Mellum2.1 moves its training weight to reinforcement learning: the 12B open model now explores repos, edits files and checks its own changes
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.