Mellum2.1 is out.
A major update to the Mellum2 we open-sourced in June. The architecture is unchanged, 12B mixture-of-experts with 2.5B active parameters under Apache 2.0. What is new is the scale of reinforcement learning in post-training: millions of sandboxed runs inside real repositories, aimed at agentic coding.
The result is a small model that works as a coding agent. On SWE-bench Verified we achieved 47%, with gains across coding, math, tool use, and general knowledge as well.
It is also the fastest model in its class. Under load it serves about twice the tokens of Qwen3.5-9B on a single H200.
Built for sub-agents, coding agents and private self-hosted deployment. Weights are on Hugging Face: both HF and GGUF formats. MTP support is on its way.
Blog: blog.jetbrains.com/ai/2026/10/mel…
🤗 collection: huggingface.co/collections/Je…
Proud of this one. Try it and tell us what breaks.
A major update to the Mellum2 we open-sourced in June. The architecture is unchanged, 12B mixture-of-experts with 2.5B active parameters under Apache 2.0. What is new is the scale of reinforcement learning in post-training: millions of sandboxed runs inside real repositories, aimed at agentic coding.
The result is a small model that works as a coding agent. On SWE-bench Verified we achieved 47%, with gains across coding, math, tool use, and general knowledge as well.
It is also the fastest model in its class. Under load it serves about twice the tokens of Qwen3.5-9B on a single H200.
Built for sub-agents, coding agents and private self-hosted deployment. Weights are on Hugging Face: both HF and GGUF formats. MTP support is on its way.
Blog: blog.jetbrains.com/ai/2026/10/mel…
🤗 collection: huggingface.co/collections/Je…
Proud of this one. Try it and tell us what breaks.
blog.jetbrains.com Mellum2.1 Gets to Work: A Fast Open Model for Coding Agents - The JetBrains Blog Trained with reinforcement learning in real environments, Mellum2.1 is built for coding agents and fast sub-agents that run on your own hardware.