Prompting Claude Fable 5.2: Declaring code ownership affects refactoring behavior

Simon Willison cites an Anthropic document: Claude Fable 5.2 is highly sensitive to how code ownership is described in prompts. If the prompt omits ownership details or attributes a codebase to someone else, the model tends to reuse existing structures even when there is duplicate code. The document recommends explicitly stating 'This codebase is yours.' In small-scale tests, adding this phrase caused the model to merge duplicate code in about half of the attempts; without it, the rate was less than one-tenth.