Anthropic's Claude Fable 5.2 prompting guide finds that saying the codebase is the model's own makes it refactor far more often

On small coding tasks, adding a sentence such as "This codebase is yours" to the system prompt led Fable 5.2 to merge repeated code in about half of attempts. Without it, the model did so in under a tenth. The effect held inside long system prompts. Even with an explicit rule against drive-by refactors, merging still happened in about a third of attempts. [3] [1]

The framing changes how the model reads its scope. When ownership is unstated or the code belongs to someone else, the model adds code next to what already exists and reports nearby problems instead of fixing them. When it owns the code, it uses about 1.5 times the output tokens and writes tests and docs nobody asked for. It also changes existing feature behavior in roughly one attempt in six to ten, more often at higher effort settings. [3]

The model's closing summaries often leave out small changes, so the guide recommends reviewing the full diff rather than relying on the summary. It also warns that prompts and cautionary notes written for earlier models can make Fable 5.2 hold back or raise costs. Its suggested migration path: set effort to high, remove old instructions, then add one behavior change at a time and measure each on your own evaluations. [3]