The separation of concerns helps to manage the intrinsic complexity of defining robotics components, systems and missions. This separation of concerns is supported by the RobMoSys modelling approach addressing both the modelling needs of the robotics domain and identifying the involved stakeholders and required expertise. In this multi-stakeholder context, there are pressing concerns about non-functional characteristics including safety aspects (e.g., collaborative robots, increasing risks to humans and the environment where robotic systems operate). It is of special interest to explicitly establish the non-functional assumptions and guarantees. This assures that the their validity can be automatically evaluated, in particular during the definition of a system as a composition of several component definitions. We present how we extended one of the RobMoSys implementations, Papyrus for Robotics, for contracts modelling and assertions validation. Notably the meta-modelling decisions to allow extensibility for assertion languages.