Prompting Fable 5.2: effort, ownership and harness changes that matter
Source
platform.claude.com
Author
Simon Willison
Date
Why it matters
Concrete, tested prompt and agent harnessThe scaffolding around a model that turns it into a working agent — the loop, the tools it can call, and the rules for when to stop.Full definition → changes for Fable 5.2, such as how effort levels behave, why an ownership sentence changes refactoring behavior, and which prompts written for earlier models to remove, with tokenThe chunk of text a model reads and writes in — roughly three-quarters of a word — and the unit AI usage is billed in.Full definition →-cost effects measured.
Terms in this piece · Glossary
token — The chunk of text a model reads and writes in — roughly three-quarters of a word — and the unit AI usage is billed in.
multi-agent — Using several AI agents on one problem — splitting work in parallel, checking each other, or filling different roles like planner and reviewer.
agent harness — The scaffolding around a model that turns it into a working agent — the loop, the tools it can call, and the rules for when to stop.
