carries a verification stamp from the course's offline pipeline; the app never asks an AI model to decide a chess fact.

  • The Italian, played calmly

    White · 11 lines · 38 positions

  • The London

    White · 5 lines · 24 positions

  • The Caro-Kann

    Black · 6 lines · 20 positions

  • The Slav setup

    Black · 5 lines · 20 positions

Fair play

  • Check only reads finished games. Before reading a Lichess account it asks the public API whether you are in a game, and refuses if you are.
  • chess.com's published-data archives contain only finished games.
  • Nothing overlays or connects to a live board. Both sites ban opening help during live games, and so do we.
  • Public data only: the chess.com published-data API, the Lichess public API, and PGN you paste. No scraping.

Privacy

Games are fetched by your browser and analysed by your browser. Nothing you check here is sent to us. This version has no accounts; saving results and your score over time come with the Home app.

Open-source notices

The analysis engine is open-source software used under the GNU General Public License v3 (source, WebAssembly build). The board is react-chessboard; move legality comes from chess.js.