# JetBrains Mellum2 Thinking (12B-A2.5B)

> JetBrains' Mellum2 Thinking is a compact reasoning-tuned sparse MoE for code and agentic workflows.

| Property | Value |
| --- | --- |
| Lab | JetBrains (Czech Republic) |
| Released | 2026-06-01 |
| Decoder type | Sparse MoE |
| Family | Sparse MoE |
| Scale | 12B total, 2.5B active (20.8% active) |
| Context window | 131,072 tokens |
| Attention | GQA with 3:1 sliding-window/full attention |
| Layer mix | 21 sliding-window + 7 full GQA |
| KV cache / token (bf16) | 56 KiB (Low) |
| License | Apache License 2.0 |

**Key detail:** Uses 64 experts with 8 active per token, a 1024-token sliding window, and YaRN scaling on full-attention layers.

## Architecture concepts

- [Grouped-Query Attention (GQA)](https://llm-atlas-7or.pages.dev/concepts/gqa.md): Several query heads share one key/value head, shrinking the KV cache with little quality loss.
- [Sliding Window Attention (SWA)](https://llm-atlas-7or.pages.dev/concepts/swa.md): Some layers only attend to a local window of recent tokens instead of the whole context.
- [Mixture of Experts (MoE)](https://llm-atlas-7or.pages.dev/concepts/moe.md): Many feed-forward experts, but a router sends each token to only a few of them.

## Links

- Architecture diagram: https://sebastianraschka.com/llm-architecture-gallery/images/architectures/mellum2.webp
- config.json: https://huggingface.co/JetBrains/Mellum2-12B-A2.5B-Thinking/blob/main/config.json
- Tech report: https://arxiv.org/abs/2605.31268
- Source card (source of truth): https://sebastianraschka.com/llm-architecture-gallery/#card-jetbrains-mellum2-thinking-12b-a2-5b
- Web page: https://llm-atlas-7or.pages.dev/models/jetbrains-mellum2-thinking-12b-a2-5b/
- JSON: https://llm-atlas-7or.pages.dev/api/models/jetbrains-mellum2-thinking-12b-a2-5b.json

_Data from Sebastian Raschka's LLM Architecture Gallery (https://sebastianraschka.com/llm-architecture-gallery/). Synced 2026-11-04._