AI systems are improving rapidly at Slay the Spire and its sequel

Slay the Spire (StS1) and its recent sequel, Slay the Spire 2 (StS2), are difficult games. Since the original’s release in 2017, the game has attracted a dedicated community of players. Top-tier players play the games on their maximum difficulty settings and chase “win streaks” that require high win rates to sustain consecutive victories. Slay the Spire’s difficulty and popularity have more recently drawn attention from AI researchers. In its release of Claude Fable 5 in June 2026, Anthropic noted that Fable 5 “reached the game’s final act three times more often” than Claude Opus 4.8, a phrasing that indicated the model still could not reliably win games. METR researcher Ajeya Cotra used win rates for Slay the Spire 2 as part of her forecasts for AI capabilities in 2026. As a single-player game with randomness, hidden information, and a huge decision tree, Slay the Spire has long remained resistant to being conquered by both classic machine learning techniques and large language models (LLMs). Here, we present findings from our new benchmark, SpireClimb, that show this is rapidly changing. Frontier models can now beat Slay the Spire 1 at base difficulty (“Ascension 0”) at least 90% of the time. Just…