GGUF

GGUF is a single-file format used to store models for inference with GGML, containing the model metadata and tensors. It supports many quantized data types (refer to the quantization type table), which saves a significant amount of memory.

Load GGUF models

from transformers import AutoModelForCausalLM, AutoTokenizer

model_id = "unsloth/Qwen3.5-4B-GGUF"
filename = "Qwen3.5-4B-Q4_K_M.gguf"

model = AutoModelForCausalLM.from_pretrained(model_id, gguf_file=filename)
tokenizer = AutoTokenizer.from_pretrained(model_id, gguf_file=filename)

The weights only stay in their GGUF blocks on Metal (MPS) devices, where the llama.cpp kernels, fetched from the Hub, run the matmuls directly on the packed blocks to keep inference fast.

Right now, the only architecture supported is Qwen3.5. Everything else falls back. On another device or quantization type, the model is dequantized at load time, and an architecture that isn’t supported yet goes through the legacy loader.

Attention

On Metal, attention has a ggml kernel too: ggml-attn, the same flash attention llama.cpp runs, for both decode and prefill.

model = AutoModelForCausalLM.from_pretrained(
    model_id, gguf_file=filename, attn_implementation="transformers-community/ggml-attn"
)

Dequantize

Dequantizing unpacks every weight at load time and gives back a plain dense model. It is the fallback whenever the fast, compressed path doesn’t apply. You can also ask for it explicitly with GgufConfig.

import torch

from transformers import AutoModelForCausalLM, GgufConfig

quantization_config = GgufConfig(dequantize=True)
model = AutoModelForCausalLM.from_pretrained(
    model_id, gguf_file=filename, quantization_config=quantization_config, dtype=torch.bfloat16
)

The model that comes out is a regular dense model, so this is also how you take a GGUF checkpoint into the dtype you passed.

Architectures other than Qwen3.5 are read by the legacy loader which dequantize the model also.

The legacy loader supports Llama, Mistral, Qwen2, Qwen2Moe, Phi3, Bloom, Falcon, StableLM, GPT2, Starcoder2, and more.

Serve

transformers serve names a GGUF model <repo>:<file>.gguf, since a repository holds several quantizations and the id has to say which one to load. Requests name it the same way.

transformers serve unsloth/Qwen3.5-4B-GGUF:Qwen3.5-4B-Q4_K_M.gguf