From the Connected Device to the Agent-based System: An Empirical Analysis of the Transformation of Design Work through a Case Study of a Next-generation Internet Box

The historical model of user-centred design (UCD) is based on the creation of deterministic graphical and physical interfaces optimised for human perception.The integration of autonomous agents based on Large Language Models (LLMs) and equipped with action capabilities (tool use) is revolutionising this paradigm: the interface no longer addresses humans directly, but becomes an intermediate semantic layer orchestrated by and for the machine.This article analyses this epistemological shift through a comprehensive industrial case study: the design of a next-generation internet box combining a hardware unit, an embedded screen, a companion mobile app and home network/home automation services.We demonstrate how the designer's workflow is transforming, shifting from the design of static screens to the engineering of ontologies, the definition of semantic action capabilities (tools) and the configuration of agency sliders.Finally, we propose a framework of mixed evaluation metrics (AIX) that enables the computational performance of agents to be assessed whilst preserving the intelligibility and decision-making autonomy of human users.