Mixed-format tests that include both multiple-choice (MC) and constructed-response (CR) items have become widely used in many large-scale assessments. When an item response theory (IRT) model is used to score a mixed-format test, the unidimensionality assumption may be violated if the CR items measure a different construct from that measured by MC items. In this simulation study, we explore the impact of different degrees of violation on the unidimensionality assumption on Rasch item parameter recovery and model-data fit statistics. Our results suggest that, when the unidimensionality assumption is violated for mixed-format tests, both estimated Rasch item parameters and model-data fit statistics are distorted, with greater impact corresponding to higher degrees of violations. The findings from this study may provide guidance for test practitioners for working with mixed-format tests within the Rasch context.