kdFlex is a powerful general-purpose simulation development toolkit for the modeling and simulation of rigid and flexible multibody dynamics. It supports system nonlinear dynamics of coupled rigid and flexible multibody systems with general and time-varying topologies. kdFlex can be used for analysis, real-time closed-loop simulations, within hardware-in-the-loop testbeds, and embedded within control and autonomy stacks for real-time mechanism modeling. A rich and modern C++ and Python API allows users to adapt and tailor kdFlex to a broad spectrum of dynamics needs.