Simon Willison· Tools· Nov 10, 2026, 13:34 GMT+8

Prompting Claude Fable 5.2: stating code ownership changes how it refactors

Original title: Quoting Prompting Claude Fable 5.2

Simon Willison quotes Anthropic documentation noting that Claude Fable 5.2 responds closely to what the prompt says about code ownership: with no ownership framing or a codebase framed as someone else's, the model tends to add to the existing structure even when repeating code. Telling the model "this codebase is yours" led it to merge repeated code in about half of small coding task tries, versus under a tenth without the sentence.

Why it matters

Quoting Anthropic docs, the piece shares a reusable prompt phrasing plus comparison data on how often the model merged duplicate code with and without it.

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Original article

10th November 2026

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. [...]

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.

— Prompting Claude Fable 5.2, Anthropic documentation

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