Major Changes

The low level Python API has changed (#3022). See the Low Level Python API documentation for more information.
Parameters such as resolution and time scale are no longer configured in the Academy; these are now passed on the mlagents-learn command line. See the Migration Guide for more details. (#2956)
Offline Behavioral Cloning training was removed. To learn from demonstrations, use the GAIL and Behavioral Cloning features with either PPO or SAC (#2969)
Agents can now use sensors of child GameObjects as observations (#3095)
The RayPerceptionSensor now supports a layerMask option that is used in raycasting. (#3111 )
The official minimum version of Unity supported by ML-Agents is now 2018.4 LTS.

For instructions on how to migrate from 0.12.0 to 0.13.0 see the Migration Guide.

Minor Fixes and Improvements

A separation of statistics collection from the trainer with the StatsReporter and StatsWriter classes (#3076, #3108)
mlagents.envs was renamed to mlagents_envs
A new AgentProcessor class and Trajectory abstraction is now live. Trainers now ingest trajectories which are assembled by the AgentProcessor. (#3067)
A bug that could cause the Academy to call a disabled agent’s _AgentReset() method is fixed (#3072)
Better error handling when the trainer configuration doesn’t contain a “default” entry (#3063)
Better error handling when there is a mismatch between the metacurriculum configuration and brains being trained (#3034)
A bug that prevented agents with different decision intervals from learning in the same scene is fixed (#3181)