Introducing JustRL II 🚀
Building on JustRL ( we took a closer look at how GRPO behaves in long-CoT RL (128k). The group-mean baseline is a great fit for short traces, but over tens of thousands of tokens it gives a coarse, response-level signal.
JustRL II keeps GRPO's group structure and adds a critic for token-level credit assignment. Just as simple, keeps improving where GRPO levels off: AIME25 61→81 on a 2B model.
The same recipe powers the RL stage of MiniCPM5-2B ( making it SOTA among models under 4B.
Data + checkpoints are open. Code lands this week. 📖
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