JetBrains’ Mellum2 open source coding model is a real step-change: a 12B-parameter, on-premises AI infrastructure model, shipped open from day one. This isn’t a warmed-over code completion tool — Mellum2’s speed, scope, and native private-deployment break new ground for agentic AI. On March 25, 2025, JetBrains announced Mellum2 with honest ambitions: go where API-locked models like Claude Code never will. For teams serious about AI infrastructure, not just code suggestions, this changes the map.
Mellum2 vs Mellum: how does the new model actually differ?
Mellum2 is not just “Mellum, scaled up.” The difference is structural. Mellum (late 2024) was a proprietary 4B-parameter model, delivered as a code completion engine for JetBrains IDEs — and eventually open-sourced in April 2025. Mellum2 multiplies the capacity (12B parameters), but the leap is in mandate: from single-task autocomplete to a practical infrastructure brain built for multi-agent AI systems.
Here’s what actually changes:
Parameter scale: Mellum2: 12B (with Mixture-of-Experts, see below). Original Mellum: 4B.
