Until recently, users of ESO’s Very Large Telescope had to prepare Observing Blocks (OBs) with a standalone desktop tool. Tool support for automated OB mass production was mostly limited to imaging public surveys. Furthermore, there was no connection between the OB preparation software and other ancillary tools, such as Exposure Time Calculators, finding chart preparation software, and observatory schedule, meaning that users had to re-type the same information in several tools, and could design observations that would be incompatible with the Service Mode schedule. To address these shortcomings, we have implemented a new programming interface (API) and a state-of-the-art web application which provide observers with unprecedented flexibility and promote the usage of instrument and science-case specific tools, from small scripts to full-blown user interfaces. In this paper, we describe the software architecture of our solution, important design concepts and the technology stack adopted. We report on first user experience in both Visitor and Service Mode. We discuss tailored API programming examples, solving specific user requirements, and explain API usage scenarios for the next generation of ESO instruments. Finally, we describe the future evolution of our new approach.