skills-writer - SKILL.md Agent Skill

name: skills-writer description: Creates and updates skill files (.claude/skills/*/SKILL.md). Triggers: creating/updating a skill, documenting a subsystem for agent context.

Skills Writer

Mental Model

A skill is a map and a list of landmines, not an encyclopedia. It points an agent at the right code and docs and warns about what isn't obvious from reading them. If a fact already lives in source or docs/, link, don't restate.

A good skill answers three things and stops: where the relevant code/docs live, what gotchas aren't obvious from reading them, and what patterns recur. Default to prose-free pointers and tables; include code only when the snippet is a pattern that can't be conveyed by pointing at a file. Every line is re-read into context on every load, so a skill that says less but points accurately beats a thorough one.

Growing a Skill — Damped Response

A skill is rarely written whole; it grows as questions pull on it. Don't answer a question 100% inside the skill. Treat demand as an elastic pull and the skill as an object resting in sand: a question pulls toward a fuller answer, the skill responds damped (moves part-way, not all the way), and retains its new position — the sand means it doesn't snap back. Over many questions this settles the skill at the best average of real demand without overfitting to any one question. It's a leaky integrator of demand, not a transcript of the last conversation.

Default: a 100% pull moves ~20%. When a question could be answered in full inside the skill, add only its broad orienting fifth — a concrete signpost plus a one-sentence shape of the answer — not the whole walkthrough. Because the position is retained, a genuinely recurring question reaches full coverage in a few pulls, while a one-off never bloats the skill past its signpost. This is also why creating a thin, broad skill is cheap and reversible: easy to add, easy to delete if demand never returns.

Why damped: chasing every specific detail down into the skill bloats it, scatters its focus, rots, and front-loads context future agents won't need. Most questions are one-offs. Let the skill grow only where demand actually, repeatedly pulls.

Two exceptions — place these by hand, at full strength, not through the elastic:

Landmines. A non-obvious, expensive-to-rediscover gotcha that isn't evident from the source you point at is a sharp fact, not a sample to be averaged. Damping it smears a precise truth into a vague gesture — the worst outcome. State it fully and firmly. (This is the "list of landmines" half of the mental model above.)
Bimodal pull. When a skill is dragged toward a low-density middle between two genuinely distinct sub-topics, don't settle in the valley — it serves neither. Split into two skills, each with its own focus. A pull toward the empty middle is the signal to fission.

Signpost quality bar. A nudge must name where to look — a file, class, or relationship — not merely assert that something exists. "Animation events interact with children" raises a question without reducing search cost; "see event dispatch in X.cs — children suppress Y because Z" reduces it. A vague signpost is worse than none: it costs context and resolves nothing.

Authoritative Sources (do not duplicate)

Before writing anything, identify where the ground truth already lives:

Source code — class outlines, property lists, method signatures, call sites.
docs/ GitBook tree — user-facing behavior, layout rules, control APIs, tutorials. If a topic has a docs page, link to it.
Other skills — cross-reference instead of copying. (gum-layout and gum-layout-engine, for example, deliberately split shallow vs. deep.)

Process

Read the relevant source files.
Check docs/SUMMARY.md for existing user-facing pages on the topic.
Skim a few existing skills in .claude/skills/ to match style and depth.
Write only the non-obvious distillation.

Skill File Rules

Length: aim under 100 lines. Hard ceiling 500. Bloat costs agent context on every load.
Naming: kebab-case noun phrases (e.g., gum-tool-undo).
Frontmatter: name and description. The description is loaded into every session's skill listing — it pays for itself in context tokens forever. Keep it brutally short. See "Writing the description" below.
Structure: ## sections. Tables for file maps. Prose for relationships and gotchas.
Progressive disclosure: keep SKILL.md to high-level architecture; spill advanced content into sibling files (e.g., [xnafiddle.md](xnafiddle.md)) only when it's bulky enough to justify a second file.

Writing the description

The description's only job is to tell future-Claude when this skill is relevant. It is a trigger, not a summary.

Hard rules:

One sentence. Under ~250 chars. Ideally under 200. The skill body covers the rest.
Drop boilerplate. No "Reference guide for…", no "Load this when working on…", no "Covers Gum's…". The fact that this is a skill is implicit — these phrases are dead weight on every entry.
Lead with the topic, then trigger identifiers. Format: <Topic> — <one-line hook>. Triggers: <distinctive identifiers, file paths, or scenarios>.
Pick the 3–8 most distinctive triggers, not all of them. Generic words ("file", "system", "behavior") don't help; specific class names, file paths, and method names do. The rest belong inside the file.
No multi-line YAML (description: >). Keep it on one line. It folds anyway, and one line is easier to scan when auditing.

Example. Same triggers, ~40% fewer tokens:

Good:

description: Gum's undo/redo. Triggers: History tab, UndoManager, UndoPlugin, UndoSnapshot, stale references after undo.

Bad (boilerplate, padded):

description: Reference guide for Gum's undo/redo system. Load this when working on undo/redo behavior, the History tab, UndoManager, UndoPlugin, UndoSnapshot, or stale reference issues after undo.

Include

Architecture: how major pieces fit together and why.
Gotchas: surprising behavior, ordering dependencies, naming mismatches, "looks like X but actually Y."
Key file map: one-line table of file → purpose.
Pointers: links to relevant docs/ pages, key source files, and related skills.
Specific identifiers only when the name itself is misleading or the behavior is surprising.

Exclude

Anything already in docs/ — link instead of restating.
Full class outlines or property lists — read source directly.
Code examples unless the snippet captures an irreplaceable pattern.
Time-sensitive info (versions, dates, migration notes).
Anything Claude already knows from general C# or .NET knowledge.

Output

Write to .claude/skills/<skill-name>/SKILL.md. Create the directory if needed. Add sibling detail files only when content is too large for the main file.