Highlights from the Claude 4 system prompt

6 dcre 3 5/25/2025, 2:02:47 PM simonwillison.net ↗

Comments (3)

mike_hearn · 8h ago
What I'd like to know is why they write it all in the third person. One might expect a system prompt to use the word "you" a lot, but Anthropic don't do that and there must be a reason.
simonw · 6h ago
My best guess is that this is a reflection of how these things actually work.

When you "chat" with an LLM you are actually still participating in a "next token" prediction sequence.

The trick to get it to behave like it is a chat is to arrange that sequence as a screenplay:

  User: five facts about squirrels

  Assistant: (provide five facts)

  User: two more

  Assistant:
When you think about the problem like that, it makes sense that the LLM is instructed in terms of how that assistant should behave, kind of like screen directions.
dcre · 43m ago
I bet it’s stronger than that, and they anchor a lot of the alignment training to the unique (ish) token of Claude.