fable-mode

Run Claude Fable 5 on Opus 4.8. The Mythos-class model the U.S. government pulled after three days — brought back as a system prompt.

     

One clone, one install: the leaked Claude Fable 5 system prompt + a measured execution playbook + verification hooks + design and agent skills — wired into Claude Code.

_{Same brief, same model. Left: stock Opus 4.8. Right: Opus 4.8 in fable-mode.}

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Why this exists

Claude Fable 5 shipped on June 9, 2026 as Anthropic's first Mythos-class model — and was suspended on June 12 under a U.S. export-control directive. You can't call the model right now.

But when its system prompt leaked, people noticed: a lot of what made Fable feel different — its taste, its directness, its tool-use instincts — lived in the prompt, not only the weights. Run that prompt on Opus 4.8 and the output changes character. The community calls it Fable 5 Lite. This is the complete version for Claude Code, plus the part a leaked prompt can't give you — measured discipline and verification, enforced by hooks.

Quickstart

git clone https://github.com/HalalifyMusic/fable-mode
cd fable-mode && ./install.sh
source ~/.zshrc
fable        # Opus 4.8 + Fable prompt + ultracode

install.sh copies everything into ~/.claude, adds the fable launcher, and merges your settings (with a backup). No model switch, no API key — it runs on the Opus 4.8 you already have.

What's in the bundle

• FABLE_PLAYBOOK.md — the core. Fable-5 vs Opus-4-8 tool traces turned into rules: reasoning density (70% vs 47%), verify-after-edit, parallelism — plus a voice layer and an evidence-ledger grounding protocol. Original work, not the leaked prompt.
• fable-system.md — the leaked Fable 5 system prompt (Anthropic's; see note below).
• Hooks — fable-trigger.py injects the playbook at xhigh/max/ultracode; test-after-edit.py runs your project's tests after each edit and reports the result back — the one habit no model keeps on willpower.
• /ground skill + grounding-verifier agent — a self-terminating grounding loop and a cold verifier that assumes every claim is wrong until the live code proves it.
• Skills — claude-design-patterns (web-UI engineering), webapp-testing, mcp-builder, skill-creator, explore-data.
• fable() launcher — Opus 4.8 + the prompt + ultracode effort.

The honest ceiling

This gives you Fable's disposition, not its raw capability. Reasoning depth, vision, long-horizon autonomy, and design intuition are weights-bound — only partly reachable by instruction. What transfers well: voice, formatting, grounding and verification, design patterns, effort configuration. On verification, parallelism, and communication it actually beats Fable, because those are enforced by hooks instead of left to intention.

About the bundled pieces

• The Fable 5 system prompt (fable-system.md) is Anthropic's IP. It is already mirrored across high-star public repos; it's bundled here only so setup is a single step. Not ours to license, removable on request. It's the consumer prompt — trim to the claude_behavior section to cut dead weight in a terminal.
• webapp-testing, mcp-builder, skill-creator are from anthropics/skills (Apache-2.0); explore-data is from anthropics/knowledge-work-plugins (Apache-2.0).

Credits

Made by me — compiled from community sources (leaked prompts, public Anthropic skills) and original measurement and tooling work.

License

MIT on the original code (see LICENSE). The bundled Fable system prompt and the Anthropic skills carry their own terms.