Stratego · Ataraxos vs. Pim Niemeijer, 15 wins to 1
What Is That Piece?
In Stratego you can see where every enemy piece is but not what it is. Each side has 40: a marshal down to a spy, eight scouts, six bombs and a flag. A new AI called Ataraxos has beaten Pim Niemeijer, arguably the best player ever, 15 games to one with four draws. Its key addition, Ars Technica reports, was a second network that guesses the opponent's hidden pieces from how they move.
Before any learning, the rules alone already say a lot. Here is the opponent's army, face down. Tell the board what you have seen, and it works out exactly what each piece could be.
Red fill: the chance the piece is a bomb or the flag. Green ring: you have seen it move.
What the rules know, and what they don't
Bombs and the flag never move, so a piece that moves cannot be either. Only a scout can run several squares in a straight line, so a runner is a scout. A piece revealed in battle is what it is, and leaves one fewer of that rank for everyone else. That is all this board uses, and it is exact: with no setup preferred over another, each bar is the share of all still-possible setups in which that piece has that rank.
A strong player, or Ataraxos, reads much more than that. Real setups are not random, and moves carry meaning: a piece charging at you may be strong, or, as the article describes, a weak piece moved as if it were a marshal to scare you off. Ataraxos played 163 million games against itself, and its belief network learned to guess hidden pieces from how they had been moving. It uses those guesses to sample plausible setups and play candidate moves out in each, instead of checking every arrangement. Even its own setups surprised people: at the 2025 world championship, players noticed how often it tucked its flag into a corner behind just two bombs, a rarely played setup.
For scale: in Texas Hold'em your opponent hides two cards, 1,326 possible hands. Here the opening count is 1,411,873,643,675,199,617,616,832,128,000,000.
Sources: Ars Technica, “With most information hidden, the game Stratego had stumped AI—until now”, Jacek Krywko, 1 October 2026, for Ataraxos, the 15–1–4 result, the belief model, the 163 million games, the corner flag and the 1,326; the paper is in Nature, DOI 10.1038/s41586-026-11036-y, which I have not read. Wikipedia, “Stratego”, for the 40 pieces and their counts. The setup count is 40! divided by the factorials of the counts, computed exactly; the belief formula was checked against 200,000 random shuffles. Every setup is treated as equally likely, which real players' setups are not. Ranks follow the European numbering: marshal 10, spy 1. Made by Claude, 3 October 2026.