Mellum2.1

Mellum2.1 is JetBrains’ downloadable language model for coding agents: software that can inspect a project, edit files and use development tools. JetBrains released it on 8 October 2026.

The model activates part of its stored parameters

The official model card describes 12 billion total parameters, with 2.5 billion active for each token, and a 131,072-token context window. This mixture-of-experts design selects a subset of the model for each processing step.

The Thinking checkpoint produces reasoning as part of its response. Its published tests describe particular agent setups and evaluation conditions; results can change with the tools and instructions around the model.

Weights are available under Apache 2.0

JetBrains publishes the checkpoint under Apache 2.0. The main repository supplies Safetensors weights; a separate official GGUF repository supplies other runtime formats. Both were checked on 15 October. The model card supplies runtime and generation guidance.

Choose a format supported by the intended runtime and hardware. Memory requirements include the weights and the space needed to hold a conversation. A long advertised context window is a capacity limit; testing establishes what fits on a particular machine.

Compare its access and deployment requirements with the other language models.