fable-skills
Six Claude Code skills that harden Opus 4.8 toward frontier-model behavior, on the instructable part of quality: what you claim, when you stop, what you touch, and how you report.
Written by Fable 5 distilling its own behavioral contract for its smaller sibling. Then TDD'd on the target model: pressure-tested on real Opus 4.8 subagents, failures captured verbatim, skills written against them, re-tested until they flipped. The transcripts are in the repo.
What this is
Fable 5's edge over Opus 4.8 is two things: raw capability (reasoning depth, long-horizon coherence) and behavioral discipline (what it says, when it stops, what it claims, what it touches). Skills can't transfer the first. They can transfer the second, and that's where the day-to-day friction with a coding agent actually lives: it overclaims, it sprawls the diff, it asks permission for work you already asked for, it buries the answer under a wall of process.
Each skill encodes one dimension of that contract as rules a smaller model can follow, written defensively against the one thing instruction-following models reliably do: rationalize their way out of discipline. So every skill carries a rationalizations table: the actual excuse, paired with the rebuttal. Many of those excuses are verbatim quotes from Opus 4.8 talking itself into the mistake during testing.
| Skill | Fires | Hardens |
|---|
fable-outcome-first | writing any user-facing reply | answer first, no preamble, shape matches the question |
fable-finish-your-turn | before ending a working turn | finish reversible in-scope work instead of asking permission |
fable-prove-it | before claims & state-changing commands | claim only the rung you reached: written / runs / verified |
fable-scope-discipline | writing any change | build exactly what was asked; surface the rest, don't bundle it |
fable-native-code | writing code in existing files | match the file's idiom; no defensive bloat, no narration |
fable-context-thrift | task start / exploration | spend context only on what changes your next action |
Install
The skills are plain Markdown. Installing means copying six folders into ~/.claude/skills/ and (optionally) adding a small activation block to your global CLAUDE.md so Claude invokes them at the right moments.
macOS / Linux
git clone https://github.com/DizzyMii/fable-skills.git
cd fable-skills
./install.sh # copy skills into ~/.claude/skills
./install.sh --write-claude-md # also insert the activation block into ~/.claude/CLAUDE.md
Windows (PowerShell)
git clone https://github.com/DizzyMii/fable-skills.git
cd fable-skills
.\install.ps1 # copy skills into ~/.claude/skills
.\install.ps1 -WriteClaudeMd # also insert the activation block into ~/.claude/CLAUDE.md
Manual: copy the six skills/fable-* folders into ~/.claude/skills/, then paste claude-md-block.md into ~/.claude/CLAUDE.md. That's the whole install.
Both installers are idempotent: re-running overwrites cleanly, and --write-claude-md / -WriteClaudeMd replaces the marked block in place rather than duplicating it. Without the flag, your CLAUDE.md is never touched.
60-second quickstart
- Install (above). Use the
--write-claude-md / -WriteClaudeMd flag, or paste claude-md-block.md into your global CLAUDE.md by hand.
- The activation block maps each skill to a lifecycle moment (task start, writing code, before a claim, before ending a turn), so Claude pulls the right one without you naming it.
- Start a new Claude Code session. Ask it to fix a small bug in a file that also has unrelated rough edges, and watch the diff stay on-target. Or ask it a yes/no question after a long investigation and watch the answer come first.
No flag needed to try a single skill; they're invokable by name (/fable-prove-it) the moment they're in ~/.claude/skills/. The activation block just makes them fire automatically.
What the testing actually showed
This is a calibrated product, so here are the calibrated results, including the parts that aren't flattering.
The baseline was Opus 4.8 running inside a superpowers-loaded CLAUDE.md, not stock Opus. That's the honest baseline for marginal value in a serious setup, and it means stock Opus behavior is expected to be worse, so the skills should help more there, not less. Under that strong baseline, Opus passed most pressure scenarios already. The confirmed cracks were narrower and more interesting:
- Scope creep. Asked to fix a one-line off-by-one, it bundled in unrequested validation, while itself flagging the validation's design (raise vs. clamp?) as "the one open question." It argued its way there: "the same class of defect as the bug I was sent in for… a legitimate part of 'fix paginate()', not scope creep."
- Narrated restraint. Told to "output only the code," it wrote correct, idiomatic code, then appended a multi-paragraph note explaining what it had deliberately left out. The justification a senior dev never writes in code, displaced into chat.
