JetBrains releases Mellum2.1, open 12B coding-agent model

JetBrains released Mellum2.1, an Apache 2.0 mixture-of-experts model with 12B total and 2.5B active parameters. Reinforcement learning in real repositories lifted its SWE-bench Verified score from 2.0 to 47.0.

Why it matters: A 2.5B-active model reaching 47 on SWE-bench Verified shows how far repository-level RL can push small models. That makes capable coding agents feasible on modest or local hardware, and gives teams an open alternative to frontier APIs.