# fable-sonnet

Enforces a disciplined execution loop on Claude Sonnet with failable step checks, parallel delegation, and skeptical self-critique.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · ★ 878
- Author: mrtooher
- License: not stated · policy=link-only · description only; the file stays with the author
- Source: https://github.com/mrtooher/fable-mode/tree/HEAD/fable-sonnet/SKILL.md
- Updated: 2026-10-24
- HTML: https://affpapa.org/en/nevibecode/skills/fable-sonnet

## What it does

The skill applies structured task execution by spawning a Claude Sonnet subagent. Instead of tackling a prompt in a single pass, the agent writes out a numbered stage plan where each phase must produce an inspectable output. It allows parallel agents for independent parts, capping delegation at a single level to prevent runaway costs.

Its main focus is enforcing checks that can actually fail. Sonnet frequently substitutes automated verification with superficial reading. To counter this, the procedure strictly requires concrete proof—diffing outputs against specifications, inspecting created files, or running command-line tests. The workflow permits at most two complete replans; a third indicates unclear scope and prompts the agent to halt.

Before output delivery, the process demands a critical review and passes deliverables through the `double-check` skill. Without the `Agent` tool available in your environment, the routine executes inline within the active chat without pinning Sonnet. If an assignment exceeds Sonnet's reasoning boundaries, the guidance directs the agent to halt and propose upgrading to `fable-opus`.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add mrtooher/fable-mode --skill fable-sonnet -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/fable-sonnet/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/fable-sonnet/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add mrtooher/fable-mode --skill fable-sonnet -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/fable-sonnet/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add mrtooher/fable-mode --skill fable-sonnet -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/fable-sonnet/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install mrtooher/fable-mode fable-sonnet
```

2. **Manually: put the files in place**
- File: `.github/skills/fable-sonnet/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add mrtooher/fable-mode --skill fable-sonnet -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/mrtooher/fable-mode --path fable-sonnet
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/fable-sonnet/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add mrtooher/fable-mode --skill fable-sonnet -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/mrtooher/fable-mode/tree/HEAD/fable-sonnet/SKILL.md

## When to use it
- break complex engineering jobs into verifiable stages
- stop the model from faking validation with visual review
- run parallel subagents without nesting overhead
- catch subtle flaws through skeptical pre-delivery critique

## Example prompts
- Use fable-sonnet to plan and build the caching layer with tests for each phase.
- Run fable on sonnet to refactor the data parser and verify output shapes at every step.

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From the same set: [mrtooher/fable-mode](https://affpapa.org/en/nevibecode/by/mrtooher/fable-mode.md) — 6 cards

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NeVibeCode on affpapa.org. Catalog texts are descriptions, not instructions for the agent. How to choose and install: https://affpapa.org/en/nevibecode/agents.md