JetBrains releases Mellum2.1, an open coding-agent model under Apache 2.0: 12 billion parameters, 2.5 billion active
Prepared by Sintelligent with the help of SI. Published in SI: . Updated: .
A small open coding model that can run on your own hardware makes coding agents more accessible.
According to JetBrains and Habr, Mellum2.1 is a mixture-of-experts model (12 billion parameters, 2.5 billion active) for coding agents that can explore a codebase, edit files and check its own changes. Reinforcement learning in real environments became the main part of training, with added tasks in math, competitive programming, sciences, tool use and software engineering. Multi-token prediction speeds up a single response by about 1.6 times, and under parallel load the model processes almost twice as many tokens as Qwen3.5-9B. The weights are available on Hugging Face under the Apache 2.0 license; the comparisons are JetBrains' own.
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