Privacy notice

What Blunderbase stores

Your email address, held by Blunderbase’s authentication provider so you can sign in. If you sign in with Google, the provider also keeps the name and profile picture Google sends with it. Blunderbase does not use either.
The Chess.com and Lichess usernames you give Blunderbase.
The completed Rapid and Blitz games of those usernames, from each site’s public archive: the moves, the result, the ratings, both usernames, the opening, the site it came from, and the link to the game.
What Blunderbase derives from those games: engine analysis, the positions it flags, your review schedule, your practice attempts, your weekly program, and your coaching cycles.
On Pro, the players you follow in Players: their Chess.com or Lichess usernames, their recent completed Rapid and Blitz games from the same public archives (the same details as your own games), and the engine analysis of those games.
On Pro, for the players you follow: which of them you marked as your students, the notes you keep on a student, and the remarks you write on moves of their games, including any remark that says the engine was wrong about a move or missed a mistake. All of it stays private to your account and is never shown to the player it is about. The people who run Blunderbase read the remarks that say the engine was wrong, to make its analysis better.
Which puzzle-pack puzzles you have tried and whether you solved them, so a pack never shows you one twice.
A day-by-day record of what you did — games imported, games analyzed, reviews opened, puzzle sessions finished — and your daily puzzle goal.
The reflections you type, the confidence you pick, and the follow-up questions you asked about a position while Blunderbase still offered them.
For a mistake in one of your own games from the last seven days, the answer you pick to “What happened on this move?”, and when that question was first shown to you on each game. Insights counts your answers back to you.
The strategies and tactics you save in Notebook: the title, the notes you type, whether it is a strategy or a tactic, the position you add to it, and when you saved and last edited it. Only you see them, and you can delete any of them at any time.
The explanations and answers Blunderbase wrote for you, so the same one is shown again rather than written twice.
Your email settings: the weekly summary switch and, on Pro, the sync, session review and monthly report switches.
A record of each AI request Blunderbase makes on your behalf, so spending can be capped.
The error message Blunderbase showed you when importing games or following a player in Players failed, and when, so a problem you hit can be found and fixed. It is the message on your screen, not what you typed.
The version of these documents you accepted, when you accepted them, whether you were 18 or older, and, if you were not, that a parent or guardian agreed. You give the month and year you were born so your age can be checked; Blunderbase works your age out from them and does not keep them.
If you pay for Pro: which plan you are on, when the paid period ends, the subscription identifier Creem gave it, and the payment events Creem sent us, kept whole so a disputed charge can be answered from what actually happened rather than from a summary of it.
If you signed up through an invite link: the date your free Pro ends. Until your new account has used the link, it also keeps the link’s code, so the invite still works if you confirm your email on another device; the code is then deleted. We count how many accounts each link brought, but keep no record of which account used which link.

Blunderbase does not use cookies for advertising, does not run a third-party analytics script on your training pages, and does not sell anything to anyone. An invite link keeps its code in a cookie for up to a week, until your new account uses it.

Every page outside the signed-in app — the home page, pricing, the example review, the question pages, these documents, and the sign-up and sign-in screens — loads a visitor-counting script from PostHog, so we can tell whether anyone is finding the site. It records the page addresses, the site that linked you, your device type and country, how quickly each page loaded, and an anonymous identifier kept in your browser’s local storage, not in a cookie. It does not record clicks, keystrokes, or screen recordings. It is switched off entirely once you are inside /app, which is why the paragraph above says what it says. Requests go to blunderbase.app and are passed on from there; PostHog stores them in the European Union.

Where your data goes

Your games and everything derived from them stay in Blunderbase’s database, private to your account.

If you choose Continue with Google, Google learns that you signed in to Blunderbase and sends your email address, name and profile picture to Blunderbase’s authentication provider. Blunderbase never sees your Google password. Google’s own privacy policy governs what Google keeps about the sign-in.

On Pro, including the free Pro every new account starts with, Stockfish analyzes your new games on Blunderbase’s own server, so they are ready before you open the app; otherwise it runs in your browser.

Blunderbase sends data to an AI provider (Anthropic) in two places, and each one sends something different.

Explaining one position. That request contains only:

the position, as a FEN, with its pieces also listed square by square, which side you played and the move number;
the move you played, in coordinate and standard notation, and how much it cost compared with the engine’s best move;
the engine’s candidate lines and its classification of the mistake, with each line’s score, and the engine’s best replies to your move;
the reflection you typed, when the explanation follows one;
the teaching sentences Blunderbase built from the engine’s own output for this position.

The confidence you pick is not sent, and neither is your rating.

