Frequently Asked Questions

Answers to common questions about how prochess.online works, what it can do, and what it cannot.

General Questions

Is prochess.online really free?

Yes — completely free. No account, no sign-up, no credit card, no trial period. The tool runs entirely in your browser using WebAssembly, so our infrastructure costs are minimal. We display non-intrusive ads to cover hosting, but they never interfere with board interaction or engine analysis.

Do I need to create an account?

No. There is no account system whatsoever. You open the page, load a game, and analyze it. Everything happens locally in your browser. We don't even have a database to store user accounts.

Do you store or upload my chess games?

No. Game analysis runs entirely inside your browser via Stockfish 18 compiled to WebAssembly. Your PGN data is never transmitted to any server. You can verify this yourself: disconnect your internet after the page loads — analysis continues working. Only the initial page assets (HTML, CSS, JS, the engine binary) are downloaded once.

Which chess engine is used?

Stockfish 18 — the official release, compiled to WebAssembly using Emscripten. Stockfish is one of the strongest chess engines in the world, consistently ranking at or near the top of computer chess championships. The WebAssembly build achieves near-native performance, typically within 85-95% of native execution speed depending on the position complexity.

How does this compare to Chess.com or Lichess analysis?

Chess.com and Lichess run their engines on powerful cloud servers. prochess.online runs Stockfish 18 directly in your browser on your device. The classification methodology (Brilliant through Blunder, accuracy scoring) is the same — but since our analysis is local, there is zero server latency, zero queuing, and your games never leave your device. The trade-off: your device's CPU determines analysis speed, not a cloud server. On most modern devices, performance is excellent.

Technical Questions

Can the engine be wrong?

Yes — and this is important to understand. Stockfish is incredibly strong but not infallible. At the Blitz preset (0.8 seconds per move), the engine may occasionally misclassify moves, especially in extremely complex tactical positions. The Deep preset (3.0 seconds) is far more reliable. At either depth, the engine may misjudge certain positional nuances or extremely deep tactical sequences. Use engine analysis as a learning aid — not as an absolute, unquestionable authority.

What do the move classifications actually mean?

Every classification is based on the evaluation delta — how much the position evaluation changed after your move compared to the engine's optimal continuation in that position:

How is accuracy calculated?

Accuracy is derived by comparing each move's evaluation against Stockfish's optimal evaluation. Each move contributes a percentage based on how much the position evaluation changed relative to the engine's best continuation. The final accuracy score is the average across all moves played by a given side. This methodology is consistent with how major platforms (Chess.com, Lichess) calculate their accuracy scores — though exact thresholds may differ slightly between implementations.

What platforms can I import games from?

Currently supported import methods:

The tool supports standard PGN format including headers, comments, and variations.

What is Sandbox mode?

Sandbox mode is the tool's most powerful learning feature. During a review, click any move in the move ledger and play an alternative move on the board. The engine immediately analyzes your alternate line — showing evaluations and the engine's top continuations. You can explore as many branches as you want, then return to the actual game continuation with one click.

Use Sandbox to answer questions like: "What if I had taken with the pawn instead of the knight?" or "Could I have played Nxf7 as a sacrifice here?" — and see the engine's actual evaluation of your idea.

Does it work on mobile devices?

Yes. The interface is fully responsive and adapts to screen widths from 320px (older phones) to 4K displays. On mobile devices, the board supports both touch-drag and tap-to-move interactions. The evaluation bar, move ledger, and accuracy scores all reflow to fit smaller screens. We recommend using the Blitz preset on mobile devices as the Deep preset may be slower on less powerful phone CPUs.

Why does the page need Cross-Origin isolation headers?

Stockfish 18's multi-threaded WebAssembly build requires SharedArrayBuffer, which modern browsers only expose when the page is served with specific HTTP headers: Cross-Origin-Opener-Policy: same-origin and Cross-Origin-Embedder-Policy: require-corp. Our .htaccess file includes these headers. If you're hosting on a platform that doesn't support custom headers, the engine falls back to a single-threaded build automatically — analysis quality is identical, just slightly slower.

Features & Limitations

Can I analyze multiple games at once?

Currently, one game at a time. Each review runs Stockfish in your browser, and running multiple engine instances simultaneously would overwhelm most consumer CPUs. We recommend reviewing games sequentially for the best experience.

Does it support chess variants?

No. prochess.online supports standard chess only (starting position, standard rules). Variants like Chess960, King of the Hill, or Atomic chess are not supported because Stockfish 18 is designed for standard chess positions.

Can I save or export my analysis?

Currently, analysis results are displayed in-browser only and are not persisted between sessions. For permanent records, we recommend taking screenshots of your review dashboard or copying the PGN with engine annotations (coming in a future update).

I found a bug or have a feature suggestion.

Please reach out via our Contact page. We read every message and prioritize fixes based on user reports. When reporting bugs, please include the PGN of the game you were analyzing — it dramatically speeds up diagnosis.