Fable 5 Advisor + Sonnet 5 Executor: Claude Code Setup, Prompts, and When to Consult
How to pair Sonnet 5 executor with Fable 5 advisor in Claude Code — /advisor fable, settings.json, prompt templates for long builds, model pairing rules, cost, vs opusplan.
Claude Fable 5Claude CodeSonnet 5Advisor ToolPrompt EngineeringAI Agents
Sonnet 5 executes. Fable 5 directs. That is Anthropic's recommended split for long coding and creative builds — not two sessions you switch manually, but one Claude Code session where Sonnet runs tools and Fable consults at decision points via the advisor tool.
The @ClaudeDevs July 8 benchmarks put numbers on it: ~92% of Fable solo on SWE-bench Pro at ~63% cost when Fable advises ~once per task. This guide is the how-to: /advisor setup, pairing rules, prompt templates (including a game-dev example), and when to use advisor vs opusplan vs subagents.
TL;DR — setup in 60 seconds
Step
Command / setting
Requirements
Claude Code v2.1.170+ · Fable 5 access · Anthropic API (not Bedrock/GCP/Foundry)
Your target pairing:Sonnet 5 main + Fable advisor — one of Anthropic's common pairings: "Fable 5 guidance at decision points without running Fable 5 throughout."
Subagents inherit the configured advisor if their model satisfies the pairing check.
Division of labor — executor vs advisor
Role
Model
Job
Executor
Sonnet 5
Write code · fix bugs · run tools · ship iterations · most tokens
Same framing as Matt Shumer's Workbench playbook: goal + bar for the project, execution for the fast model — except Anthropic wires Fable in as a server tool instead of you pasting advice manually.
Prompt principle: Tell Sonnet it is the implementation agent. Tell it to consult the advisor proactively — not only on errors, but on creative, architectural, pacing, progression, and UX decisions when multiple valid directions exist.
Prompt template — executor instructions
Paste this at the top of a long session (game, refactor, greenfield app):
markdown
You are the implementation agent (Sonnet 5).
## Your job- Execute code, fix bugs, implement systems, ship working improvements.
- Do not stop to overthink — keep moving.
- Never ask me for implementation decisions unless truly blocked.
## Advisor (Fable 5)
Consult the advisor:
- Before any major feature or architecture change
- Before progression, boss, AI Director, UI redesign, or rendering systems
- Before marking a task complete (quality review)
- After 2+ failed attempts on the same issue
- Whenever multiple valid design directions exist — proactively, not only on errors
Flow: consult advisor → pick strongest solution → implement.
Advisor owns direction. You own execution.
Add project-specific tasks below (combat, weapons, story acts, etc.). Keep house rules and definition of done in the same doc — same pattern as Field Guide to Fableunhobbling: give Sonnet room to execute, fence with rules Fable helps refine.
Worked example — indie game AAA pass
A common failure mode: one giant prompt to Fable solo — expensive, or one giant prompt to Sonnet solo — shallow UI and weak systems design. Advisor split maps cleanly:
Workstream
Executor (Sonnet)
Advisor consult (Fable)
Cooldown UI on abilities
Implement HUD timers
Review readability + juice before merge
Weapon attach to hands
Fix transforms, sockets
Review animation pipeline + sprite sheet plan
Weapon orientation (mace handle)
Patch all weapon prefabs
Confirm convention once for all weapons
Boss drops + legendary weapons
Data model + drop tables
Pacing: when Thor-like weapon unlocks
AI Director
Implement telemetry + spawn logic
Balance philosophy — fun not max difficulty
Story acts vs "level 20"
Act flags, checkpoints
Narrative structure + checkpoint art briefs
Asset gen (sprites, audio)
Subagents or APIs in parallel
Art direction bar before replacing placeholders
UI redesign
Rebuild HUD, game over, menus
UX review against target (Hades / Dead Cells tier)
Delegate assets without blocking code: Sonnet keeps implementing with placeholders while image/audio agents run — same multi-agent pattern as loop engineering and official /loop guidance.
Story note: Acts (Escape → Mutation → Hunt) beat numbered levels. Advisor helps define narrative checkpoints; Sonnet wires when they trigger. Use image APIs at act boundaries — advisor drafts what each panel must communicate.
Enable advisor — three ways
1. /advisor command (recommended)
snippet
/model sonnet
/advisor fable
Saves to user advisorModel. If org availableModels blocks Fable, pick an allowed advisor or request access — see Fable 5 status.
2. settings.json
json
{"advisorModel":"fable"}
3. Launch flag
bash
claude --advisor fable
Session-only; overrides saved setting for that run.
Anthropic docs: Claude decides when — tends toward:
Before committing to an approach
Recurring errors (same failure twice)
Before declaring done
You can force rhythm in prompts: consult the advisor before you continue — same as any tool request.
In session UI:Advising line with model name → Ctrl+O for full Fable guidance. Sonnet follows advice unless evidence contradicts it (file contents, failed step) — then it surfaces the conflict.
Cost: Advisor tokens bill at Fable rates; executor at Sonnet rates. Advisor runs at decision points, not every turn — see cost breakdown. Track with /usage.
Advisor vs opusplan vs subagents vs switching model
Approach
When stronger model runs
Best for
Advisor (this guide)
Decision points mid-task
Long builds — mostly Sonnet turns, Fable at forks
opusplan
Plan mode only, then Sonnet executes
Up-front plan you approve once
Subagents
Whole delegated subtask on chosen model
Parallel asset gen, isolated modules
/model fable
Every subsequent turn
When every turn needs frontier reasoning
For month-long game polish: Sonnet + Fable advisor + subagents for assets beats Fable solo on cost and beats Sonnet solo on direction.
Pairing rules, version gates, and availability reflect Anthropic's Claude Code documentation as of July 2026. Org allowlists and Fable access may limit /advisor fable on your account — verify with /advisor picker output.
