---
name: write-skill
preamble-tier: 2
version: 0.1.0
description: |
  Write a new agent skill or improve one without bloat: the edit ladder,
  levels, drift and size checks, full diets. Use when asked to "write a skill",
  "improve this skill", "add this lesson to the <name> skill" or "put this
  skill on a diet". (gstack)
codex_default_prompt: "Use $gstack-write-skill to change this skill the lean way: fix the tool first, edit the line that already covers the idea, and record the size before and after."
triggers:
  - write a skill
  - improve this skill
  - add this lesson to the skill
  - put this skill on a diet
allowed-tools:
  - Bash
  - Read
  - Write
  - Edit
  - Grep
  - Glob
  - AskUserQuestion
---

{{PREAMBLE}}

# /write-skill: write and improve skills, lean

## 1. Find the skill

Run `~/.claude/skills/gstack/bin/gstack-skill-bill <name|folder>`. It resolves exact names only and prints the
skill's kind and its size per level. A new skill: make the folder first (in a gstack checkout, follow CONTRIBUTING.md
"Adding a new skill"), then write every line by sections 2 and 3.
- `user`: edit it in place.
- `gstack-install`: never edit; gstack-upgrade overwrites it. Edit `<skill>/SKILL.md.tmpl` in a gstack checkout, or
  copy the skill to a folder the user owns.
- `gstack-checkout`: edit `SKILL.md.tmpl`, never `SKILL.md`, then `bun run gen:skill-docs`.

## 2. Levels

Put each fact on the lowest level that still reaches the agent before it needs it.

| Level | Holds | Loaded |
|---|---|---|
| description | one trigger per case | every session |
| SKILL.md | what every task needs | every use |
| `references/` | what some tasks need, behind a line saying when to read it | that case |
| the tool's `--help` | flags, values, examples | when the agent picks the command |
| the tool's replies, errors, tests | the right name, the fix, the next step | when a guess is wrong |

## 3. Edit ladder

Stop at the first step that handles the change.
1. **Fix it below the skill.** A clearer error, a flag, a check or a test in the tool. Prose is for judgment calls.
2. **Check the model needs it.** If a fresh session does the right thing without the line, add nothing.
3. **Edit, don't append.** Change the line that already covers the idea; each meaning lives in one place.
4. **Pick its level** (section 2).
5. **Rule, not story.** One positive rule under a short name; the incident goes in the commit message.

Keep two or three safety lines inline for each risky area: writes, data leaving the machine, paid spend.
When code changes a name or flag a skill teaches, update the skill in the same change.

## 4. Check and record

- Run `~/.claude/skills/gstack/bin/gstack-skill-bill` before and after, and put `Skill-Size: <before>→<after> bytes (<file>)` in the commit
  message. DUE means it grew over 10% since its last `Skill-Diet:` commit: diet it instead of adding.
- In a gstack checkout: `bun test test/skill-bin-drift.test.ts test/parity-suite.test.ts` and the budgets in
  CONTRIBUTING.md. A red context-budget ratchet means diet, not a higher ceiling.
- Rerun the case that prompted the change in a fresh session.

## Look up the rest

- Full diet of a large, bloated or drifted skill → read `references/diet.md` in this skill's folder.