“Building and evaluating model diffing agents” by bilalchughtai, Josh Engels, Neel Nanda

This is the second in a series of research updates from the Google DeepMind Language Model Interpretability team, in interpretability and adjacent areas. The first post can be found here. TL;DR It is possible to build extremely simple agents that reliably find interesting behavioural differences between distinct models. We call these ‘diffing agents’.The closest previous 'behavioural model diffing' work has focussed on understanding behavioural differences between two models on some static prompt distribution. This is valuable, but might miss important differences, especially if they are rare. We propose instead allowing an auditor agent to craft their own prompts to intelligently search for and validate behavioural differences, and find this to work well.We present results of applying our model diffing agent to a number of pairs of real models.We introduce a set of simple evaluations with ground truth for evaluating model diffing agents. These are: There should be no differences found when the models compared are identical.In model organisms with a conditional system instruction, the only difference found by the agent should be the intended behavioural change specified by the conditional system instruction.We validate that our diffing agents outperform standard auditing agents [...] ---Outline:(00:24) TL;DR(02:10) Introduction(04:24) Method(07:08) Realistic Findings(11:27) Evaluations with ground truth(11:38) Zero FPR on identical models(12:05) System prompted model organisms(16:38) Single model baseline(18:38) Example: Python Indentation Style(19:01) Finetuned model organisms(20:37) Conclusion(22:54) Appendix(22:57) Diffing Agent Prompts(23:16) Single Model Auditing Agent Prompts(23:35) Seed Prompts --- First published: June 12th, 2026 Source: https://www.lesswrong.com/posts/qi4mNbZYAFDYwfRba/building-and-evaluating-model-diffing-agents --- Narrated by TYPE III AUDIO. ---Images from the article:Apple Podcasts and Spotify do not show images in the episode description. Try Pocket Casts, or another podcast app.