skill-authoring

Authoring a skill for the FGDH catalog

Experimental. This skill has not been verified against a real task since its last change. Verify its output before relying on it.

Preconditions

This skill is pure convention. It needs no toolchain and makes no assumption about where it is running, so it works whether you are authoring inside the catalog repository or drafting a skill somewhere else.

Detect which of the two you are in, because it changes only the verification step:

Inside the catalog repository — scripts/build_index.py exists at the repository root. The mechanical gate is available; use it, and treat its exit code as the answer.
Anywhere else — no scripts/build_index.py. The checklist in Verification below is then the whole gate, and the skill is not finished until it has been opened as a pull request against the catalog and passed CI there. Say so explicitly rather than reporting the skill as done.

Do not attempt to install or fetch the validator when it is absent. Fall back to the checklist and state that you did.

Procedure

Decide the tier before writing anything. Classification has three outcomes and only two of them may be stored:

Classification	May assume	Stored as `fgdh.tier`
`universal`	nothing — conventions, checklists, review procedures	`universal`
`domain`	a toolchain exists; project layout is discovered, not assumed	`domain`
`project`	specific files at specific paths	never — the skill belongs in the consuming repository

A skill drafted as project can usually be lifted to domain by replacing each hard-coded path with a discovery step plus an explicit failure branch. That rewrite is the work. It is what makes the skill survive installation into a repository nobody anticipated. If it genuinely cannot be lifted, stop and say so — do not change the value to get past the validator.

Copy the template. templates/SKILL.md.template in the catalog repository. The directory name and the name: field must be identical; the specification requires it. Replace every <angle-bracket> placeholder and delete the instructional comment.
Write the description last, and write it as the triggering decision it is. An agent sees only name and description before deciding whether to load the skill. The body arrives after that decision, so nothing in the body can rescue a description that does not match how people actually ask.

Read the description policy before writing it. The short form: say what the skill does and the situations that should trigger it, use the vocabulary a user would type rather than the vocabulary you would choose, and end with a delimitation clause naming the neighbouring skill (Do not use for X; see <other-skill>.).

Under-triggering is the common failure mode. Be explicit rather than coy.
Fill the four mandatory sections in order: ## Preconditions, ## Procedure, ## Verification, ## Scope and delimitation. Prefer a command with an exit code over a judgement call at every step where one is possible.

4a. Measure on more than one instance before writing anything down as normative. A skill turns measurements into rules, and a measurement licenses a claim about the artefact it was taken on — nothing more. That cuts both ways: "X cannot be done here" must not become "X cannot be done", and "X has this shape here" must not become the shape every instance has. The positive direction is the dangerous one, because a false negative stops while a false shape keeps running and looks green.

So: name the class the claim is about, measure a **second** instance chosen as the one
most likely to differ (the irregular one, a controlled negative, or the whole set where it
is small), and record the sample — instances, versions, date, numbers — beside the claim.
Where only one instance is available, bound the claim to it and say the generalisation is
unverified. The rule, the three shipped defects it is made of, and the checklist are in
[the measurement rule](references/measurement-rule.md).

4b. State a dependency on another skill as a checked precondition, never as an install. allowed-tools grants permission; it does not express a dependency, and the two are routinely confused because a grant broad enough to run the skill's own tooling is usually broad enough to install something. If your procedure hands off to a sibling skill, detect whether it is present and, when it is not, emit the exact pinned install command and stop — the consumer installs it. Installing it yourself writes to the user's project as a side effect of an unrelated run, makes that run non-hermetic (the version that executes is not the version anybody reviewed or recorded), and contradicts the catalog's static-by-design stance: installation is an explicit, reviewed act, recorded in skills-lock.json.

Set the metadata. fgdh.tier, fgdh.domain and fgdh.owner are required; fgdh.language and fgdh.status default to en and stable. All values are strings — quote anything YAML would coerce, and quote "@handle" because @ is a reserved YAML indicator. The full registry with allowed values is in the frontmatter contract.
Set fgdh.status: "experimental" and open the body with the banner. A newly authored skill has not been exercised against a real task. The field alone is invisible to an agent, so the caution has to be mirrored where the agent will read it:
```
> **Experimental.** This skill has not been verified against a real task since its last change. Verify its output before relying on it.
```
Put no warning in the description. Caution must not suppress triggering: the skill should activate, and the agent should then be told to verify its output. That instruction only has to survive until activation, so the body is the right place and the description stays a clean matching surface.

