Mellum2.1 is the next version of Mellum2, with the architecture unchanged: still a 12B mixture-of-experts model with 2.5B active parameters under Apache 2.0, with the changes concentrated after pre-training. Mellum2 was fast but could not work inside a repository at the level its developers wanted; this version, trained with reinforcement learning in real environments across thousands of environments and millions of sandboxed runs, can explore a codebase, edit files and check its own changes. Under one shared evaluation setup it is compared with Mellum2 and with the similar open models Qwen3.5-9B and Gemma 4 E4B, and the largest gain is in agentic coding, with improvements also reported in coding, competitive programming, math, tool calling and general knowledge.
Mellum2.1 is the next version of Mellum2, with the architecture unchanged: still a 12B mixture-of-experts model with 2.5B active parameters under Apache 2.0, with the changes concentrated after pre-training. Mellum2 was fast but could not work inside a repository at the level its developers wanted; this version, trained with reinforcement learning in real environments across thousands of environments and millions of sandboxed runs, can explore a codebase, edit files and check its own changes. Under one shared evaluation setup it is compared with Mellum2 and with the similar open models Qwen3.5-9B and Gemma 4 E4B, and the largest gain is in agentic coding, with improvements also reported in coding, competitive programming, math, tool calling and general knowledge.