Today more and more software operates in a software ecosystem where software products and services are shared over a common technological platform between business actors. In such a setting, a software platform API becomes the interface between the platform and its owner (s) on the one side and other actors and their extensions on the other side. In such ecosystem context, platform API fitness for application development has a huge impact on the success of the ecosystem. However, software ecosystems are usually evolving at a high pace given their dynamic structure and the continuously changing business strategies of ecosystem actors. The design of APIs for software ecosystems therefore has to co-evolve. In this position paper, we propose a continuous (re-)assessment method for platform API's that takes into account ecosystem needs. We illustrate the method with insights from an ongoing case study.