About prochess.online

A free, privacy-first chess game review tool — built for players who want to truly understand their games, not just see a final score. Every move is classified by Stockfish 18 running directly in your browser.

What prochess.online Is

prochess.online is a fully client-side chess analysis platform. You import a finished chess game — from Chess.com, Lichess.org, or by pasting a PGN string — and the tool analyzes it move-by-move using the Stockfish 18 engine, compiled to WebAssembly and running entirely inside your browser.

Each move receives a classification: Brilliant, Best, Excellent, Good, Inaccuracy, Mistake, or Blunder. These are not arbitrary labels — they are derived from real engine evaluation deltas, calculated the same way top-tier review platforms do it.

Beyond classification, the tool provides an accuracy score for each player, opening recognition powered by thousands of ECO-coded lines, a tally table summarizing move quality at a glance, an evaluation bar tracking positional advantage throughout the game, and a Sandbox mode that lets you play out "what if" variations and see the engine's live response.

How It Works — The Technology

The core of prochess.online is Stockfish 18, one of the strongest chess engines in existence, compiled from C++ to WebAssembly via Emscripten. WebAssembly allows near-native execution speed inside a browser sandbox — no server round-trips, no cloud processing, no data leaving your device.

The analysis pipeline works as follows:

• Game import — PGN is parsed client-side using the chess.js library, which validates move legality and reconstructs the full game tree
• Position dispatch — Each position in the game is sent to Stockfish for evaluation at your chosen time control (Blitz: 0.8s, Balanced: 1.5s, Deep: 3.0s, or fully Custom)
• Move classification — The engine evaluation delta between consecutive moves determines whether a move was Brilliant (material sacrifice that holds advantage), Best (within 5 centipawns of top engine move), Excellent (within 15cp), Good (within 50cp), Inaccuracy (within 150cp), Mistake (within 300cp), or Blunder (300cp+ loss)
• Opening detection — The move sequence is matched against a database of thousands of named openings sourced from ECO (Encyclopedia of Chess Openings) codes A through E
• Accuracy calculation — Each player's accuracy is derived from comparing their actual move evaluations against the engine's optimal evaluations, using a statistical model consistent with industry-standard methodologies

Key Features in Detail

Move-by-Move Classification

Every single move in your game is classified. You see at a glance where the game turned — which moves held up under engine scrutiny and which ones cost you material or positional advantage. Hovering over any move in the ledger reveals the engine's top alternative continuation.

Sandbox Mode

During review, click any move in the ledger and play a different move on the board. The engine analyzes your alternate line live — showing evaluations and the engine's preferred responses for both sides. This is the single most powerful learning feature: you can explore "what if I had played this instead?" and see exactly what would have happened, move by move.

Opening Recognition

prochess.online identifies the opening you played from thousands of named variations — from the well-known (Sicilian Defense, Ruy Lopez, Queen's Gambit) to the obscure (Caro-Kann Fantasy Variation, King's Indian Sämisch Attack). The opening name and ECO code are displayed prominently so you always know where you stand in opening theory.

Accuracy Scoring

Each player receives a 0-100% accuracy score reflecting how closely their moves matched the engine's top choices. This is the same methodology used by major chess platforms. Track your accuracy across games to measure genuine improvement — or spot when you have plateaued and need to change your study approach.

Analysis Presets

Four analysis depth options let you balance speed against precision:

• Blitz (0.8s/move) — Fast, ideal for quick post-game reviews
• Balanced (1.5s/move) — Good depth without long waits
• Deep (3.0s/move) — Maximum precision for tournament preparation
• Custom — Set any milliseconds-per-move value you want

Who This Is For

prochess.online is built for any chess player who wants to improve through post-game analysis:

• Club players reviewing their weekly games to find recurring mistakes
• Online players wanting a deeper look at their Chess.com or Lichess games
• Students studying master games move-by-move with engine guidance
• Coaches preparing annotated games for their students
• Casual players curious about how their games hold up against engine analysis

Why It's Free

prochess.online is free because the heavy computation happens on your device, not ours. We don't run servers that do engine analysis — Stockfish runs in your browser via WebAssembly. There's no infrastructure cost per analysis, which means there's no reason to charge. The site is supported by minimal, non-intrusive advertising that doesn't interfere with the analysis experience.

No account required. No sign-up. No data collection. Just open the page, load a game, and start reviewing.