Policy & terms

The engine runs on your device

Everything that needs a chess engine (the computer's moves, analysis, reviews, hints, learn-from-mistakes) runs right in your browser. Our server doesn't run an engine: it only keeps accounts and games, pairs online players and handles payments.

• Downloads about 52 MB by itself (a 51 MB neural network and an engine program of almost 1 MB) right after you sign in, or when you open play the computer, analysis, game review, learn from mistakes or solve your own position. Nothing to click; it's ready once downloaded.
• Downloaded only once per engine version: the data is kept in your browser's Cache Storage on your device and is ready right away next time.
• If you're on mobile data, this download counts towards your plan. When the browser is in data saver mode, the engine doesn't download by itself and waits for you to tap download.
• The engine uses your device's CPU while the computer plays, analyses or reviews (up to 6 threads, always leaving one core for the interface), so your device may warm up and use more battery.
• The engine only receives the position on screen and returns a move; it doesn't read or send any other data on your device.
• Game review: your games are reviewed on your device. The result (scores and best lines for each position) is sent to the server to be kept with the game; the server only checks that the moves in it are legal.
• A recent browser with multi-threaded WebAssembly is needed: a recent Chrome, Edge, Firefox or Safari. Browsers built into some apps (Zalo, Facebook) may not work; open the page in your device's browser instead.
• Remove it from your device at any time: Settings → Engine on this device. Next time you play, the engine downloads again by itself.
• The engine is free software under the GPL-3.0 licence (name, authors and licence are in the source files). Source code of the exact in-browser build: /engine/SOURCE.txt.

Data we keep

• Account: email, display name (each name belongs to one player); if you sign in with Google, also your Google account ID and the link to your profile picture. We store no passwords (you sign in with a code sent by email, or with Google).
• Sign-in sessions: IP address and browser type at sign-in, so you can spot unknown sessions. Session tokens are stored only as hashes.
• Games: moves, clocks, results, Elo and reviews. Anyone with the link can view a game; online games show both players' names and others can watch them live.
• Payments: orders, amounts, transfer references and the incoming transactions on the bank account that receives the money. We never receive or store your cards or bank passwords.
• Profile picture: a picture you upload or copy from Gravatar when you click, stored as a 256×256 WebP; you can delete it in Settings.
• Learning progress: puzzles solved, puzzle rating, day streak, puzzle rush, stars for each Puzzle Path level, lessons, learning from mistakes, daily missions.
• Friends: your friend list and friend requests; friends see whether you're online or playing.
• Feedback: your message, the page you were on and your email (if you give it), forwarded to the administrators' email.
• Abuse prevention: request counts per account or IP (sign-in codes, new games, reviews…), deleted automatically after 24 hours.
• Security log: when a request arrives that our pages never send (calls from another site, probing the admin pages, forged puzzle proofs…), the account or IP address, the kind of request and the time are recorded and kept for 90 days.
• Error log: when a page hits an error on your device, the error, the page and the browser type are written to the server log so we can fix it.
• Quick chat messages in online games only go to the players in the game and are not kept.
• Retention: guests' games against the computer that nobody claims within 180 days, expired sessions and old sign-in codes are deleted daily. Database backups (hourly, keeping 48 hourly and 30 daily copies) are kept on our own storage server.
• Download your data: Settings → Account → My data gives you a file with everything we store about your account.

Cookies & browser storage

• pkf_session — keeps you signed in; 30 days.
• pkf_guest — links the games you play against the computer before signing in, so they move to your account when you sign in; 1 year.
• pkf_oauth — only for 10 minutes while signing in with Google.
• Browser storage: board settings when not signed in, lesson progress, background music position, and the engine (see above).
• We don't use advertising cookies or third-party trackers.

Third-party services

• Resend — sends sign-in code and receipt emails.
• Google — sign-in with Google (if you choose it), and Google Fonts for the fonts.
• MB Bank — the account that receives top-ups; we read the incoming transactions of this account to know when money has arrived. The payment QR code is made by us.
• Gravatar — only when you choose to take your picture from Gravatar: the server sends a hash (SHA-256) of your email to Gravatar to fetch the picture.

Delete account

Go to Settings → Account → Delete account. Your email, name, picture, Elo, progress, friends and wallet balance are deleted for good. Games you played with others stay for them to replay, but your name in them becomes “Deleted player”. If you ever topped up, orders and the wallet ledger (amounts, dates) are kept without any name or email, as accounting records. For your mailbox we keep only a one-way hash (it can't be turned back into the email), so an account made again on the same mailbox doesn't get the first-game gifts or the mission reward of the day it was deleted again.

Terms of use

• Fair play: in rated online games, don't use an engine, another person or any tool to suggest moves. Accounts that break this may be locked and their Elo adjusted.
• Games against the computer count towards your vs-computer Elo (separate, never used for pairing or rankings), using the Elo on the character's card — except games that used Undo or Hint and games started from a chosen position (those are just practice: no Elo, rank stars, unlocks or win missions). Leaving an online game for more than 45 seconds while playing counts as a loss; when the server restarts, both players have 90 seconds to come back.
• Abuse: an account that deliberately attacks the service (forged requests, probing admin rights, flooding) may be blocked; its wallet and games are kept — contact us to have it reviewed.
• Puzzles: a puzzle only counts as solved when you play the whole solution yourself; puzzles where you viewed the solution or used a hint don't earn puzzle points.
• Free: the first 3 characters (Little Na, Uncle Tư, Teacher Lan), custom computer levels 1–3, the coach for 1 game a day, analysis to depth 12. Each character unlocks after you beat the one before it once without Undo or Hint.
• Paid games: 100₫ per game against Ông Ba, Anh Khoa, Kiện tướng Hùng, Kỳ Ngư, the custom engine from level 4, every online game and every Puzzle Path try from stage 2, taken from your wallet when the game starts. An aborted online game (no one moved, or a server interruption) is refunded to your wallet; a 3-star Puzzle Path try is refunded too. The first game with each paid character is free, once per email inbox. While your wallet has money, coach comments on every game, hints, learning from mistakes and unlimited analysis cost nothing extra.
• Top-ups: from 20.000₫ by bank transfer; the money reaches your wallet when it arrives in our account, exactly as sent; wallet money never expires. For payment problems (a missing reference…), contact us with a picture of the transaction.
• Gift cards: enter the code on the Top-up page and the card amount goes into your wallet at once, like a top-up. Each code works once; after several wrong codes in a row you must wait 15 minutes to try again.
• The service is provided as is; prices and features may change, and changes to game prices will be announced by email in advance.