The repository contains the code for the SEPO algorithm presented in the paper:

Fine-Tuning Discrete Diffusion Models with Policy Gradient Methods.

SEPO is an efficient, broadly applicable, and theoretically justified policy gradient algorithm, for fine-tuning discrete diffusion models over general rewards.

Note : The repo is not complete at the moment.

Full implementation of the GRPO version of SEPO on a masked difusion language model MDLM (Sahoo et al., 2023), with an application on fine-tuning a masked diffusion language model on DNA sequences. Extensible and modular codebase to facilitate further research.

• grpo_train.py: Contains the full iterative SEPO algorithm (GRPO version).
• diffusion_gosai_update_new.py: Provides helper functions for the algorithm.
• eval_plots.ipynb: Reproduces the tables and plots presented in the paper.

👉 Note: You must download the fine-tuned models from Hugging Face to reproduce these results.
They are available at huggingface.co/Xssama/SEPO-DNA.

You can download the models directly using the huggingface_hub Python library:

from huggingface_hub import hf_hub_download

# Example: Download the SEPO fine-tuned model checkpoint
ckpt_path = hf_hub_download(
    repo_id="Xssama/SEPO_DNA",
    filename="finetuned_sepo_kl.ckpt",  # finetuned_sepo_kl_gf.ckpt for SEPO with gradient flow
    cache_dir="./checkpoints"  # Optional: specify your preferred local directory
)

print(f"Checkpoint downloaded to: {ckpt_path}")

Alternatively, use wget

wget https://huggingface.co/Xssama/SEPO-DNA/resolve/main/finetuned_sepo_kl.ckpt -P ./checkpoints/

The GRPO_MDLM_DNA folder is built on top of DRAKES (Wang et al., 2024).

This section will be updated with the full reproducible code for the experiments in the paper. Stay tuned!

• Upload training scripts for SEDD (Lou et al., 2023) fine-tuning with PPO (experiments in the paper).

If you find this work useful in your research, please consider citing:

@article{zekri2025fine,
  title={Fine-Tuning Discrete Diffusion Models with Policy Gradient Methods},
  author={Zekri, Oussama and Boull{\'e}, Nicolas},
  journal={arXiv preprint arXiv:2502.01384},
  year={2025}
}

• The current codebase for DNA sequence modelling with discrete diffusion model is provided by DRAKES (Wang et al., 2024). We thank them for their really clear and reproducible code.
• MDLM (Sahoo et al., 2023).
• SEDD (Lou et al., 2023).
• minChatGPT (Li, 2023).