OctoThinker: Mid-training Incentivizes Reinforcement Learning Scaling
Authors: name: Zengzhi Wang, name: Fan Zhou, name: Xuefeng Li, name: Pengfei Liu

Different base language model families, such as Llama and Qwen, exhibit
divergent behaviors during post-training with reinforcement learning (RL),
especially on reasoning-intensive tasks. What makes a base language model
suitable for reinforcement learning? Gaining deeper insight into this question
is essential for developing RL-scalable foundation models of the next
generation. In this work, we investigate how mid-training strategies shape RL
dynamics, focusing on two representative model families: Qwen and Llama. Our
study reveals that (1) high-quality mathematical corpora, such as
MegaMath-Web-Pro, significantly improve both base model and RL performance,
while existing alternatives (e.g., FineMath-4plus) fail to do so; (2) further
adding QA-style data, particularly long chain-of-thought (CoT) reasoning
examples, enhances RL outcomes, and instruction data further unlocks this
effect; (3) while long-CoT improves reasoning depth, it can also induce
verbosity of model responses and unstability of RL training, underscoring the
importance of data formatting; (4) scaling mid-training consistently leads to
stronger downstream RL performance. Building on these insights, we introduce a
two-stage mid-training strategy, Stable-then-Decay, in which base models are
first trained on 200B tokens with a constant learning rate, followed by 20B
tokens across three CoT-focused branches with learning rate decay. This yields
OctoThinker, a family of models demonstrating strong RL compatibility and
closing the performance gap with more RL-friendly model families, i.e., Qwen.
We hope our work will help shape pre-training strategies for foundation models
in the RL era. To support further research, we release our open-source models
along with a curated math reasoning-intensive corpus of over 70 billion tokens
(i.e., MegaMath-Web-Pro-Max).