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The World's Hardest Sudoku

In 2006 Finnish mathematician Arto Inkala designed a sudoku he called AI Escargot, built specifically to be as hard as a single-solution 9×9 puzzle can be for a human. It has just 23 clues and needs deep chains of logic to crack. Here it is, playable, with the story and the full solution below.

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Why AI Escargot is so hard

A sudoku's difficulty is not about how few clues it has, it is about the depth of reasoning required. AI Escargot was engineered so that almost every step depends on a long, forced chain of deductions rather than a quick scan. There are very few easy placements; you have to hold several possibilities in mind and follow them to a contradiction.

Inkala used his own solving algorithms to search for the puzzle that took the most steps of the hardest techniques. The name comes from the snail-shell pattern the solving path traces across the grid. He later published even harder designs, but AI Escargot remains the most famous.

How to actually solve it

Fill in every candidate first, you will need full pencil marks. Then look for the advanced techniques: X-wings and coloring to trim candidates, then XY-wings and forcing chains to break the logjam. Expect to spend most of the puzzle in chain logic.

If you get stuck, our step-by-step solver will show the next logical move, and the difficulty rater confirms just how extreme this grid is. There is exactly one solution, and it can be reached by logic alone, no guessing required.

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Frequently asked questions

What is the world's hardest sudoku?
The most famous is AI Escargot, designed by Finnish mathematician Arto Inkala in 2006 to be the hardest single-solution sudoku for a human solver. It has 23 clues and requires long chains of advanced logic. Inkala later published harder puzzles, but AI Escargot is the best known.
How many clues does the hardest sudoku have?
AI Escargot has 23 given clues. Difficulty is not about clue count, though, hard puzzles can have more clues than easy ones. What makes a puzzle hard is the depth of logic needed, not the number of starting digits.
Can the world's hardest sudoku be solved without guessing?
Yes. AI Escargot has exactly one solution and can be solved with pure logic using advanced techniques like X-wings, coloring, XY-wings, and forcing chains. You never have to guess, but you do have to follow long chains of deduction.
What is the hardest sudoku technique?
For puzzles like this you need chain-based techniques: forcing chains and Nishio, supported by coloring, XY-wings and unique-rectangle logic. Our strategy guides walk through each with worked, verified examples.