Many AI models that power chatbots advertise themselves as “open source,” but do not fully release the code and training data. A new study shows that many large companies describe their models as “open weights”, meaning that researchers can use them, but have no access to the underlying data and can’t make fundamental changes to them. The lack of transparency in the training data is a particular obstacle. Smaller companies and research groups, on the other hand, rely on truly open source models that enable the advancement of AI research.