Inside the "Brain" of Claude

How researchers are dissecting large language models to understand what makes them tick

This substack is a summary of "On the Biology of a Large Language Model" by Jack Lindsey, Wes Gurnee, Emmanuel Ameisen, et al. (2025) which I am teaching today and I wrote up the lecture notes. It is a fascinating research paper exploring the internal mechanisms of Claude 3.5 Haiku through experiments made using circuit tracing and attribution graphs.

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