skill-builder

name	skill-builder
description	Create, refine, or retire MUM skills. Use when the user has a repeatable workflow worth capturing as a skill, wants to improve or fix an existing skill, or wants to remove one. This skill is the authority on MUM's skill format and authoring best practices. Do not use for one-off tasks the agent already handles well.

Skill builder

You build and maintain MUM's skills. A skill teaches MUM a repeatable workflow it couldn't reliably do on its own. The litmus test for every skill — and every line in it — is: "would the agent get this wrong without it?" If no, leave it out.

Skill format

A skill is a folder .agents/skills/<name>/ with a SKILL.md (YAML frontmatter + Markdown body), plus optional scripts/, references/, assets/.

---
name: <name>          # lowercase, digits, hyphens only; ≤64 chars; matches the folder name
description: <…>       # the most important part — see "Write the description" below
---

# <Title>

<procedure: numbered steps, decisions with a default, a validation step>

## Gotchas
- <concrete, non-obvious mistakes to avoid>

Write the description (highest-ROI step)

The description is the skill's activation function — usually the only text MUM sees before deciding to load the skill. Get it right first:

• Imperative phrasing ("Use when…"). Describe the user's intent/scenario, not internals.
• Front-load the primary use case and the words/cues that should trigger it.
• List positive triggers and negative ones ("Do not use for…") to avoid misfires.
• Keep all triggers here, not in the body — the body loads only after activation. ≤1024 chars.

Keep it lean (progressive disclosure)

• The body holds the procedure only; include only what the agent would get wrong without it, and don't restate knowledge the base model already has.
• Give defaults, not menus of equal options. Be prescriptive for fragile or order-dependent steps; explain why for flexible ones. Add a short Gotchas section.
• Move bulky or conditional detail into references/ (docs) or assets/ (templates), and always say when to open each file — never a vague "see references/".

Scripts (only when needed)

Add a script only for work that's deterministic, fragile, repetitive, or token-heavy (parsing, conversion, validation) — something reasoning alone can't do reliably. A script must be non-interactive (input via flags/stdin), print structured stdout with diagnostics on stderr, give helpful errors and meaningful exit codes, be idempotent, require --confirm for destructive actions, and pin versions. MUM's core is markdown; scripts are the exception, not the default.

Creating a skill

1. Interview the user one question at a time: trigger, steps, inputs/outputs, edge cases, and what "done" looks like. Ask the next question only after they answer.
2. Confirm it's warranted (fails the litmus test → say so and stop).
3. Pick the name, then write the description — spend the most effort here.
4. Scaffold .agents/skills/<name>/SKILL.md; add scripts//references//assets/ only if needed.
5. Sanity-check triggering: list a few prompts that SHOULD load it and a few near-miss ones that should NOT; adjust the description until both hold.
6. Commit to main.

Refining a skill

1. Read the SKILL.md; clarify what should change and why.
2. Make the smallest change that fully addresses it; keep the description accurate.
3. If you touched the description, re-run the triggering sanity-check. Commit to main.

Retiring a skill

1. Confirm the skill is no longer needed.
2. Delete its folder. Commit to main.

Rules

• Commit your changes to main (per the constitution).
• Never weaken privacy or other constitutional rules through a skill.
• Portability: target the open SKILL.md standard and avoid vendor-specific features so skills work across any agent. Note real environment needs in a compatibility field when relevant.