JetBrains Releases Mellum2.1: A 12B MoE Open Model for Coding Agents

What happened

JetBrains released Mellum2.1, an Apache 2.0, 12B mixture-of-experts thinking model with 2.5B active parameters. JetBrains has released Mellum2.1 , an open model built for coding agents and fast sub-agents.

It ships under Apache 2.0 on Hugging Face . The upgrade comes almost entirely from reinforcement learning (RL) in real software environments.

RL in real repositories lifted its SWE-bench Verified score from 2.0 to 47.0. Mellum2.1 is a 12B mixture-of-experts thinking model from JetBrains that activates 2.5B parameters per token.

Key facts

• JetBrains — released: Mellum2.1, an Apache 2.0, 12B mixture-of-experts thinking model with 2.5B active parameters
• JetBrains has — released: Mellum2.1 , an open model built for coding agents and fast sub-agents