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Custom Trainer Plugin

How to write a custom trainer plugin

Step 1: Write your custom trainer class

Before you start writing your code, make sure to use your favorite environment management tool(e.g. venv or conda) to create and activate a Python virtual environment. The following command uses conda, but other tools work similarly:

conda create -n trainer-env python=3.10.12
conda activate trainer-env

Users of the plug-in system are responsible for implementing the trainer class subject to the API standard. Let us follow an example by implementing a custom trainer named "YourCustomTrainer". You can either extend OnPolicyTrainer or OffPolicyTrainer classes depending on the training strategies you choose.

Please refer to the internal PPO implementation for a complete code example. We will not provide a workable code in the document. The purpose of the tutorial is to introduce you to the core components and interfaces of our plugin framework. We use code snippets and patterns to demonstrate the control and data flow.

Your custom trainers are responsible for collecting experiences and training the models. Your custom trainer class acts like a coordinator to the policy and optimizer. To start implementing methods in the class, create a policy class objects from method create_policy:

def create_policy(
    self, parsed_behavior_id: BehaviorIdentifiers, behavior_spec: BehaviorSpec
) -> TorchPolicy:

    actor_cls: Union[Type[SimpleActor], Type[SharedActorCritic]] = SimpleActor
    actor_kwargs: Dict[str, Any] = {
        "conditional_sigma": False,
        "tanh_squash": False,
    if self.shared_critic:
        reward_signal_configs = self.trainer_settings.reward_signals
        reward_signal_names = [
            key.value for key, _ in reward_signal_configs.items()
        actor_cls = SharedActorCritic
        actor_kwargs.update({"stream_names": reward_signal_names})

    policy = TorchPolicy(
    return policy

Depending on whether you use shared or separate network architecture for your policy, we provide SimpleActor and SharedActorCritic from mlagents.trainers.torch_entities.networks that you can choose from. In our example above, we use a SimpleActor.

Next, create an optimizer class object from create_optimizer method and connect it to the policy object you created above:

def create_optimizer(self) -> TorchOptimizer:
    return TorchPPOOptimizer(  # type: ignore
        cast(TorchPolicy, self.policy), self.trainer_settings  # type: ignore
    )  # type: ignore

There are a couple of abstract methods(_process_trajectory and _update_policy) inherited from RLTrainer that you need to implement in your custom trainer class. _process_trajectory takes a trajectory and processes it, putting it into the update buffer. Processing involves calculating value and advantage targets for the model updating step. Given input trajectory: Trajectory, users are responsible for processing the data in the trajectory and append agent_buffer_trajectory to the back of the update buffer by calling self._append_to_update_buffer(agent_buffer_trajectory), whose output will be used in updating the model in optimizer class.

A typical _process_trajectory function(incomplete) will convert a trajectory object to an agent buffer then get all value estimates from the trajectory by calling self.optimizer.get_trajectory_value_estimates. From the returned dictionary of value estimates we extract reward signals keyed by their names:

def _process_trajectory(self, trajectory: Trajectory) -> None:
    agent_id = trajectory.agent_id  # All the agents should have the same ID

    agent_buffer_trajectory = trajectory.to_agentbuffer()

    # Get all value estimates
    ) =  self.optimizer.get_trajectory_value_estimates(
        trajectory.done_reached and not trajectory.interrupted,

    for name, v in value_estimates.items():
            f"Policy/{self.optimizer.reward_signals[name].name.capitalize()} Value Estimate",

    # Evaluate all reward functions
    self.collected_rewards["environment"][agent_id] += np.sum(
    for name, reward_signal in self.optimizer.reward_signals.items():
        evaluate_result = (
            reward_signal.evaluate(agent_buffer_trajectory) * reward_signal.strength
        # Report the reward signals
        self.collected_rewards[name][agent_id] += np.sum(evaluate_result)


A trajectory will be a list of dictionaries of strings mapped to Anything. When calling forward on a policy, the argument will include an “experience” dictionary from the last step. The forward method will generate an action and the next “experience” dictionary. Examples of fields in the “experience” dictionary include observation, action, reward, done status, group_reward, LSTM memory state, etc.

Step 2: implement your custom optimizer for the trainer.

We will show you an example we implemented - class TorchPPOOptimizer(TorchOptimizer), which takes a Policy and a Dict of trainer parameters and creates an Optimizer that connects to the policy. Your optimizer should include a value estimator and a loss function in the update method.

