FLUX.2 klein 4B: GPTQ 4-bit for MLX / mflux (generate and edit)
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This is FLUX.2 klein 4B (Black Forest Labs, Apache 2.0) quantized for Apple Silicon and mflux. It does two things:
- text to image, in 4 steps;
- image editing from an instruction: "make it winter", "turn it into a watercolor", "give him glasses". You pass a reference image and the model changes what you asked while keeping the rest.
It fits comfortably on a 16 GB Mac:
- 5.3 GB on disk (bf16 original: ~16 GB);
- ~7.3 GB peak for text to image at 1024²;
- ~9.4 GB peak for an edit with one reference image.
What's inside
| component | precision | size |
|---|---|---|
| transformer (3.9B) | GPTQ 4-bit, group 64, Hessian-corrected | 2.2 GB |
| text encoder (Qwen3-4B) | 8-bit, cut to the 27 layers klein actually reads | 3.3 GB |
| VAE | as in the original | 0.16 GB |
- GPTQ, not plain rounding. The transformer was calibrated on the model's real 4-step denoising loop: text to image and edits with a reference image. Each Linear's input Hessian was summed over every token of every step. The weights are the same size as mflux's own
--quantize 4, but closer to bf16. - Text encoder cut to its used layers. klein conditions only on Qwen3 hidden states 9, 18 and 27, so layers 27–35 never affect the image. They're dropped from the checkpoint (−1 GB). Prompt embeddings and the final image are bit-identical to the full encoder.
- 8-bit text encoder, 4-bit transformer. An earlier sweep found this split works best. The text encoder is the conditioning signal: at 4-bit it shifted text layout and faces; at 8-bit it matched bf16.
GPTQ vs plain 4-bit (same size)
Test set: 6 text-to-image prompts and 3 edits, none of them in the calibration set, 1024², seed 7, same text encoder for every model.
| transformer 4-bit g64 | velocity error, text to image | velocity error, edit | PSNR vs bf16, text to image | PSNR vs bf16, edit |
|---|---|---|---|---|
RTN (mflux --quantize 4) | 0.244 | 0.171 | 18.7 dB | 25.7 dB |
| GPTQ (this repo) | 0.202 (−17%) | 0.136 (−20%) | 19.6 dB | 26.2 dB |
Velocity error is ‖v_q − v_bf16‖ / ‖v_bf16‖ with teacher forcing: at every step, the quantized model gets exactly the bf16 run's latents. It measures how far the denoiser itself is from bf16, without trajectory drift compounding into it. GPTQ is lower on all 9 samples, by 12–22%.
Usage
In LLMTray (easiest)
LLMTray runs this model for you.
- Open Settings › Image generation › Image editing and choose FLUX.2 klein 4B.
- Attach a photo in the chat, or generate one, and ask: "make it night", "put a hat on the cat". The chat model calls the edit tool for you.
It shows live step previews and keeps nothing on disk for temporary chats.
With mflux (Python)
pip install mflux==0.20.0
hf download roman220220/flux2-klein-4b-mlx-mixed --local-dir klein4b
from mflux.models.flux2.variants.txt2img.flux2_klein import Flux2Klein
from mflux.models.flux2.variants.edit.flux2_klein_edit import Flux2KleinEdit
from mflux.models.common.vae.tiling_config import TilingConfig
# Text to image
model = Flux2Klein(model_path="klein4b")
model.text_encoder.layers = model.text_encoder.layers[:27] # the layers this checkpoint carries
model.tiling_config = TilingConfig(vae_decode_tile_size=256) # halves the VAE decode peak
model.generate_image(seed=1, prompt="A cozy cabin by a mountain lake at golden hour",
num_inference_steps=4, width=1024, height=1024).image.save("cabin.png")
# Edit (a path or a PIL image as the reference)
edit = Flux2KleinEdit(model_path="klein4b")
edit.text_encoder.layers = edit.text_encoder.layers[:27]
edit.tiling_config = TilingConfig(vae_decode_tile_size=256)
edit.generate_image(seed=1, prompt="Make it winter, keep everything else", image_paths=["cabin.png"],
num_inference_steps=4, width=1024, height=1024).image.save("cabin-winter.png")
- Keep the
layers[:27]line. mflux builds all 36 text-encoder layers and leaves the missing ones uninitialized. They don't change the output, but cutting them saves ~1 GB of memory and some time. - Speed (M5, 1024²): text to image ~25–30 s, an edit ~1 min.
How it was made
The code and the full write-up (method, pitfalls, every number above) are in the flux2-quant project of quant-ternary:
klein_gptq.py: the calibration and GPTQ passes;klein_gptq_eval.py: the evaluation;klein_truncate_te.py: cuts the text encoder to its used layers;docs/FINDINGS.md: the write-up.
The GPTQ core is the same gptq_nbit used for our Z-Image-Turbo releases, on MLX's exact affine grid.
The IPSupport local-AI stack
Local-first AI tools for macOS by IPSupport — nothing leaves your Mac.
- LLMTray — your local AI workstation for macOS: chat with local LLMs, generate and edit images, make music, run agents, and serve an OpenAI-compatible API. Downloads models from Hugging Face in-app, with per-model profiles.
- IPSupport Code — your AI coding agent for real repositories: analyze, fix, test, report.
LLMTray uses this model for in-chat image generation and editing (it drives mflux under the hood).
License
Licensed under the Apache License 2.0, the same license as the base model. See LICENSE.
This is a modified version of black-forest-labs/FLUX.2-klein-4B:
- the transformer's Linear layers were GPTQ-corrected and quantized to 4-bit;
- the text encoder was quantized to 8-bit, and its layers 27–35 were removed;
- everything was saved in mflux's MLX checkpoint format.
The weights and configuration files in this repo are therefore modified versions of the original, not the original files.