Characteristic Formulae (CF) offer a productive, principled approach to generating verification conditions for higher-order imperative programs, but so far the soundness of CF has only been considered with respect to an informal specification of a programming language (OCaml). This leaves a gap between what is established by the verification framework and the program that actually runs. We present a fully-fledged CF framework for the formally specified CakeML programming language. Our framework extends the existing CF approach to support exceptions and I/O, thereby covering the full feature set of CakeML, and comes with a formally verified soundness theorem. Furthermore, it integrates with existing proof techniques for verifying CakeML programs. This validates the CF approach, and allows users to prove end-to-end theorems for higher-order imperative programs, from specification to language semantics, within a single theorem prover.