Software contracts empower programmers to describe functional properties of components. When it comes to constraining effects, though, the literature offers only one-off solutions for various effects. It lacks a universal principle. This paper presents a unifying theory of effectful software contracts in the context of algebraic effect handlers. A key metatheorem shows that contracts cannot unduly interfere with a program’s execution. Furthermore, an evaluation demonstrates that the theory covers existing approaches. Two implementations—one for an effect-handler language and one for Racket—show that the theory can guide practice.