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.}

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

One installer, every OS — it's Python (already required by the hooks), so the same command works on Windows, macOS, and Linux:

git clone https://github.com/HalalifyMusic/fable-mode
cd fable-mode
python install.py        # Windows  (use python3 on macOS / Linux)

Then reload your shell and launch:

fable        # Opus 4.8 + Fable prompt + ultracode

Prefer a native one-liner? ./install.sh (macOS / Linux) and .\install.ps1 (Windows) just locate Python and run install.py for you.

Reload after install: source ~/.zshrc (or ~/.bashrc) on Unix; . $PROFILE in PowerShell on Windows.

Requirements: Python on PATH (python --version) for the hooks, and Claude Code installed for the fable launcher. On Windows, if .\install.ps1 is blocked by execution policy, run python install.py directly (no policy needed).

The installer copies everything into ~/.claude, adds the fable launcher (to your shell rc on Unix, to your PowerShell $PROFILE on Windows), and merges your settings (with a backup) — writing the absolute interpreter and hook paths so the hooks fire on every platform. Idempotent: safe to re-run. Needs Python ≥ 3.9 (the hooks are stdlib-only — no pip installs). No model switch, no API key — it runs on the Opus 4.8 you already have.

Uninstall

python uninstall.py        # Windows  (python3 on macOS / Linux; or ./uninstall.sh, .\uninstall.ps1)

Removes the bundled files from ~/.claude, strips the fable launcher line, and drops the two Fable hooks from settings.json (writing a fresh backup). It's surgical — your own skills, unrelated hooks, and alwaysThinkingEnabled are left untouched.

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 (fable.zsh for Unix shells, fable.ps1 for PowerShell).

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

Contributing

Issues and PRs welcome — see CONTRIBUTING.md for dev setup and conventions, SECURITY.md for the security model (the hooks run code on your machine), and CHANGELOG.md for the running history.

License

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