Deprecation is the opposite case and the one exception — see the frontmatter contract.
Keep SKILL.md under 500 lines and the body under roughly 5000 tokens. The body is paid for on every activation; references/ is paid for only when actually needed. Move tables, mappings and long procedures into references/, one level deep, referenced by a path relative to the skill root.
Never write an absolute path, and never traverse upward with ... Not in a link, not in a command. Everything the skill needs lives inside the skill directory. This is the rule that decides whether the skill still works after installation, and it fails silently when broken.
Declare the language axes. Metadata is always English. The body may be another language when the artifacts the skill operates on are — declare that in fgdh.language, and put the foreign-language terms a user would actually type inside the English description instead of translating the description. If the required output language is not English, say so in the first sentence of ## Procedure; it does not follow from either of the other two axes.
Write the trigger prompts. Three to five realistic prompts that should activate the skill, phrased as a user would phrase them rather than as the description is phrased, plus two that should activate a neighbouring skill instead. Store them in references/triggers.md. They are the evidence required to promote the skill from experimental to stable, and the next annual re-validation reuses them.

Verification

Inside the catalog repository, in this order. The first two are the gate; the rest is what a reviewer will check anyway, so checking it yourself is cheaper.

python scripts/build_index.py          # regenerates the index and the catalog
python scripts/build_index.py --check  # exit 0 means the committed artefacts are in sync
python scripts/check_descriptions.py   # trigger-collision report

build_index.py reports all violations at once, each naming the file and the rule. Rule names are stable and greppable: tier/rejected, name/directory, path/absolute, path/traversal, path/depth, path/missing, mirror/experimental-banner, mirror/deprecated-description, mirror/stale-experimental. Fix the cause, not the check.

Commit skills/index.json and CATALOG.md in the same change as the skill. CI regenerates both and fails if the working tree moves, so a skill whose generated artefacts were not refreshed will not merge.

Outside the catalog repository, or if the validator is unavailable, confirm by hand:

directory name equals name; lowercase, digits and single hyphens only, ≤ 64 chars
description is 1–1024 characters, states what and when, ends with a delimitation clause
fgdh.tier is universal or domain — never project
fgdh.domain and fgdh.owner are set; fgdh.owner is quoted and starts with @
status is experimental and the body opens with the banner; the description does not mention it
all four mandatory sections are present, in order
every normative claim about a class of artefacts was measured on more than one instance, or is explicitly bounded to the single one it was measured on; the sample is recorded beside the claim — see the measurement rule
a dependency on another skill is a detected precondition that emits the pinned install command, not an install the skill performs
SKILL.md is under 500 lines
no absolute path, no upward .. traversal, every relative reference exists and is one level deep
bundled scripts are referenced relatively but invoked through a resolved $SKILL_DIR
if allowed-tools is declared, every command the body instructs running — bundled scripts included — is covered by a grant (a bash script needs Bash(bash:*) + a bash … invocation, not Bash(python3:*)); see the frontmatter contract
references/triggers.md exists

Scope and delimitation

This skill covers writing the artefact: the frontmatter contract, the tier decision, the description, the body structure, and the mechanical checks that gate it.

It deliberately does not cover:

Whether a skill should exist, who owns it, how it is reviewed, when it is re-validated, and how it is deprecated and removed. That is governance, and the catalog's docs/lifecycle.md is the normative answer.
Release mechanics — which change is a MAJOR, MINOR or PATCH, and how a release is cut. See the catalog's CONTRIBUTING.md.
Any domain content. This skill tells you how to shape a skill, never what to put in one. If you do not have source material for a domain, the correct output is a stub with TODO(owner): markers, not plausible-sounding instructions.

If a skill of the same name is provided both by this catalog and locally, the local one wins: precedence between server-provided and locally-defined skills is unspecified upstream, so it is stated here rather than left to the host.

About this skill

Preconditions

Procedure

Verification

Scope and delimitation