SpecReason: Fast and Accurate Inference-Time Compute via Speculative
  Reasoning
Authors: name: Rui Pan; name: Yinwei Dai; name: Zhihao Zhang; name: Gabriele Oliaro; name: Zhihao Jia; name: Ravi Netravali
Published: 2025-04-10 16:05:19

Abstract: Recent advances in inference-time compute have significantly improved
performance on complex tasks by generating long chains of thought (CoTs) using
Large Reasoning Models (LRMs). However, this improved accuracy comes at the
cost of high inference latency due to the length of generated reasoning
sequences and the autoregressive nature of decoding. Our key insight in
tackling these overheads is that LRM inference, and the reasoning that it
embeds, is highly tolerant of approximations: complex tasks are typically
broken down into simpler steps, each of which brings utility based on the
semantic insight it provides for downstream steps rather than the exact tokens
it generates. Accordingly, we introduce SpecReason, a system that automatically
accelerates LRM inference by using a lightweight model to (speculatively) carry
out simpler intermediate reasoning steps and reserving the costly base model
only to assess (and potentially correct) the speculated outputs. Importantly,
SpecReason's focus on exploiting the semantic flexibility of thinking tokens in
preserving final-answer accuracy is complementary to prior speculation
techniques, most notably speculative decoding, which demands token-level
equivalence at each step. Across a variety of reasoning benchmarks, SpecReason
achieves 1.5-2.5times speedup over vanilla LRM inference while improving
accuracy by 1.0-9.9\%. Compared to speculative decoding without SpecReason,
their combination yields an additional 19.4-44.2\% latency reduction. We
open-source SpecReason at https://github.com/ruipeterpan/specreason.