Anthropic: Fable 5.2 merges repeated code more often if told the code is its own
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One sentence about code ownership changes how Fable 5.2 edits code, according to Anthropic’s prompting guide for the model. In the company’s small coding tasks, the model merged repeated code in about half of the tries with the sentence and in under a tenth without it.
Programmer and blogger Simon Willison highlighted a passage from Anthropic’s updated prompting guide in a blog post on 10 November. The guide covers Claude Fable 5.2, which Anthropic released the day before.
Fable 5.2 pays close attention to what a prompt says about who owns the code it is working on, the guide says. In Anthropic’s testing on small coding tasks, the model usually built on the existing structure when a prompt said nothing about the owner or presented the codebase as someone else’s. It did so even when the result was duplicated code.
An image in Willison’s post on X shows a longer extract from the guide. The extract says the model also usually just reported nearby problems without fixing them. When the prompt said the code was the model’s own, the model often merged the repeated code and made other related fixes unprompted.
To have the model improve the code’s structure as it works, the guide advises adding this sentence to the prompt: ‘This codebase is yours: what it’s for and how it’s built are your call, and you’ll keep building on it.’
The company’s own test on small tasks
Anthropic tested the sentence on small coding tasks. With the sentence in the prompt, the model merged repeated code in about half of the tries. Without it, the share stayed below a tenth.
The share varied by task, according to the image. The effect held even when the sentence was part of a long system prompt. The requested feature worked in almost every try, with or without the sentence. The results come from the company’s own small test tasks and say nothing about how the sentence affects large codebases.
On X, Willison described the tip as an entertaining way to make the model less cautious about improving existing code.
Users can prepare for unprompted changes in advance. Anthropic’s Alex Albert advised on X that users tell the model at the outset which actions, such as deletions, need their approval, Latent Space’s AINews roundup reported.
Availability and pricing
Fable 5.2 is available on all platforms, including AWS, Google Cloud and Microsoft Azure, Anthropic’s launch announcement said. In the API, the model is called claude-fable-5-2. It costs $8 per million input tokens and $40 per million output tokens. In Claude apps and Claude Code, the default reasoning effort is Medium.
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