The promise
Every puzzle here has exactly one answer, and every puzzle here can be
finished by reasoning — never by guessing. That is not a slogan: the
generator refuses to ship a grid its own technique library cannot finish,
and you can watch it refuse, in The Forge.
Playing
Tap a square, then a digit. The notes button switches the pad to
pencil marks; fill notes writes every legal candidate for you and
keeps them tidy as you go. Undo is unlimited. Nothing is timed unless you
want it to be, and Zen mode turns off the clock, the mistake ink and the
counting altogether.
Keyboard: arrows or WASD to move ·
1–9 to write · shift+1–9 for a pencil mark · backspace to erase ·
N notes · A fill notes · U undo · H hint · space pause
Hints that teach
The hint button is a ladder, not an answer. Press it once and the room
names the technique that is available. Press again and it shows you where
to look. Press a third time and it explains the argument in plain words and
plays the move. Nothing is ever revealed by peeking at the answer — the room
finds the move the same way you would.
Learning
Each technique the room uses on your behalf joins your codex, with its
idea and a real position to practise on. Find a technique yourself three
times — in a puzzle that genuinely needed it, without hints — and it is
marked mastered, and the room stops explaining it.
What it costs
Nothing. There are no ads, no accounts, no subscriptions, no analytics
and no server: your progress lives in this browser and nowhere else, and
the whole room works offline once loaded. If you clear your browser data it
is gone, because there is nowhere else for it to be.