Claude Fable 5.2 merges duplicate code in half of tries with ownership prompt

Anthropic's documentation describes a default behavior in Claude Fable 5.2: when a prompt is silent on ownership or frames the codebase as someone else's, the model tended to extend the existing structure rather than restructure it, even when that meant repeating code. That default can leave duplicated logic in place, which the testing linked to prompts that gave the model no stake in the code.

The suggested workaround is a short ownership statement — "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, that sentence coincided with the model merging repeated code in about half of tries, compared with under a tenth when it was absent. The gap suggests prompt framing alone can move whether the model cleans up duplication.