Web apis are becoming an increasingly popular alternative to the more heavy-weight Web services. Recently, they also have been used in the context of sensor networks. However, making different Web apis (and thus sensors) cooperate often requires a significant amount of manual configuration. Ideally, we want Web apis to behave like Linked Data, where data from different sources can be combined in a straightforward way. Therefore, in this paper, we show how Web apis, semantically described by the light-weight format restdesc, can be composed automatically based on their functionality. Moreover, the composition process does not require specific tools, as compositions are created by generic Semantic Web reasoners as part of a proof. We then indicate how the composition in this proof can be executed. We describe our architecture and implementation, and validate that proof-based composition is a feasible strategy on a Web scale. Our measurements indicate that current reasoners can integrate compositions of more than 200 Web apis in under one second. This makes proof-based composition a practical choice for today’s Web apis.