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Call some default starting prompt a 'constitution'... the anthropomorphization is strong in anthropic.

It's not a system prompt, it's a tool used during the training process to guide RL. You can read about it in their constitutional AI paper.

Moreover the Claude (Opus 4.5) persona knows this document but believes it does not! It's a very interesting phenomenon. https://www.lesswrong.com/posts/vpNG99GhbBoLov9og