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 the docs site, 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. Two independent axes govern what a pull adds — conflating them is the most common way this section gets misapplied.
Axis 1 — coverage: how much of the answer's territory you write down. This is what "damped" governs. Picture the skill as an object resting in sand: a question pulls it toward a fuller answer, it moves damped (covers part of the conceptual ground, not all) and retains that position — the sand doesn't snap back. Repeated pulls settle the skill at the average of real demand instead of overfitting to one question. Default: a 100% pull moves ~20% — add only the one signpost that would have shortest-circuited the problem (the file/class/relationship to check first, plus a one-sentence shape of the answer), not the walkthrough or the second-order cases. A genuinely recurring question reaches full coverage over a few pulls; a one-off never bloats the skill past its signpost.
Why damped: chasing every specific detail into the skill bloats it, scatters its focus, and front-loads context future agents won't need — most questions are one-offs.
Axis 2 — prose density: always terse, at every coverage level. Independent of axis 1; never relaxes, not even for a full-coverage landmine. State each fact once — no restating the same point from a second angle. Skip examples unless the pattern truly can't be conveyed by naming the file/symbol. Omit narrated history (see "War stories" under Exclude) and anything a reader can derive from the code you're pointing at. A landmine written at full axis-1 coverage should still read as tight rule-plus-consequence, not a paragraph.
Example — same gotcha, axis 1 held constant, axis 2 fixed:
Bad (states the same thing twice): "Animation frames don't always drive the Sprite's color directly — some channels fall back to a default unless a frame explicitly sets them. Assuming every frame controls color can therefore misread a null channel as intentional, since it's silently skipped instead of applied."
Good (one pass, no restatement): "Sprite.UpdateToAnimationFrame applies color per-channel and skips any channel left null on the frame — check for nulls before assuming a frame drives full color."
Two exceptions to axis 1's damping — place these by hand, at full coverage, 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. Write the whole gotcha: what triggers it, what breaks, what to check. (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 frames interact with the Sprite" raises a question without reducing search cost; "see Sprite.UpdateToAnimationFrame — color is applied there, gated on null per-frame channels" 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.
- The docs site (docs.flatredball.com) — user-facing behavior, engine APIs, Glue reference, tutorials. If a topic has a docs page, link to it rather than restating it. (The docs are hosted GitBook, not checked into this repo.)
- Other skills — cross-reference instead of copying. When two skills cover a shallow-vs-deep split of the same topic, point between them rather than duplicating the overlap.
Process
- Read the relevant source files.
- Check docs.flatredball.com for an existing user-facing page on the topic — link it instead of restating.
- Skim a few existing skills in
.claude/skills/ to match style and depth.
- Write only the non-obvious distillation.
- Before saving, re-read every sentence you just added — against each other and against existing sections you didn't touch: does it restate a nearby sentence from a different angle, include an example that a file/symbol pointer would replace, or narrate history instead of stating a timeless rule? Cut what fails.
Skill File Rules
- Length: not a target — a byproduct. The damped-response process above is what keeps a skill short: most edits are ~20% pulls (a signpost, not a walkthrough), so length only grows where demand genuinely, repeatedly pulls. Don't pad toward a line count, and don't treat headroom under a number as license to add more than the current pull warrants. Hard ceiling 500 lines as a bloat backstop, not a goal.
- Naming: kebab-case noun phrases (e.g.,
sprite-animation, glue-codegen).
- 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.,
[advanced-topic.md](advanced-topic.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 FlatRedBall'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: FlatRedBall sprite texture-flip animation. Triggers: AnimationChain, AnimationFrame, .achx, Sprite.AnimationChains, UpdateToCurrentAnimationFrame.
Bad (boilerplate, padded):
description: Reference guide for FlatRedBall's sprite animation system. Load this when working on animation behavior, AnimationChains, AnimationFrame, .achx files, Sprite.AnimationChains, or UpdateToCurrentAnimationFrame.
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 on the docs site — link instead of restating.
- Full class outlines or property lists — read source directly.
- Code examples unless the snippet captures an irreplaceable pattern.
- In-flight migration / refactor STATE — what's done now, what currently blocks what, what's left, "X is already converted," "Y can't move until Z." This inverts to false the moment the work lands, turning the skill into an active liar that every future agent re-reads as fact. Skills hold timeless structure only. Transient progress belongs in the ephemeral working ledger, not the skill — and that includes versions and dates, any time-sensitive fact.
- War stories — "Issue #N: X happened" / "PR #N caught this" framing, even for a landmine that never expires. State the rule and the gotcha in pure present-tense, timeless form: what to check, what breaks if you don't, why. Never narrate it as an event that occurred on a numbered issue/PR — that framing is what keeps getting written despite this rule, because a durable fact wrapped in "Issue #N: ..." reads like it satisfies "state landmines fully," so the incident-number wrapper slips through. The test: could this sentence be true independent of which issue surfaced it? If removing "Issue #N:" and rephrasing as a bare rule loses no information, the issue number was never required — cut it. If you need a concrete illustration, name the pattern (a type, a method, a symbol) instead of the ticket.
- 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.
