What Characterizes Effective Reasoning? Revisiting Length, Review, and
  Structure of CoT
Authors: name: Yunzhen Feng; name: Julia Kempe; name: Cheng Zhang; name: Parag Jain; name: Anthony Hartshorn
Published: 2025-09-23 17:50:54

Abstract: Large reasoning models (LRMs) spend substantial test-time compute on long
chain-of-thought (CoT) traces, but what *characterizes* an effective CoT
remains unclear. While prior work reports gains from lengthening CoTs and
increasing review (revisiting earlier steps) via appended *wait* tokens, recent
studies suggest that shorter thinking can outperform longer traces. We
therefore conduct a systematic evaluation across ten LRMs on math and
scientific reasoning. Contrary to the "longer-is-better" narrative, we find
that both naive CoT lengthening and increased review are associated with
*lower* accuracy.
  As CoT unfolds step by step, token-level metrics can conflate verbosity with
process quality. We introduce a graph view of CoT to extract structure and
identify a single statistic-the *Failed-Step Fraction (FSF)*, the fraction of
steps in abandoned branches-that consistently outpredicts length and review
ratio for correctness across models. To probe causality, we design two
interventions. First, we rank candidate CoTs by each metric at test time, where
FSF yields the largest pass@1 gains; second, we edit CoTs to remove failed
branches, which significantly improves accuracy, indicating that failed
branches bias subsequent reasoning. Taken together, these results characterize
effective CoTs as those that *fail less* and support *structure-aware*
test-time scaling over indiscriminately generating long CoT.