The rise of global challenges, such as climate change, is pushing global governance to evolve. In result thereof, the traditionally state-centric diplomatic sphere is experiencing an increasing number of non-state actors entering the arena. Geoffrey Wiseman (1999) describes this phenomenon as the shift from traditionally bilateral and multilateral diplomacy to polylateral diplomacy. This study looks at how non-state actors can be fitted in frontline diplomacy in relation to sustainable development. By means of a qualitative, inductive case study of the Dutch embassy in the Philippines, it looks at how state actors perceive this non-state actor inclusion by means of data triangulation: a document analysis, semi-structured interviews and participant-observation. In particular, it zooms in on a specific example of multi-stakeholder partnership, which Sustainable Development Goals 17.16 and 17.17 hail as a tool for sustainability: the Manila Bay Sustainable Development Master Plan. Through the theoretical lenses of collaborative governance and the function-sensitive approach, this thesis concludes that the functions non-state actors can fulfil in the diplomatic activities of global governance vary depending on the three contingencies of time, trust and interdependence.