In the distributed development of modern IT systems, contracts play a vital role in ensuring interoperability of components and adherence to specifications. The design of embedded systems, however, is made more complex by the heterogeneous nature of components, which are often described using different models and interaction mechanisms. Composing such components is generally not well-defined, making design and verification difficult. Several frameworks, both operational and denotational, have been proposed to handle heterogeneity using a variety of approaches. However, the application of heterogeneous operational models to contract-based design has not yet been investigated. In this work, we adopt the operational mechanism of tag machines to represent heterogeneous systems and construct a full contract model. We introduce heterogeneous composition, refinement, dominance, and compatibility between contracts, altogether enabling a formalized and rigorous design process for heterogeneous systems.