MO-Gymnasium is a standardized API and a suite of environments for multi-objective reinforcement learning (MORL)

MO-HalfCheetah gif

MO-Gymnasium is an open source Python library for developing and comparing multi-objective reinforcement learning algorithms by providing a standard API to communicate between learning algorithms and environments, as well as a standard set of environments compliant with that API. Essentially, the environments follow the standard Gymnasium API, but return vectorized rewards as numpy arrays.

The documentation website is at, and we have a public discord server (which we also use to coordinate development work) that you can join here:


As for Gymnasium, the MO-Gymnasium API models environments as simple Python env classes. Creating environment instances and interacting with them is very simple - here’s an example using the “minecart-v0” environment:

import gymnasium as gym
import mo_gymnasium as mo_gym
import numpy as np

# It follows the original Gymnasium API ...
env = mo_gym.make('minecart-v0')

obs, info = env.reset()
# but vector_reward is a numpy array!
next_obs, vector_reward, terminated, truncated, info = env.step(your_agent.act(obs))

# Optionally, you can scalarize the reward function with the LinearReward wrapper
env = mo_gym.LinearReward(env, weight=np.array([0.8, 0.2, 0.2]))

For details on multi-objective MDP’s (MOMDP’s) and other MORL definitions, see A practical guide to multi-objective reinforcement learning and planning.

You can also check more examples in this colab notebook! MO-Gym Demo in Colab


To install MO-Gymnasium, use:

pip install mo-gymnasium

This does not include dependencies for all families of environments (some can be problematic to install on certain systems). You can install these dependencies for one family like pip install "mo-gymnasium[mujoco]" or use pip install "mo-gymnasium[all]" to install all dependencies.


If you use this repository in your research, please cite:

  author = {Lucas N. Alegre and Florian	Felten and El-Ghazali Talbi and Gr{\'e}goire Danoy and Ann Now{\'e} and Ana L. C. Bazzan and Bruno C. da Silva},
  title = {{MO-Gym}: A Library of Multi-Objective Reinforcement Learning Environments},
  booktitle = {Proceedings of the 34th Benelux Conference on Artificial Intelligence BNAIC/Benelearn 2022},
  year = {2022}