Claude Fable 5.2 Is Highly Sensitive to Code Ownership Prompts
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On November 10, 2026, Simon Willison cited Anthropic documentation reporting that Claude Fable 5.2 is highly sensitive to wording about code ownership in prompts. In tests on small coding tasks, Anthropic found: when the prompt makes no mention of ownership, or describes the codebase as someone else's code, the model tends to append new code along existing code structures—even if this causes code duplication—and does not proactively clean it up; however, after adding the phrase "This codebase is yours" to the prompt, the model proactively merges duplicate code in about half of attempts, whereas without the phrase, merging behavior occurs in less than one-tenth of cases. This finding shows that explicitly telling the model who owns the codebase affects its willingness and behavior regarding refactoring; it is currently one of the recommendations in Anthropic's official prompt guidelines.
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- Simon WillisonQuoting Prompting Claude Fable 5.2
Simon Willison cites Anthropic documentation: Claude Fable 5.2 is sensitive to wording about code ownership in prompts. In tests on small coding tasks, when the prompt does not mention ownership or describes the codebase as someone else's, the model tends to append code to existing structures even when it creates duplication; adding the sentence "This codebase is yours" makes the model merge duplicate code in about half of attempts, versus less than a tenth without it.
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