Unity-Technologies/ml-agents · error · UnityCommunicationException

The EnvironmentParametersChannel received a message from Uni

Error message

The EnvironmentParametersChannel received a message from Unity, this should not have happened.

What it means

EnvironmentParametersChannel is outbound-only: Python sends float parameter values to Unity; Unity should never reply on this channel. Any incoming message indicates a Unity/Python protocol mismatch, so on_message_received raises UnityCommunicationException.

Source

Thrown at ml-agents-envs/mlagents_envs/side_channel/environment_parameters_channel.py:29

    You can send parameters to an environment with the command
    set_float_parameter.
    """

    class EnvironmentDataTypes(IntEnum):
        FLOAT = 0
        SAMPLER = 1

    class SamplerTypes(IntEnum):
        UNIFORM = 0
        GAUSSIAN = 1
        MULTIRANGEUNIFORM = 2

    def __init__(self) -> None:
        channel_id = uuid.UUID("534c891e-810f-11ea-a9d0-822485860400")
        super().__init__(channel_id)

    def on_message_received(self, msg: IncomingMessage) -> None:
        raise UnityCommunicationException(
            "The EnvironmentParametersChannel received a message from Unity, "
            + "this should not have happened."
        )

    def set_float_parameter(self, key: str, value: float) -> None:
        """
        Sets a float environment parameter in the Unity Environment.
        :param key: The string identifier of the parameter.
        :param value: The float value of the parameter.
        """
        msg = OutgoingMessage()
        msg.write_string(key)
        msg.write_int32(self.EnvironmentDataTypes.FLOAT)
        msg.write_float32(value)
        super().queue_message_to_send(msg)

    def set_uniform_sampler_parameters(
        self, key: str, min_value: float, max_value: float, seed: int

View on GitHub (pinned to 3ecb446f75)

Solutions

Match Unity ML-Agents package and Python mlagents versions.
Remove custom Unity code that sends messages on the environment-parameters channel id.
Update the Unity environment binary to the matching release.
Catch UnityCommunicationException around stepping and log both tool versions.

Defensive patterns

Strategy: try-catch

Validate before calling

# EnvironmentParametersChannel is send-only; ensure Unity code never sends on
# UUID 534c891e-810f-11ea-a9d0-822485860400

Try / catch

from mlagents_envs.exception import UnityCommunicationException
try:
    env.step()
except UnityCommunicationException as e:
    print("Unexpected message on EnvironmentParametersChannel; check versions:", e)

Prevention

Match Unity ML-Agents and Python versions
Use only float_parameter/IntegerParameter setters from Python side
Avoid custom Unity writes to this channel id
Verify env binary version before training runs

When it happens

Trigger: env.step()/reset() when Unity sends data on channel UUID 534c891e-810f-11ea-a9d0-822485860400 (e.g. custom Unity code or wrong version writing environment parameters back).

Common situations: Version-skewed ML-Agents Unity package vs Python package; custom environment code mistakenly sending on this channel id.

Related errors

AI-assisted analysis of Unity-Technologies/ml-agents@3ecb446f75 (2026-09-02). Data as JSON: /api/errors/0c724c7bd4e06bbf. Report an issue: GitHub.