meta:
  title: "Mellum2-12B-A2.5B-Thinking"
  slug: "mellum2-12b-a2.5b-thinking"
  provider: "JetBrains"
  description: "JetBrains' reasoning-augmented code MoE (12B total / 2.5B active) that emits explicit <think> chains for debugging, planning, and agentic coding"
  date_added: 2026-06-02
  date_updated: 2026-06-02
  tasks:
    - text
  performance_headline: "69.9 LiveCodeBench v6, 58.4 AIME — fits on a single GPU"
  related_recipes:
    - "JetBrains/Mellum2-12B-A2.5B-Instruct"
  hardware:
    h200: verified

model:
  model_id: "JetBrains/Mellum2-12B-A2.5B-Thinking"
  min_vllm_version: "0.23.0"
  architecture: moe
  parameter_count: "12B"
  active_parameters: "2.5B"
  context_length: 131072
  base_args:
    - "--max-model-len"
    - "131072"
  base_env: {}

features:
  reasoning:
    description: "Parse the model's <think>...</think> reasoning blocks (Qwen3-style parser)"
    args:
      - "--reasoning-parser"
      - "qwen3"
  tool_calling:
    description: "Hermes tool-call parser for function calling"
    args:
      - "--enable-auto-tool-choice"
      - "--tool-call-parser"
      - "hermes"

  spec_decoding:
    description: "DFlash speculative decoding with an externally trained draft model; the served checkpoint is unchanged."
    args:
      - "--speculative-config"
      - '{"model": "RedHatAI/Mellum2-12B-A2.5B-Thinking-speculator.dflash", "num_speculative_tokens": 7, "method": "dflash"}'

opt_in_features:
  - spec_decoding

variants:
  default:
    precision: bf16
    vram_minimum_gb: 29
    description: "Native bfloat16 weights; fits comfortably on a single H200/H100/A100"

  fp8:
    model_id: "RedHatAI/Mellum2-12B-A2.5B-Thinking-FP8-Dynamic"
    precision: fp8
    vram_minimum_gb: 15
    description: "RedHatAI FP8 (E4M3) with dynamic per-token activation quantization"
    extra_args:
      - "--kv-cache-dtype"
      - "fp8"

  nvfp4:
    model_id: "RedHatAI/Mellum2-12B-A2.5B-Thinking-NVFP4"
    precision: nvfp4
    vram_minimum_gb: 8
    description: "RedHatAI NVFP4 quantization"

compatible_strategies:
  - single_node_tp
  - single_node_tep
  - single_node_dep

strategy_overrides:
  single_node_tp:
    tp: 1

guide: |
  ## Overview

  [Mellum2-12B-A2.5B-Thinking](https://huggingface.co/JetBrains/Mellum2-12B-A2.5B-Thinking)
  is JetBrains' reasoning-augmented code assistant. It uses a Mixture-of-Experts architecture
  with 64 experts (8 activated per token) — 12B total parameters, 2.5B active — combining
  sliding-window and full-attention layers for a 131,072-token context. The model emits its
  chain-of-thought inside `<think>...</think>` blocks before the final answer, making it
  suited to complex debugging, multi-step planning, and agentic workflows. For direct,
  low-latency answers without reasoning traces, use the
  [Instruct](https://huggingface.co/JetBrains/Mellum2-12B-A2.5B-Instruct) variant instead.

  ## Prerequisites

  - Hardware: a single H200, H100, or A100 (~29 GB at bf16) is plenty
  - vLLM **nightly** — `MellumForCausalLM` support landed after v0.22.0 and is not yet in a
    stable release. Install the nightly wheels until the next tagged release ships.

  ### Install vLLM (nightly)

  ```bash
  uv venv
  source .venv/bin/activate
  uv pip install -U vllm --extra-index-url https://wheels.vllm.ai/nightly
  ```

  ## Launch command

  ```bash
  # With reasoning (recommended for the Thinking checkpoint)
  vllm serve JetBrains/Mellum2-12B-A2.5B-Thinking \
    --max-model-len 131072 \
    --reasoning-parser qwen3

  # Add tool calling
  vllm serve JetBrains/Mellum2-12B-A2.5B-Thinking \
    --max-model-len 131072 \
    --reasoning-parser qwen3 \
    --enable-auto-tool-choice \
    --tool-call-parser hermes
  ```

  ## Client usage

  JetBrains recommends sampling at `temperature=0.6`, `top_p=0.95`, `top_k=20` for the
  Thinking checkpoint.

  ```python
  from openai import OpenAI
  client = OpenAI(base_url="http://localhost:8000/v1", api_key="EMPTY")

  resp = client.chat.completions.create(
      model="JetBrains/Mellum2-12B-A2.5B-Thinking",
      messages=[{"role": "user", "content": "Is 1024 a power of 2? Explain your reasoning."}],
      max_tokens=81920,
      temperature=0.6,
      top_p=0.95,
      extra_body={"top_k": 20},
  )
  print(resp.choices[0].message.content)
  ```

  ## References

  - [Model card](https://huggingface.co/JetBrains/Mellum2-12B-A2.5B-Thinking)
  - [vLLM support PR #43992](https://github.com/vllm-project/vllm/pull/43992)
  - [Mellum2 Technical Report](https://arxiv.org/abs/2605.31268)