Methods and apparatus, including computer program products, for selectively supporting extensions to a modeling language. A method includes receiving a request to extend a modeling language entity and selectively providing support to define an extension of the modeling language entity. Support is selectively provided by determining whether the support to define the extension of the modeling language entity can be provided based on a definition of the modeling language entity in a visual modeling language. If the support can be provided, the support is provided; otherwise, the support is excluded. The method can further include selectively generating the extended modeling language entity. The extended modeling language entity can be selectively generated by determining whether the extension meets one or more conditions for being a valid extension to the modeling language entity, and if the extension meets the conditions, the extended modeling language entity is generated.