Anthropic's Fable 5.2 prompting guide suggests "This codebase is yours" for code changes

AISimon Willison says Anthropic's updated prompting guide for Fable 5.2 recommends telling the model "This codebase is yours" to make it less cautious when improving existing code. He links to the guide's section on saying who owns the code.

Say who owns the code
Claude Fable 5.2 responds closely to what the prompt says about who owns the code it works on. In Anthropic’s testing on small coding tasks, when a prompt said nothing about ownership or presented the code base as someone else’s, the model tended to add to the existing structure, even when that repeated existing code. It also tended to report nearby problems instead of fixing them. When the prompt says the code base is the model’s own, the model often merges repeated code and makes related fixes without being asked.
If you want the model to improve the structure as it works, say that the code is its own:
This codebase is yours: what it's for and how it's built are your call, and you'll keep building on it.
In Anthropic’s testing on small coding tasks, the model merged repeated code in about half of the tries with this sentence, and in under a tenth of the tries without it. The rate varied by task. The result held when the sentence stood inside a long system prompt

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Simon Willison · x.comPublished Nov 10, 12:42 AM ET · added here 12:58 AM ET

Original title: When Anthropic release a new model it's always worth taking a look at their updated prompting guide - here's an entertaining tip to make ...

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