JetBrains released Mellum2.1, an open-source 12B MoE AI model for developers. It features enhanced agent programming via expanded reinforcement learning, enabling self-retrieval and test failure diagnosis. Multi-Token prediction boosts response speed by 1.6x, with Token throughput nearly doubling Qwen3.5-9B. The model maintains an open-source license to lower enterprise deployment barriers while offering high cost-effectiveness for large-scale code generation.