Reasoning Quality Emerges Early

Curating reasoning data is expensive because scoring a trace usually means reading it to the end, but this UCLA work shows the quality of a trace is largely decided in its opening tokens. A short prefix predicts whole-trace quality well enough to rank and filter on, and difficulty can be detected from the loss of the first 100 tokens at a perturbed checkpoint. That turns curation into a cheap early-stopping problem, outperforming baselines while being far more token efficient at building SFT data for reasoning models.