Programmers rely on the correctness of their tools. Semanticists have long studied the correctness of compilers, but we make the case that other tools deserve semantic models, too, and that using these models can help in developing these tools. We examine these ideas in the context of DrScheme's stepper. The stepper operates within the existing evaluator, placing breakpoints and reconstructing source expressions from information placed on the stack. We must ask whether we can prove the correspondence between the source expressions emitted by the stepper and the steps in the formal reduction semantics. To answer this question, we develop a high-level semantic model of the extended compiler and run-time machinery. Rather than modeling the evaluation as a low-level machine, we model the relevant low-level features of the stepper's implementation in a high-level reduction semantics. The higher-level model greatly simplifies the correctness proof. We expect the approach to apply to other semantics-based tools.