The paper reports the results of a design science research study that develops design principles for information systems (IS) supporting environmental sensemaking by generative artificial intelligence (AI) in the use case of a personal assistant for sustainability reporting. We identify initial design principles based on the concept of sensemaking and related prior research on generative AI assistants. Afterward, we revise the design principles in two rounds of developing, demonstrating, and evaluating a prototypical implementation. Through the second round, we incorporate the knowledge of experts about the requirements of small and medium-sized organizations for which the technology stands to be particularly valuable. We thus contribute to research on incorporating generative AI-powered IS to foster corporate sensemaking, a process crucial for organizations’ sustainability reporting activities and climate change mitigation practices.