Although blockchain protocols have existed for some time now, a focused analysis on smart contracts, as Application Programming Interfaces (APIs) for user driven and web based applications, is clearly missing. APIs as abstract interfaces can inspire us in designing smart contract based applications and information infrastructures. Such an approach has an impact both on the architecture and coding of applications. In this article, we will use our pilot on managing building rights within the City of Geneva to demonstrate how the architecture, design and implementation of smart contracts can be advanced. Initiating the creation of new applications and services based on the smart contracts characteristics, such as forced temporality and immutability and transparency, comes with new opportunities and challenges. Blockchain could be more than an innovative technology, a building block of new forms of social applications and infrastructures through the design of smart contracts as APIs.