We propose a normative approach to collaborative support system design in distributed tangible environments. Based on activity theory, our goal is to mediate rather than drive human activity and to integrate components from the physical, numerical and social environments. We propose an original architecture coupling a physical space (tools supporting human activity), a processing space (agent performing activity, be it human or artificial), an informational space (traces reflecting activity), and a normative space (filters regulating activity). We further consider collaboration as a conversational process grounded in the objects of the working space. To this end, tangible objects of various kinds are designed to support multi-threaded activity. Heterogeneous trace properties may then be fused to situate activity and ground the filtering process. Interface agents are designed to provide appropriate visual feedback and support mutual awareness. Beyond the mere sharing of individual or group activity, we approach awareness as involving mutual knowledge of the activity constraints. We show through simple examples from the RISK game the potential richness of the proposed approach.