Integrating circular design principles directly into the early stages of the product development process presents significant challenges, primarily due to the scarcity of accessible information and the need for proactive engagement with designers and their Computer-Aided Design (CAD) tools. To bridge this expectation, this paper presents a framework—PROTHEUS—that leverages a cognitive chatbot to support circular design strategies within a closed-loop system. Drawing on artificial intelligence technologies, including Large Language Models (LLMs), PROTHEUS aims to ecologically support designers in making informed and actionable choices. This research begins with an overview of existing integrations of chatbots into CAD systems, methodologies and tools that advocate sustainable design and associated sustainability metrics. We then define the architecture of the proposed chatbot model, with its operational mechanisms, information requirements and main objectives. Finally, we use a case study to demonstrate the application of the chatbot and validate the effectiveness of the PROTHEUS model in improving circular design practices.