Integrated assessment models (IAMs) are a cornerstone of the climate change mitigation. Each IAM exhibits different methodologies for characterising the energy system, land use change, economics, and climate response. To date, no existing IAM can simulate the heterogeneity of investor or consumer decision-making in an integrated assessment context.This paper presents an open-source modelling framework designed to fill that gap. Named MUSE (ModUlar energy systems Simulation Environment), this new model supports flexible characterisation of agents’ decision-making, including imperfect foresight. It is modular, technology-rich, recursive-dynamic, and partial-equilibrium in nature. The structure of MUSE creates the ability to produce climate change mitigation assessments that are more grounded, and more tangible model outputs for conceiving effective approaches to mitigation.