We propose a novel approach and tool for collaborative software engineering and development. In model-based software engineering, the underlying data structure is a complex, directed and labeled graph. Collaborative engineering requires that developers be able to copy the graph, make independent changes, compare them, detect conflicts, and merge non-conflicting graphs. To support different collaboration and development styles requires a very flexible toolset. Worldwide, loosely-coupled development teams require the support of large-scale networks of users, possibly disconnected, in a decentralised fashion. No matter how the graph replicas evolve, they must eventually converge. We describe and evaluate C-Praxis, a tool that satisfies these requirements.