Before writing your optimizer class, first define setting class class PPOSettings(OnPolicyHyperparamSettings) for your custom optimizer:

class PPOSettings(OnPolicyHyperparamSettings):
    beta: float = 5.0e-3
    epsilon: float = 0.2
    lambd: float = 0.95
    num_epoch: int = 3
    shared_critic: bool = False
    learning_rate_schedule: ScheduleType = ScheduleType.LINEAR
    beta_schedule: ScheduleType = ScheduleType.LINEAR
    epsilon_schedule: ScheduleType = ScheduleType.LINEAR

You should implement update function following interface:

def update(self, batch: AgentBuffer, num_sequences: int) -> Dict[str, float]:

In which losses and other metrics are calculated from an AgentBuffer that is generated from your trainer class, depending on which model you choose to implement the loss functions will be different. In our case we calculate value loss from critic and trust region policy loss. A typical pattern(incomplete) of the calculations will look like the following:

run_out =

log_probs = run_out["log_probs"]
entropy = run_out["entropy"]

values, _ = self.critic.critic_pass(
policy_loss = ModelUtils.trust_region_policy_loss(
loss = (
    + 0.5 * value_loss
    - decay_bet * ModelUtils.masked_mean(entropy, loss_masks)

Finally update the model and return the a dictionary including calculated losses and updated decay learning rate:

ModelUtils.update_learning_rate(self.optimizer, decay_lr)

update_stats = {
    "Losses/Policy Loss": torch.abs(policy_loss).item(),
    "Losses/Value Loss": value_loss.item(),
    "Policy/Learning Rate": decay_lr,
    "Policy/Epsilon": decay_eps,
    "Policy/Beta": decay_bet,

Step 3: Integrate your custom trainer into the plugin system

By integrating a custom trainer into the plugin system, a user can use their published packages which have their implementations. To do that, you need to add a file. In the call to setup(), you'll need to add to the entry_points dictionary for each plugin interface that you implement. The form of this is {entry point name}={plugin module}:{plugin function}. For example:


Some key elements in the code:

ML_AGENTS_TRAINER_TYPE: a string constant for trainer type
your_trainer_type: name your trainer type, used in configuration file
your_package: your pip installable package containing custom trainer implementation

Also define get_type_and_setting method in YourCustomTrainer class:

def get_type_and_setting():
    return {YourCustomTrainer.get_trainer_name(): YourCustomTrainer}, {
        YourCustomTrainer.get_trainer_name(): YourCustomSetting

Finally, specify trainer type in the config file:

    trainer_type: your_trainer_type

Step 4: Install your custom trainer and run training:

Before installing your custom trainer package, make sure you have ml-agents-env and ml-agents installed

pip3 install -e ./ml-agents-envs && pip3 install -e ./ml-agents

Install your custom trainer package(if your package is pip installable):

pip3 install your_custom_package

Or follow our internal implementations:

pip3 install -e ./ml-agents-trainer-plugin

Following the previous installations your package is added as an entrypoint and you can use a config file with new trainers:

mlagents-learn ml-agents-trainer-plugin/mlagents_trainer_plugin/a2c/a2c_3DBall.yaml --run-id <run-id-name>
--env <env-executable>

Validate your implementations:

Create a clean Python environment with Python 3.10.12 and activate it before you start, if you haven't done so already:

conda create -n trainer-env python=3.10.12
conda activate trainer-env

Make sure you follow previous steps and install all required packages. We are testing internal implementations in this tutorial, but ML-Agents users can run similar validations once they have their own implementations installed:

pip3 install -e ./ml-agents-envs && pip3 install -e ./ml-agents
pip3 install -e ./ml-agents-trainer-plugin

Once your package is added as an entrypoint, you can add to the config file the new trainer type. Check if trainer type is specified in the config file a2c_3DBall.yaml:

trainer_type: a2c

Test if custom trainer package is installed by running:

mlagents-learn ml-agents-trainer-plugin/mlagents_trainer_plugin/a2c/a2c_3DBall.yaml --run-id test-trainer

You can also list all trainers installed in the registry. Type python in your shell to open a REPL session. Run the python code below, you should be able to see all trainer types currently installed:

>>> import pkg_resources
>>> for entry in pkg_resources.iter_entry_points('mlagents.trainer_type'):
...     print(entry)
default = mlagents.plugins.trainer_type:get_default_trainer_types
a2c = mlagents_trainer_plugin.a2c.a2c_trainer:get_type_and_setting
dqn = mlagents_trainer_plugin.dqn.dqn_trainer:get_type_and_setting

If it is properly installed, you will see Unity logo and message indicating training will start:

[INFO] Listening on port 5004. Start training by pressing the Play button in the Unity Editor.

If you see the following error message, it could be due to trainer type is wrong or the trainer type specified is not installed:

mlagents.trainers.exception.TrainerConfigError: Invalid trainer type a2c was found