Lucas Maes*, Quentin Le Lidec*, Damien Scieur, Yann LeCun and Randall Balestriero

Abstract: Joint Embedding Predictive Architectures (JEPAs) offer a compelling framework for learning world models in compact latent spaces, yet existing methods remain fragile, relying on complex multi-term losses, exponential moving averages, pretrained encoders, or auxiliary supervision to avoid representation collapse. In this work, we introduce LeWorldModel (LeWM), the first JEPA that trains stably end-to-end from raw pixels using only two loss terms: a next-embedding prediction loss and a regularizer enforcing Gaussian-distributed latent embeddings. This reduces tunable loss hyperparameters from six to one compared to the only existing end-to-end alternative. With ~15M parameters trainable on a single GPU in a few hours, LeWM plans up to 48× faster than foundation-model-based world models while remaining competitive across diverse 2D and 3D control tasks. Beyond control, we show that LeWM's latent space encodes meaningful physical structure through probing of physical quantities. Surprise evaluation confirms that the model reliably detects physically implausible events.

[ Paper | Data | Website ]

If you find this code useful, please reference it in your paper:

@article{maes_lelidec2026lewm,
  title={LeWorldModel: Stable End-to-End Joint-Embedding Predictive Architecture from Pixels},
  author={Maes, Lucas and Le Lidec, Quentin and Scieur, Damien and LeCun, Yann and Balestriero, Randall},
  journal={arXiv preprint},
  year={2026}
}

This codebase builds on stable-worldmodel for environment management, planning, and evaluation, and stable-pretraining for training. Together they reduce this repository to its core contribution: the model architecture and training objective.

Installation:

uv venv --python=3.10
source .venv/bin/activate
uv pip install stable-worldmodel[train,env]

Datasets use the HDF5 format for fast loading. Download the data from the Drive and decompress with:

tar --zstd -xvf archive.tar.zst

Place the extracted .h5 files under $STABLEWM_HOME (defaults to ~/.stable-wm/). You can override this path:

export STABLEWM_HOME=/path/to/your/storage

Dataset names are specified without the .h5 extension. For example, config/train/data/pusht.yaml references pusht_expert_train, which resolves to $STABLEWM_HOME/pusht_expert_train.h5.

jepa.py contains the PyTorch implementation of LeWM. Training is configured via Hydra config files under config/train/.

Before training, set your WandB entity and project in config/train/lewm.yaml:

wandb:
  config:
    entity: your_entity
    project: your_project

To launch training:

python train.py data=pusht

Checkpoints are saved to $STABLEWM_HOME upon completion.

For baseline scripts, see the stable-worldmodel scripts folder.

Evaluation configs live under config/eval/. Set the policy field to the checkpoint path relative to $STABLEWM_HOME, without the _object.ckpt suffix:

# ✓ correct
python eval.py --config-name=pusht.yaml policy=pusht/lewm

# ✗ incorrect
python eval.py --config-name=pusht.yaml policy=pusht/lewm_object.ckpt

Pre-trained checkpoints are available on Google Drive. Download the checkpoint archive and place the extracted files under $STABLEWM_HOME/.

Each tar archive contains two files per checkpoint:

• <name>_object.ckpt — a serialized Python object for convenient loading; this is what eval.py and the stable_worldmodel API use
• <name>_weight.ckpt — a weights-only checkpoint (state_dict) for cases where you want to load weights into your own model instance

To load the object checkpoint via the stable_worldmodel API:

import stable_worldmodel as swm

# Load the cost model (for MPC)
cost = swm.policy.AutoCostModel('pusht/lewm')

This function accepts:

• run_name — checkpoint path relative to $STABLEWM_HOME, without the _object.ckpt suffix
• cache_dir — optional override for the checkpoint root (defaults to $STABLEWM_HOME)

The returned module is in eval mode with its PyTorch weights accessible via .state_dict().

Feel free to open issues! For questions or collaborations, please contact lucas.maes@mila.quebec