Bingxiang He(@HBX_hbx):Introducing JustRL II 🚀 Building on JustRL (https://t.co/lwt1gYNDpG), 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 (https://t.co/bMA9848id4), making it SOTA among models under 4B. Data + checkpoints are open. Code lands this week. 📖

2026.09.08 09:05

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. 📖