Isabelle/UTP: a mechanised theory engineering framework. We introduce Isabelle/UTP, a novel mechanisation of Hoare and He’s unifying theories of programming (UTP) in Isabelle/HOL. UTP is a framework for the study, formalisation, and unification of formal semantics. Our contributions are, firstly, a deep semantic model of UTP’s alphabetised predicates, supporting meta-logical reasoning that is parametric in the underlying notions of values and types. Secondly, integration of host-logic type checking that subsumes the need for typing proof obligations in the object-language. Thirdly, proof tactics that transfer results from well-supported mathematical structures in Isabelle to proofs about UTP theories. Additionally, our work provides novel insights towards reconciliation of shallow and deep language embeddings.