Monte Carlo Tree Search


Monte Carlo Tree Search (MCTS) describes the class of algorithms used by the current crop of strong research Go playing programs.


A playout is the basis of all Monte Carlo methods. It is a fast game played with random moves from a starting position to the end of the game, generating either a score or simply a result (win/loss). A playout can be light (completely random moves) or heavy (moves are biased based on heuristics, such as pattern libraries, shape, group status, move history, killer moves, etc.). (See also: playout analysis)

Win rate is a statistic kept by nodes in the game tree, tracking how many of the playouts from this position resulted in a win for the color to move. Research before 2020 suggested that this is a more effective statistic to optimize than average score. However, KataGo has obtained better results by optimizing a combination of winrate and score. The use of winrate instead of score results in the characteristic Monte Carlo behavior of winning games by small margins.

CrazyStone started the new wave of MCTS programs by winning the 9x9 gold medal at the ICGA Computer Olympiad, in 2006. The principles of MCTS algorithms were published in the proceedings of the Computer and Games conference, which was held together with the Olympiad.

UCT is a simple and effective form of MCTS, used successfully by MoGo in 2006. It is a best-first search which tries to balance deep searches of high-winrate moves with exploration of untried moves.

RAVE (rapid action value estimate) is a heuristic which takes into account the frequent transpositions found in the game of Go. Normally, only the first move of a playout determines which node's statistics are updated. RAVE updates all children where the move was played at any point in the playout. AMAF (all moves as first) is a general name for this type of heuristic.


  • [ext] Gobble was one of the first programs to try Monte Carlo methods (but without the search tree on top). It introduced AMAF around 1993.
  • Indigo started experimenting with Monte Carlo methods in 2003.
  • Valkyria
  • CrazyStone, by [Rémi Coulom] introduced the modern form of MCTS in 2006
  • MoGo, by Sylvain Gelly?, introduced UCT and RAVE
  • lib EGO contains optimized routines for performing playouts
  • AyaMC is about 3-4 stones stronger than classical Aya
  • GNU Go has an experimental MC configuration for 9x9 Go
  • Leela, the first MC program for sale to the public
  • Many Faces of Go version 12 includes a MCTS engine
  • Pachi
  • Fuego
  • Zen (go program) was the first program to maintain a 1d rating on KGS, in 2009


Monte Carlo Tree Search last edited by xela on June 30, 2024 - 03:01
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