LFM2.5-8B-A1B-4bit

LFM2.5-8B-A1B (4-bit NF4 Quantized)

This is a bitsandbytes 4-bit NF4 quantized version of LiquidAI/LFM2.5-8B-A1B, an 8.3B-parameter Mixture-of-Experts model with 1.5B active parameters and a 128K-token context window.

• Disk size: 4.4 GB (model) + 125 MB (expert quant state)
• VRAM at inference: ~5 GB
• Compute dtype: bfloat16

Architecture

The model uses a hybrid architecture: 19 conv-L layers (liquid neural network cells with an internal cache of 3) and 5 full-attention layers, arranged in a repeating pattern.

Quantization

Quantized with bitsandbytes NF4 using double quantization and bf16 compute dtype:

• Non-expert weights (q_proj, k_proj, v_proj, o_proj, gate_proj, up_proj, down_proj, embed_tokens, norm, router): loaded natively as Linear4bit / Params4bit by from_pretrained
• Expert weights (experts.gate_up_proj, experts.down_proj): stored as raw uint8 Parameter with a companion expert_quant_state.pt file containing the per-weight QuantState needed for dequantization

The expert weights require explicit QuantState restoration after loading. See Usage below.

Usage

This model requires Unsloth for the MoE inference forward path. The expert 4-bit weights must be patched after from_pretrained.

Installation

pip install unsloth bitsandbytes transformers safetensors

Load

from unsloth import FastLanguageModel
from bitsandbytes.nn import Params4bit as BnbParams4bit
from safetensors import safe_open
from pathlib import Path
import torch

cache_path = "models/lfm2.5-8b-4bit-hf"

model, tokenizer = FastLanguageModel.from_pretrained(
    model_name=cache_path,
    max_seq_length=2048,
    load_in_4bit=True,
    device_map=0,
)

# Restore expert quant states
qs_path = Path(cache_path) / "expert_quant_state.pt"
st_path = Path(cache_path) / "model.safetensors"
quant_states = torch.load(str(qs_path), weights_only=False)
device = next(model.parameters()).device

# Move quant state tensors to GPU once
for name, qs in quant_states.items():
    qs.absmax = qs.absmax.to(device, non_blocking=True)
    qs.code = qs.code.to(device, non_blocking=True)
    if qs.nested:
        if qs.offset is not None:
            qs.offset = qs.offset.to(device, non_blocking=True)
        if hasattr(qs.state2, "absmax") and qs.state2.absmax is not None:
            qs.state2.absmax = qs.state2.absmax.to(device, non_blocking=True)
        if hasattr(qs.state2, "code") and qs.state2.code is not None:
            qs.state2.code = qs.state2.code.to(device, non_blocking=True)

with safe_open(str(st_path), framework="pt", device="cpu") as f:
    for name, param in model.named_parameters():
        if name not in quant_states or "experts" not in name:
            continue
        original_data = f.get_tensor(name)
        new_param = BnbParams4bit(
            data=param.data,
            requires_grad=False,
            quant_state=quant_states[name],
            blocksize=64,
            compress_statistics=True,
            quant_type="nf4",
            bnb_quantized=True,
        )
        new_param.data.copy_(original_data.to(new_param.device))
        parts = name.split(".")
        parent = model
        for p in parts[:-1]:
            parent = getattr(parent, p)
        setattr(parent, parts[-1], new_param)

model.config.use_cache = True
model.eval()
torch.cuda.empty_cache()

Generate

@torch.inference_mode()
def generate(prompt: str, max_new_tokens: int = 128, temperature: float = 0.7):
    messages = [{"role": "user", "content": prompt}]
    inputs = tokenizer.apply_chat_template(
        messages, tokenize=True, add_generation_prompt=True, return_tensors="pt"
    ).to(model.device)
    attention_mask = torch.ones_like(inputs)
    outputs = model.generate(
        inputs,
        attention_mask=attention_mask,
        max_new_tokens=max_new_tokens,
        max_length=inputs.shape[1] + max_new_tokens,
        temperature=temperature,
        do_sample=temperature > 0,
        pad_token_id=tokenizer.eos_token_id,
    )
    return tokenizer.decode(outputs[0][inputs.shape[1]:], skip_special_tokens=True).strip()

print(generate("What is 2+2?"))

Benchmarks

Benchmark scores from the original LFM2.5-8B-A1B model card. Quantization to 4-bit NF4 should preserve these scores near-identically.

Caveats

• Unsloth required: the MoE forward pass uses unsloth_zoo's MoE utilities. Loading without Unsloth will produce incorrect outputs for MoE layers.
• Two-step load: expert weights need explicit QuantState restoration (see above). A future bitsandbytes release with native Experts4bit support will eliminate this step.
• Expert quant state: the expert_quant_state.pt file (125 MB) is required. Without it, expert weights are un-dequantizable raw uint8 tensors.

Citation

@article{liquidAI20268BA1B,
  author  = {Liquid AI},
  title   = {LFM2.5-8B-A1B: Personal Assistant On Your Laptop},
  journal = {Liquid AI Blog},
  year    = {2026},
  note    = {www.liquid.ai/blog/lfm2-5-8b-a1b},
}

@misc{liquidAI2025LFM2,
  title  = {LFM2 Technical Report},
  author = {Liquid AI},
  year   = {2025},
  eprint = {2511.23404},
  archivePrefix = {arXiv},
}

License

This quantized distribution inherits the LFM 1.0 License from the original model.