LFM2-8B-A1B — MLX 3-bit (Apple Silicon)

Maintainer / Publisher: Susant Achary
Upstream model: LiquidAI/LFM2-8B-A1B
This repo (MLX 3-bit): mlx-community/LFM2-8B-A1B-3bit-MLX

This repository provides an Apple-Silicon-optimized MLX build of LFM2-8B-A1B at 3-bit quantization.
3-bit is an excellent size↔quality sweet spot on many Macs—very small memory footprint with surprisingly solid answer quality and snappy decoding.

🔎 What is LFM2-8B-A1B?

Architecture: Mixture-of-Experts (MoE) Transformer.
Size: ~8B total parameters with ~1B active per token (the “A1B” naming commonly indicates ~1B active params).
Why MoE? Per token, only a subset of experts is activated → lower compute per token while retaining a larger parameter pool for expressivity.

Memory reality on a single device: Even though ~1B parameters are active at a time, all experts typically reside in memory in single-device runs. Plan RAM based on total parameters, not just the active slice.

📦 What’s in this MLX build

config.json (MLX), mlx_model*.safetensors (3-bit shards)
Tokenizer: tokenizer.json, tokenizer_config.json
Metadata: model_index.json (and/or processor metadata as applicable)

Target: macOS on Apple Silicon (M-series) using Metal/MPS.

✅ Intended use

General instruction following, chat, and summarization
RAG back-ends and long-context assistants on device
Schema-guided structured outputs (JSON) where low RAM is a priority

⚠️ Limitations

3-bit is lossy: tiny improvements in latency/RAM come with some accuracy trade-off vs 6/8-bit.
For very long contexts and/or batching, KV-cache can dominate memory—tune max_tokens and batch size.
Add your own guardrails/safety for production deployments.

🔢 RAM planning (3-bit, MoE, MLX)

You asked to assume and decide realistic ranges. The numbers below are practical starting points—verify on your machine.

Rule-of-thumb components

Weights (3-bit): ≈ total_params × 0.375 byte → for 8B params ≈ ~3.0 GB
Runtime overhead: MLX graph/tensors/metadata → ~0.6–1.0 GB
KV-cache: grows with context × layers × heads × dtype → ~0.8–2.5+ GB

Indicative peak RAM (batch=1)

Context window	Estimated peak RAM
4k tokens	~4.4–5.5 GB
8k tokens	~5.2–6.6 GB
16k tokens	~6.5–8.8 GB

For ≤2k windows you may see ~4.0–4.8 GB. Larger windows/batches increase KV-cache and peak RAM.

🧭 Precision choices for LFM2-8B-A1B (lineup planning)

While this card is 3-bit, teams often publish multiple precisions. Use this table as a planning guide (8B MoE LM; actuals depend on context/batch/prompts):

Variant	Typical Peak RAM	Relative Speed	Typical Behavior	When to choose
3-bit (this repo)	~4.4–8.8 GB	🔥🔥🔥🔥	Direct, concise, great latency	Default on 8–16 GB Macs
6-bit	~7.5–12.5 GB	🔥🔥	Best quality under quant	Choose if RAM allows
8-bit	~9.5–12+ GB	🔥🔥	Largest quantized size / highest fidelity	When you prefer simpler 8-bit workflows

MoE caveat: MoE lowers compute per token; unless experts are paged/partitioned, memory still scales with total parameters on a single device.

🚀 Quickstart (CLI — MLX)

Deterministic generation

python -m mlx_lm.generate \
  --model mlx-community/LFM2-8B-A1B-3bit-MLX \
  --prompt "Summarize the following in 5 concise bullet points:\n<your text>" \
  --max-tokens 256 \
  --temperature 0.0 \
  --device mps \
  --seed 0