Building energy systems account for one third of the primary energy demand in the OECD member countries. Due to global warming and growing energy scarcity, a higher energy efficiency of these systems is required. As a result, building energy systems become more complex and often contain multiple heat and cold suppliers. The selection of the ideal supplier based on the current system state requires new control strategies. Multi agent systems depict a promising technology for the problem. There are agent-based frameworks available for many programming languages, but not for the modelling language Modelica, which is commonly used for building simulation. In the course of this work, a Modelica library for the agent-based control of building energy systems based on market mechanisms is introduced. The structure of the library, different types of agents, the concepts of agent communication and the trading of capacity adjustments are discussed. The library offers a variety of cost functions in order to realize different optimization goals. The functionality of the concept is proven with a simulation example of a building energy system controlled with agents from the introduced library. The system depicts a plug&play solution to optimize the performance of complex building energy systems with interchangeable optimization goals. Due to communication via Ethernet, the control system can be coupled to real energy systems. (C) 2017 Elsevier Ltd. All rights reserved.