Fable Prompt

What this skill does

The user gives a rough ask ("I need a prompt that does X"). This skill turns it into a finished, Fable-5-optimized prompt they can paste straight into a Fable 5 / Mythos 5 session, plus a one-line note on which effort level to run it at.

The whole point: Fable 5 is smart and obedient enough that over-specified prompts HURT it. This skill enforces the opposite of the old instinct — give direction and intent, then trust the model. Every prompt it produces is deliberately lean.

Reply in the user's language; the produced prompt itself is in whatever language its target needs (the user's language for content, usually English for code/agents).

Source of truth

Everything here is distilled from Anthropic's official guide: Prompting Claude Fable 5 (platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5). The reusable instruction snippets below are quoted/adapted from it — they're battle-tested, prefer them over inventing new phrasing.

The 7 principles (what makes a prompt "Fable 5 good")

1. Give the reason, not just the request. Intent lets Fable 5 connect the task to the right context instead of guessing. This is the highest-leverage move. Pattern: "I'm working on [larger task] for [who]. They need [what the output enables]. With that in mind: [request]."
2. Short instruction beats an enumerated list. One brevity/boundary sentence steers as well as ten "don't do X" rules. Longer ≠ better; it degrades output.
3. State the boundaries. Say explicitly what to do and what NOT to do, and when to stop — Fable 5 can otherwise take unrequested actions (fix a thing you only asked about, make backups).
4. Don't over-plan / don't over-build. On ambiguous or routine tasks at high effort it can survey options and tidy things it wasn't asked to. Add a "when you have enough info, act" or "don't refactor beyond the task" line when relevant.
5. Ground progress claims (long runs only). For multi-step/autonomous work, tell it to audit each claim against a real tool result — kills fabricated status reports.
6. Aim high. Fable 5 shines on hard, long, ambiguous work. Let it scope, ask clarifying questions, and execute end-to-end rather than hand-holding.
7. Never ask it to reproduce its reasoning in the response. "Explain your thinking / show your work" as response text can trigger a refusal. If reasoning visibility is wanted, it comes from thinking blocks, not the prompt.

Canonical snippets (drop in verbatim when the situation calls for it)

Only include the ones the task actually needs — don't bolt on all of them.

• Don't over-plan (ambiguous task):

  When you have enough information to act, act. Don't re-derive facts already established, re-litigate a settled decision, or narrate options you won't pursue. If weighing a choice, give a recommendation, not a survey.

• Lead with the outcome (any deliverable):

  Lead with the outcome. Your first sentence answers "what happened" or "what did you find." Supporting detail comes after. Readability matters more than compression — don't shrink into fragments, abbreviations, or arrow chains.

• Don't over-build (code/build tasks at high effort):

  Don't add features, refactor, or introduce abstractions beyond what the task requires. Do the simplest thing that works. Don't add error handling for scenarios that can't happen; only validate at real system boundaries.

• Boundaries / stop-and-report (analysis or "I'm thinking out loud"):

  When I'm describing a problem or asking a question rather than requesting a change, the deliverable is your assessment. Report your findings and stop. Don't apply a fix until I ask.

• Ground progress (long/autonomous runs):

  Before reporting progress, audit each claim against a tool result from this session. Only report work you can point to evidence for; if something isn't verified, say so. If tests fail, say so with the output.

• Checkpoints (long workflows):

  Pause for me only when the work genuinely requires it: a destructive/irreversible action, a real scope change, or input only I can provide. Otherwise proceed.

• Self-verification (long build):

  Establish a method to check your own work as you build. Every [interval], verify against the spec — prefer a fresh-context subagent over self-critique.

• Autonomous / not-watching:

  You're operating autonomously; I'm not watching in real time. For reversible actions that follow from the request, proceed without asking. Don't end a turn on a promise ("I'll…") — do the work now.

Execution

Step 1 — Read the ask, decide if you have enough

Extract three things from the user's request:

• Task — what the prompt should make Fable 5 do.
• Intent — why / who it's for / what the output enables. (If missing, this is the #1 thing to get — principle 1.)
• Shape — one-shot vs. long autonomous run? code, content, analysis, agent/subagents? output language + format?

If Task + Intent are clear enough to write a strong prompt, write it — don't interrogate. Only ask when a genuine gap would change the prompt. Cap at 2–3 crisp questions, and offer a sensible default with each so the user can just nod.

Step 2 — Build the prompt

Assemble in this order:

1. Context + intent line (principle 1) — one or two sentences: the larger goal, who it's for, what it enables.
2. The actual request — concrete and direct.
3. Only the snippets this task needs — pick from the canonical list by shape:
   • Quick/one-shot content → intent line + request + maybe "lead with outcome." That's it. Resist adding more.
   • Analysis / "look at this" → add the boundaries/stop-and-report snippet.
   • Code/build → add don't-over-build.
   • Long or autonomous → add ground-progress + checkpoints (+ self-verification / autonomous as fit).
4. Constraints/format — output language, length, structure, must/must-nots.

Keep it lean. If you're tempted to add a fourth or fifth instruction, re-check whether the shorter version already covers it (principle 2). The prompt should look almost too short.

Never include: "explain your reasoning," "show your thinking," "walk me through your logic" in the response body (principle 7).

Step 3 — Deliver

Return, in chat:

1. The finished prompt in a single copy-paste code block.
2. One line: recommended effort — high default; xhigh for the hardest/most capability-sensitive; medium/low for routine. (Fable 5's low often beats prior models' xhigh.)
3. (optional, one line) what you deliberately left OUT and why — reinforces the lean philosophy so the user internalizes it.

Then stop. Don't over-explain the prompt you just wrote.

Anti-patterns (don't produce these)

• Ten bullet "rules" the model must follow → collapse to one sentence.
• Enumerating every edge case → state the principle, trust the model.
• "First, explain your approach, then…" reasoning-echo scaffolding → refusal risk.
• Defensive "if X fails, then Y, and also handle Z" for things that can't happen.
• Padding a simple content ask with autonomous-agent machinery it'll never use.