JetBrains Mellum2: The Open 12B MoE Coding Model Designed to Be a Sub-Agent, Not a Star

JetBrains open-sourced Mellum2 on June 1, 2026 — a 12B Mixture-of-Experts coding model with 2.5B active parameters, 131K context, and built-in speculative decoding. It is explicitly designed to be a fast component inside larger AI pipelines, not a standalone frontier replacement. Here is the technical review.