The recent turn toward core practices and practice-based teacher education has been accompanied by a growing literature on the definitions, pedagogies to teach, and assessments of core practices. Despite these developments, the field lacks core practices performance assessments designed to be used across course sections, courses, and subjects. This paper provides an existence proof of this type of assessment and investigates the affordances and constraints of the approach. The study describes three types of mixed-reality simulation-based performance tasks of three core practices. More than 400 novices in 64 teacher preparation programs in the United States reported that they were able to use the simulation environment and believed the tasks measure important teaching skills. Scores on the tasks were positively related to novices' prior academic and teacher education experiences. Implications for the formative use of such simulations are discussed.