Abstract. In 2021, a team of developers from the Deutscher Wetterdienst (DWD), the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt, DLR), the German Climate Computing Center (Deutsches Klimarechenzentrum, DKRZ), and the Forschungszentrum Jülich (FZJ) started the ICOsahedral Non-hydrostatic (ICON) model system Community Interface (ComIn) project: ICON ComIn is a library with multi-language support for connecting third-party modules (“plugins”) to the ICON model using the dynamic loader of the operating system. ComIn is intended for a wide range of use cases, from the integration of simple diagnostic Python scripts to chemistry model components into ICON. ICON ComIn is distributed with the ICON model code under an open-source license. Its application programming interface (API) provides a low barrier for code extensions to ICON and reduces the migration effort in response to new ICON releases. ComIn's main design principles are that it is lightweight, interoperable (Fortran, C/C++, Python), and flexible, and required changes in ICON are minimised. During the development of ComIn the ease of getting started and the experience during plugin development were guiding principles to provide a convenient tool. The extensive documentation and a variety of test and example plugins are results of this process. This paper motivates the underlying design principles and provides some concrete reasoning for their selection. Further, current limitations are discussed and the vision for the future is presented.