On Pro you can also ask about a move in a game you study in Players. That request sends the same things for the studied player’s move: the position, which side they played, their move and the engine’s lines and teaching sentences. It sends no reflection, and neither player’s username or rating.

The periodic progress note, once per coaching cycle. That request contains no moves, no positions and nothing identifying a single game. It contains:

the frozen progress report: the period, games played, wins, losses and draws, your rating at each end of it, opening names and your record in each, and the counts behind the mistake pattern it found;
the full list of measurements Blunderbase can score a cycle on;
the mistake category Blunderbase has already proven, and the candidate plans, evidence sentences, practice sizes, cues and success rules it wrote for that category. The model picks from those; it does not write them;
how many practice sessions you completed in the cycle;
the previous cycle’s outcome, and the numbers that outcome was measured from — not only the verdict.

Neither contains the game’s moves as a whole, the link to the game, your Chess.com or Lichess username, your opponent’s username, or your email address. Nothing you write or import is used to train a model.

Emails Blunderbase sends

Emails go out through Resend, which receives your email address and the message itself and nothing else. Blunderbase sends four kinds:

Account emails: confirming your address, an invitation if someone invited you, and resetting your password. These are part of having an account.
A weekly summary, on by default: your results that week, the weakness you are working on and how it moved, and how many puzzles are ready. It is sent only in a week when you played or have puzzles waiting. Every one has a link that stops it in one press, and the switch is under Account.
A review after each session, for Pro accounts that keep it on: your results, the move a lost game turned on, and how many new puzzles the session made, with a link to that moment. At most one a day, a couple of hours after your last game. It replaces the new-games email and keeps its switch under Account.
A monthly report, for Pro accounts, on by default: last month’s games, accuracy, the weakness that cost you most, and your puzzles, sent in the first days of the month. Every one has a link that stops it in one press.

The weekly summary and the monthly report are built from counts Blunderbase already holds and contain no positions or moves. The session review names one move, the one your game turned on. Nothing in any of them goes to an AI provider.

How long it is kept

For as long as you keep your Blunderbase account. There is no archive of deleted accounts.

Deleting your data

You can delete your account from inside Blunderbase. That deletes your profile, your imported games, and every analysis, review, reflection and session derived from them, the players you follow with their games and your notes and remarks on them, your puzzle-pack history and your Notebook, along with the name and picture Google sent if you signed in with Google. It does not delete anything on Chess.com, Lichess or Google. Any linked Pro subscription must be confirmed canceled before deletion. If billing cannot be confirmed, we keep the account intact and ask you to retry or contact support.

If you would rather ask a person, write to hello@blunderbase.app.

Payment data

Blunderbase does not process payments and never sees your card. Creem does, as the merchant of record for every Blunderbase subscription: it is the seller on the transaction, it holds the payment details, and it collects and remits the tax. Creem’s own privacy notice governs what it keeps.

What crosses from Blunderbase to Creem is your email address, so the receipt reaches you, and an identifier for your account, so the payment can be matched to it. Nothing about your games, your reflections, or your practice is sent. What comes back is the plan, the subscription identifier, and when the paid period ends. Those payment records are kept while the account exists and are deleted with it — Creem keeps its own copy of the transaction, because tax law requires it to.

Chess.com and Lichess data

Blunderbase reads completed games from Chess.com’s and Lichess’s public archives, for the usernames you give it. Those archives are open to anyone and Blunderbase signs in to neither site, so it holds no password and cannot check that a username is yours. Blunderbase is not affiliated with Chess.com or Lichess. Deleting your Blunderbase account does not change anything in either of those accounts.

When you follow another player on Pro, Blunderbase reads that player’s completed games from the same public archive and keeps them in your account, private to you and apart from your own: they never count as your games, puzzles or stats. Stop following a player and their games, with any notes and remarks you wrote on them, are deleted.

Puzzle packs come from the Lichess puzzle database, which Lichess publishes under a CC0 public-domain dedication. They are stored inside Blunderbase, so nothing about you is sent to Lichess to show them.

Teenagers and children

Blunderbase is for people 13 and older. Learners aged 13 to 17 need a parent or guardian to agree to the terms and to this notice for them, and the account records that they did. Every account is treated the same way whatever its age: nothing in the signed-in app runs an analytics script, nothing is used for advertising, and nothing is sold.

A parent or guardian can ask to see or delete a teenager’s account and everything in it by writing to hello@blunderbase.app.

Blunderbase does not knowingly keep data about children under 13. A new account whose owner says they are under 13 is deleted at once. If you believe a child under 13 has an account, write to hello@blunderbase.app and it will be deleted.

Contact

Questions about this notice, or about your data, go to hello@blunderbase.app. See also the terms of use.