It is increasingly common that public construction projects are procured and managed with explicit intentions to establish closer collaboration between partners. Opportunities for negotiation and the freedom to choose partners are more restricted in a public setting than in a private one. Thus, many relational requirements that are generally considered essential to trust-based collaboration are more difficult to fulfil. Based on two case studies from the Netherlands and one from Sweden, we investigate how this formalized context, including formal partnering models to support relationship management, influences relational development in public sector construction contracts. Collaboration is primarily needed in projects where uncertainty is high and exchange conditions cannot be comprehensively defined. The contractual ambiguities, together with the limited opportunities for pre-contractual sense-making and relationship-building, imply that the early post-contractual phase is of significantly higher importance in public projects than in private ones. After signing a contract, it is important that the parties use the interpretive flexibility to signal trust and commitment and also that joint learning processes are organized to resolve ambiguities and enable joint learning. Formal partnering models are helpful in structuring this interaction, but also incentive arrangements may be important in shaping interaction and signalling trust.