fable-skills is a library of 35 composable instruction files that encode senior engineering habits—such as root‑cause debugging, security reflexes, and edge‑case sweeps—to lift LLM outputs toward Fable‑level discipline regardless of the underlying model.

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2026-07-10

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FR-AI / ANALYSIS

Why it is worth attention

It provides auditable, benchmarked evidence (n=5 blind‑graded per cell) that adding these skill prompts to a lighter model like Sonnet 5 can match or exceed the bare performance of a stronger model like Opus 4.8, reducing variance and closing the model gap through explicit habits rather than model scale.

Who it is for

  • Developers using Claude Code or similar LLM‑driven coding tools
  • Engineering teams deploying LLM agents for code review, debugging, and planning
  • AI researchers interested in prompt composition and model alignment
  • Senior engineers evaluating structured vs. ad‑hoc LLM output quality

Use cases

  • Improving code review thoroughness by activating security and correctness skills
  • Guiding LLMs through structured root‑cause debugging with a reproducible checklist
  • Enforcing edge‑case and numerical correctness when processing financial or concurrent logic
  • Enabling a cheaper model to produce work equal to a more expensive one on specific tasks

Strengths

  • Benchmarked with blind grading and full audit trail (62 raw outputs, answer keys, grader prompts)
  • Skills are small, composable, and activated situationally—no monolithic prompts
  • Proven lift across multiple models and task areas (debugging, concurrency, edge cases)
  • Reduces output variance, making results more consistent and predictable

Considerations

  • Benchmarks use n=5 per cell—directional but not statistically conclusive
  • The strongest subtle insights (e.g., JSON null cache guard) still require the most capable model even with skills
  • Grading is performed by an LLM applying an objective checklist, not by a human expert

README quick start

fable-skills

A library of 35 engineering-discipline skills — small, composable instruction files that encode how a careful senior engineer works, not what any particular framework does.

The premise

The most capable Claude models don't just know more — they work more carefully. Fable 5 reaches for a codebase's own conventions before writing, proves a bug's cause before fixing it, sweeps edge cases before declaring done, and reports what it actually verified rather than what it assumes. A strong-but-lighter model like Opus 4.8 can do all of that too — it just doesn't always reach for those habits unprompted.

These skills close that gap. Fable identified the places where a less deliberate run tends to fall short of its own ceiling — orientation, root-cause discipline, security reflexes, edge-case coverage, honest reporting — and wrote each habit down as an explicit, always-available checklist. Load them into a session and a model follows the discipline on purpose instead of leaving it to chance. The goal is simple: Fable-level output from whatever model is driving.

Each skill is one focused habit. They fire situationally — a debugging task pulls in root-cause-debugging, a diff touching user input pulls in security-reflexes, a schema change pulls in data-migration-safety — so the model carries the right discipline into the right moment without drowning in guidance.

Does it actually help? Benchmarks

Same method throughout: run an identical task through fresh agents with no guidance vs. with the relevant skill files, then score the outputs against a fixed answer key. Benchmark 1 uses Opus 4.8; Benchmark 2 runs both Opus 4.8 and Sonnet 5. These are directional signals, not statistical proofs.

The full assets are in benchmark/ — seeded tasks, answer keys, all 62 raw run outputs, the blind-grader prompts, and the per-item score tables — so every number below is auditable and re-gradable rather than take-my-word-for-it. (They were reconstructed verbatim from the original session transcript; each file carries a provenance header, and the recovery manifest in benchmark/README.md lists the known gaps.)

Benchmark 1 — Security code review (n=1, blind-graded)

The same task was run through two fresh agents and both reviews were scored blind by an independent third agent. Task: review a Python ord

Description

35 engineering-discipline skills authored by fable 5, with a blind-graded benchmark: sonnet 5 loaded with the skills beat opus 4.8 running bare. no deps, mit.

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