C1 by Thesys is an API that transforms large language model outputs into live, interactive user interfaces. It allows developers to create adaptive UIs in real time without hardcoding each response. The system supports multiple LLMs and integrates easily with existing design frameworks.

The article discusses the evolution of user interfaces (UI), application programmable interfaces (API), and the emerging user agent interface (UAI) design, emphasizing the importance of treating all three as equal in application development. It highlights the need for centralized business logic to ensure consistency across interfaces and illustrates this with an example of managing a reservation system's date selection. The author stresses the significance of designing features that are accessible and legible across all interfaces while maintaining a clear separation between business logic and interface-specific patterns.