Mellum 2.1
JetBrains' open 12B mixture-of-experts coding model, retrained with reinforcement learning to explore, edit, and verify its own code changes for fast local coding agents.
Overview
JetBrains released Mellum 2.1 on October 8, 2026, the next version of the 12B mixture-of-experts coding model it open-sourced in June, with 2.5B active parameters under the Apache 2.0 license. After a summer of reinforcement learning across millions of sandboxed runs in math, competitive programming, science, tool use, and software engineering, Mellum 2.1 jumped from 2% to 47% on SWE-bench Verified and can explore a codebase, edit files, and check its own changes. Multi-token prediction makes it roughly 1.6x faster per token than its predecessor, and JetBrains positions it as a compact sub-agent model that fits on a single high-end GPU instead of a cluster.
This is a sourced directory entry. Features and capabilities are reported in the linked sources; this entry does not document independent Aymorphic testing.
Key features
- 12B parameters, 2.5B active (mixture-of-experts), Apache 2.0 license, no commercial-use restrictions
- SWE-bench Verified score jumped from 2% (Mellum2) to 47%
- Trained via reinforcement learning across millions of sandboxed runs: file exploration, editing, self-verification
- Multi-token prediction yields ~1.6x faster per-token throughput, fastest under heavy load versus Mellum2, Qwen3.5-9B, and Gemma 4 E4B
- Fits on a single high-end GPU for local/private deployment
Use cases
- Fast, self-hosted sub-agent for coding-agent pipelines (file exploration, editing, change verification)
- On-prem or private coding assistant where cluster-scale models aren't an option
- Low-latency tool-calling and agentic coding steps inside IDE workflows
Pricing
Free and open weight under Apache 2.0; no commercial-use restrictions. Self-hosted, so cost is infrastructure-dependent.
Pricing reflects the source record and may change. Confirm current terms with the provider.
Dates and sources
- Released
- 8 Oct 2026
- Record updated
- 12 Oct 2026
- Record last checked
- 12 Oct 2026