COBS is an open-source, modular co-simulation platform for developing and comparing building control algorithms. It integrates various simulators and agent models with EnergyPlus and supports fine-grained and occupant-centric control of building subsystems. COBS provides the ability to access the state after each simulation step in EnergyPlus and to use its value to determine the control action for the next time step(s). The interface is designed similar to OpenAI Gym so that the environments written for Gym can be easily used with the simulator.