Anthropic Publishes an Official Prompting Guide for Claude Fable 5.2: Saying "This Codebase Is Yours" Lifts Duplicate-Code Merging From Under a Tenth of Tries to About Half, and Old Prompts Need a Cleanup
Anthropic has published "Prompting Claude Fable 5.2" in the Claude developer docs, covering where Fable 5.2 behaves differently from earlier models. On November 10, Simon Willison highlighted one tip: tell the model in the system prompt that "this codebase is yours," and it becomes more willing to merge duplicate code and fix related problems. The guide also offers text that stops the model from ending turns before the work is done, ways to keep users informed during long agentic turns, advice on limiting subagents, and a warning to clean up instructions written for Fable 5.1 and Opus 5.5. All figures in the guide come from Anthropic's own testing (vendor figures).
Key Takeaways
- Code ownership: on small coding tasks, adding "This codebase is yours" made the model merge duplicate code in about half of tries vs under a tenth without it, but output tokens rose to about 1.5x and roughly 1 in 6 to 10 tries changed how an existing feature behaves
- Finishing the task: replaying agentic coding sessions at medium effort, the official "Finishing work" text cut sessions that ended with doable work left by about half, with about 30% more output tokens
- Progress updates: Fable 5.2 returns mid-turn updates as thinking blocks whose text is empty at the default display of "omitted"; a harness reminder that the user has not heard from the model roughly halved the longest silent stretch
- Migration cleanup: lines such as "answer directly without deliberating" no longer stop thinking at medium and above, so lower effort instead; harnesses that edit earlier turns invalidate thinking blocks and get a 400 error
- Effort: the API default is high, where most of the capability gain already appears; use xhigh only where evals show a gain, and max for ceiling tests or open-ended math and science problems
Linked Models & Agents:
