The past few years have been marked by the rapid increase in popularity and use of Web APIs as indicated by the growing number of available APIs and the multitude of applications built on top of them. The development and evolution of applications based on Web APIs is, however, hampered by the lack of automation achievable with current technologies. In this paper, we focus in particular on invocation, which as of now requires manual implementation of custom-tailored clients for each individual API. We present an approach for employing lightweight semantics for supporting the automated invocation of Web APIs. We investigate current Web API description forms and conduct an analysis of the requirements for a description model capable of supporting unified API invocation. In the light of these results, we propose a shared API description model that overcomes the current heterogeneity of the documentation and provides common grounds for enhancing APIs with semantic annotations that facilitate a general automated invocation solution. We evaluate the applicability of our approach by determining the coverage provided by our description model and via a prototypical implementation of an invocation engine.