The creation of adaptive and reconfigurable Service Oriented Architectures (SOA) must take into account the unpredictability of the Internet and of potentially buggy software, and thus requires monitoring subsystems for detecting degradations and failures as soon as possible. In this paper we propose EXOGEM, a novel and lightweight monitoring framework for REpresentational State Transfer (REST) Application Programming Interfaces (APIs). EXOGEM is an extension to the mainstream code generator OpenAPI Generator, and it allows to create a monitoring subsystem for generated APIs with limited changes to the usual API development workflow. We showcase the approach on a smart grid testbed, where EXOGEM monitors the interaction of a heatpump with a system that optimizes its operations. Our measurements estimate EXOGEM’s comparable to the usage of HTTPS when the server is not flooded with requests. Moreover, in one experiment EXOGEM was used to identify high load, and to activate computational elasticity. Together, this suggests that EXOGEM can be a useful monitoring framework for real-life systems and services.