A little faster.
A move sharper.

Inspired by Chess Tactics Server

The original Chess Tactics Server (CTS) inspired the six-second preview, time-weighted credit, and evolving player and puzzle ratings. This is a new implementation, independent of the original project.

The archived CTS time rules and rating guide document that foundation. The curve after ten seconds is reconstructed from the published graph and break-even rule; the original server code has not been recovered.

How the points work

You get six seconds to study the position before the opponent’s opening move. Solve the full line in an average of three seconds or less per move for full credit. Credit declines linearly to 50% at ten seconds per move, then falls more gradually. A wrong move or giving up earns zero.

Both player and puzzle ratings update after each confirmed attempt using Glicko, which accounts for uncertainty. A slow correct answer can still lose points. New players start at 1500 with high uncertainty, so early answers estimate their level quickly. Each game has an independent rating pool, separate from Lichess and FIDE.

Built for uninterrupted play

Moves and opponent replies run immediately on your device. Up to four upcoming puzzles are kept ready; their preview clocks start only when you open them. The thinking clock excludes previews, opponent reply delays, and time waiting for the network.

Completed results wait on this device until the server checks the move sequence and saves both ratings. Pending points are marked provisional. If the connection drops, keep playing cached puzzles; syncing retries automatically when the connection returns. Leave this browser’s data intact until everything is synced. An unfinished puzzle resumes on this device; confirmed account ratings and history are available across devices.

Good puzzles, openly shared

The 50,000 puzzles come from the Lichess open puzzle database, released under CC0. They come from real games, and every solution review links to its original game.

The collection includes one to three solver moves, at least 1,000 recorded attempts per puzzle, popularity of at least 85, and rating deviation no higher than 85. Lichess ratings provide the initial estimates; this site’s chess puzzle ratings then evolve with play. Selection favors fresh puzzles and mixes themes, colors, and solution lengths.

Go Tactics

Go shares your account with chess and keeps its own rating and history. There are 1,897 problems across two sources.

The 408 annotated problems are by An Younggil and David Ormerod, Go Game Guru, from their open problem collection. They include life-and-death and tesuji problems with professional explanations and alternative continuations.

The Go Game Guru collection uses Creative Commons Attribution–NonCommercial–ShareAlike 4.0. This free trainer credits the authors, preserves their license, and offers the converted problem data under the same license. Original SGF downloads are linked from each Go review. Conversion changes the file format, identifies winning branches from the authors’ annotations, and supplies initial difficulty estimates. Those estimates evolve with play.

For 27 additional easy and hard problems, training follows the author’s main line to an explicit success annotation. Their other branches could not all be classified reliably, so these additions accept only the recorded main solution. Full original SGFs remain available in reviews, and the hard main lines and easy main lines use the same CC BY-NC-SA license.

The other 1,489 problems are classical move records from Guan Zi Pu, Xuan Xuan Qi Jing, and Qi Jing Zhong Miao (Gokyo Shumyo), via Ulrich Görtz’s classical collection. Credit goes to Flygo, Jean-Pierre Vesinet, and Ulrich Görtz for the transcriptions and assembled files. Modern explanatory prose is not imported. The normalized classical records and individual SGFs are downloadable. There are also 8 longer classical solutions for deeper reading practice.

The 84 Gokyo Shumyo kyu problems use solution records by Ulrich Görtz. Their recorded winning alternatives are accepted; marked wrong variations are excluded. These cover shorter life-and-death and shape problems, with difficulty estimates that adjust through play.

Easier difficulty selects from the lowest-rated quarter of the Go collection. Expert difficulty selects from the highest-rated quarter of the Go collection, with fresh problems preferred. It uses this site’s evolving puzzle ratings; starting estimates are not official kyu/dan ranks. Your next problem follows the selected level.

Duplicates, including rotated, reflected, and color-swapped positions, are excluded from the additions. Each accepted setup and complete line passes capture, suicide, ko, and turn-order checks. Many classical records supply one solution; the Gokyo Shumyo kyu set also includes recorded alternatives. Unlisted valid moves may still be missing; legal replay does not independently establish tactical optimality.

Unclassified variations are not used as winning answers. Go scores follow the supplied solution trees, including documented winning alternatives where available; an unlisted move is not independently adjudicated by a Go AI. The timer uses the number of solver moves actually played. Your Go rating is not a kyu/dan rank.

One account, separate games

Chess and Go use the same sign-in and guest identity, but each has its own rating, calibration, and history. A new account keeps this guest’s progress in both games. Finish syncing before switching accounts. An unfinished puzzle and unsynced results remain on this device until you return to that game.

Open-source credits

Chess pieces by Colin M. L. Burnett (Cburnett), used under GPL-2.0-or-later. The unmodified SVG source files are available from this site, including the white knight, and from Lichess’s piece collection.

Chess move rules use chess.js, under the BSD-2-Clause license. Thank you to these projects and their contributors.