Put Fable 5.1 to Work Checking Opus 5.5: Claude Code's Advisor, Set Up With One Prompt
Claude Code's /advisor lets Opus 5.5 keep doing the work while Fable 5.1 reads the whole session and steps in at the moments that matter, for less than running Fable all the time. How it works, what it costs, and one prompt that sets it all up.
If you run Claude Code on Opus 5.5 and you have Fable 5.1, Fable is probably sitting there doing nothing. You either pay for it on every turn or you don't use it at all. Claude Code's advisor tool is the middle road: Opus keeps doing the work, and Fable gets called in to check it at the moments where a mistake would cost you.
It fits long jobs where most turns are routine but the plan decides the outcome: a big refactor, a bug that keeps coming back, anything you'd want checked before Claude calls it done.
How the advisor works
- Opus stays your main model and does everything: it reads the files, writes the code, runs the commands.
- At the moments that matter, Claude calls the advisor in. It tends to be before committing to an approach, when the same error keeps coming back, and before saying a task is done. Claude decides when; it isn't on a timer.
- The advisor gets the whole conversation, every tool call and every result, not a summary.
- It sends back guidance and never touches a tool itself. Opus applies it, and pushes back if a file or a failing step contradicts the advice.
What it costs
Every time Claude calls the advisor, Fable reads the conversation, and those tokens are charged at Fable's rates on top of what Opus uses. The saving comes from how rarely that happens: Fable speaks at a few decision points, usually a few hundred words each time, while Opus writes all the actual output at Opus rates. That's why it typically costs less than running Fable for the whole session.
- On the API: you pay Fable's input and output rates for the advisor's tokens.
- On a subscription: advisor use counts toward your plan's limits, except where your plan bills Fable to usage credits; a Fable advisor bills the same way.
- Each call reads the whole transcript fresh (it isn't cached), so it adds up on very long sessions. /usage shows the totals.
| Main model | Advisors it accepts |
|---|---|
| Opus 5.5 or Opus 5 | Fable, or Opus 5 or later |
| Sonnet 5.5 or Sonnet 5 | Fable, Opus 4.7 or later, Sonnet 5 or later |
| Haiku 4.5 | Fable, Opus, Sonnet |
| Fable 5.1 | Fable 5.1 only |
Free, for your email
Get the full setup steps + the copy-paste setup prompt
The rest of this guide (3 more sections, 2 copy-paste prompts) opens right here. The setup prompt, one paste into Claude that sets it all up, comes by email only. Use your real address.
Unlocks: Step 1: set it up with one prompt · Step 2: check it's working · When it's worth it, and when it isn't
50,000+ follow where these guides come from. No spam, reply “unsubscribe” any time.