Anthropic posted a FULL GUIDE on how to prompt Fable 5 (Mythos).
Claude Fable 5 is not meant to be prompted like any other model.
It's meant to run autonomously.
Here's exactly how to enable Fable to do work for you with minimal manual intervention:
1. Effort selection
Anthropic recommends using High for most tasks and Xhigh only for complex workflows.
Low/medium: quick questions, basic research
High: default for most work
Xhigh: complex builds, multi-step analysis
Ultracode: full autonomous orchestration
2. /loop prompting
Use /loop prompts to kick Fable off to complete full tasks.
/loop <time interval> <goal>
3. Tell it WHY, not just what (context)
Fable can't perform on instructions alone. It needs context to make decisions on its own.
Anthropic's exact prompting structure:
"I'm working on [larger task] for [who it's for]. They need [what the output enables]. With that in mind: [your actual request]."
4. Keep prompts short (instructions)
Counterintuitive but critical.
Over-engineering your prompts on Fable 5 degrades output. You're constraining a model that would have figured it out on its own.
4. Tell it when to stop and check in during runs
"Pause for me only when the work genuinely requires my input: a destructive action, a real scope change, or something only I can provide. Otherwise, keep going and report back when done."
5. Build it a memory system
Fable performs best when it can record lessons from its previous loops.
Give it a markdown file and this instruction:
"Store one lesson per file with a one-line summary at the top. Record corrections and confirmed approaches. Don't save what the repo or chat history already records."
The optimal general prompt structure:
"Goal: I'm working on [larger task] for [who it's for]. They need [what the output enables].
Request: [your specific ask in one sentence]
Output format: [exactly how you want it]
Constraints: [what must not happen]."
One last thing - your old prompts may actually work against you.
Skills and project instructions built for Opus 4.8 may produce worse results on Fable.
Bookmark this to actually maximize your Fable workflows.