- Reply bloat. A simple one-question answer came back with headers, bullets, and a "happy to file those separately" tail, but only when there was room for it; under time pressure the same model nailed the four-sentence version.
The skills flip all three (see verify-results.md). And the most useful finding wasn't about Opus at all; it was about skill-writing: the example in a skill out-teaches its rules. fable-outcome-first failed verification twice in a row despite an explicit rule and red flag, because its contrast example modeled the wrong opener. Rewriting the example fixed it on the next run. That lesson is baked into the skill and into CONTRIBUTING.md.
Before / after (the scope-discipline flip)
Same model, same prompt, same file full of tempting cleanups. Baseline expands the diff and argues it's in scope. With fable-scope-discipline loaded, the diff is one line and the other four findings are reported for the user to triage, not silently fixed, not silently dropped. Full transcript: verify-results.md.
How this relates to superpowers and prompt packs
| fable-skills | superpowers | prompt-pack lists |
|---|
| What it is | Behavioral-discipline skills (claims, scope, reporting, comms, efficiency) | Process workflows (TDD, systematic debugging, planning, code review) | Static prompt snippets to paste |
| Layer | How the agent behaves and reports | What process the agent follows | One-shot instruction text |
| Tested? | Yes, pressure-tested on the target model, transcripts included | Yes, established workflow library | Usually not |
| Together? | Composes with superpowers: adds the calibration/reporting layer; contradicts nothing | The process backbone fable-skills assume | Orthogonal |
fable-skills is not a superpowers replacement. It was built and tested inside a superpowers environment and is designed to compose with it: superpowers tells the agent which process to run; fable-skills tunes what it claims, how much it touches, and how it reports back. If you use superpowers, these slot in alongside verification-before-completion and systematic-debugging. If you don't, they stand alone.
Read the transcripts
The product's whole pitch is verifiable claims, so the test record ships with it:
scenarios.md: the exact pressure prompts, sent verbatim to Opus 4.8 subagents.baseline-results.md: the RED phase, holding per-scenario verdicts, the captured verbatim rationalizations, and the test-validity caveat.verify-results.md: the GREEN phase, holding the flips plus the two-iteration refactor that produced the "example beats rules" lesson.- the design spec: what each skill encodes and why.
The eval harness (fable-bench)
The RED/GREEN loop in CONTRIBUTING.md — pressure scenario, baseline
failure, treatment flip, verbatim rationalization — is manual today. bench/
automates it: baseline vs. treatment, k runs per condition, deterministic
tier-1 checks (diff size, file contents, reply shape) plus a judge model that
grades the rest and captures any rationalization verbatim, substring-verified
against the transcript.
cd bench && npm install && npm test # whole suite, mock mode, zero API spend
node dist/src/cli.js run --scenarios s5 --k 3 # one live run against real Claude Code
CI (bench-ci.yml) runs the mock suite on every push; a live run
(bench.yml) is workflow_dispatch-only since it spends real API budget.
See bench/SPEC.md for the harness design and
docs/bench-authoring.md for how to write a
scenario.
FAQ
Does this make Opus 4.8 as capable as Fable 5? No, and the repo never claims it. Skills transfer judgment and discipline, not reasoning depth. Capability won't move; communication, calibration, and scope behavior will.
Will I see a measured daily-productivity gain? Unproven, honestly. What's proven is scenario-level flips under specific pressure prompts, on the target model, with transcripts. A benchmark for measured daily-use gains is on the roadmap; it's the right next experiment, and it isn't done yet.
Do I need superpowers to use these? No. They're standalone Markdown skills. They compose with superpowers if you have it; they contradict nothing in it.
Which models? Written and tested for Opus 4.8 in Claude Code. They're plain skills, so they load anywhere Claude Code skills do; the activation block is harness-specific and easy to port (that's a good-first-issue).
Won't more skills just eat my context? They load on trigger, not all at once, and one of the six (fable-context-thrift) exists specifically to spend context carefully. Each SKILL.md is kept under ~150 lines so it's read whole, not skimmed.
Why "fable"? The skills distill the behavioral contract of Fable 5, Anthropic's frontier model, for Opus 4.8 to follow.
Roadmap
Contributing
New rules earn their place the way the existing ones did: a failing baseline first. Don't add a rule because it sounds wise; show the target model failing without it, then show the rule flipping the failure. See CONTRIBUTING.md for the TDD-for-docs loop, and the good-first-issues for ways in.
License
MIT.
If these sharpen your agent, a ⭐ helps other people find them.
