Skill Creator

Skill Creator

Guidance for creating effective skills, and for improving them by reflecting on what you wrote.

This skill is deliberately harness-agnostic: it relies on no specific runtime, CLI, or vendor tooling. The optional helper scripts in scripts/ use only the Python standard library, and every step has a plain "do it by hand" fallback, so any agent in any harness can follow it.

About skills

A skill is a modular, self-contained folder that extends an agent's capabilities with specialized knowledge, workflows, or tools — an "onboarding guide" for a task or domain that turns a general-purpose agent into one equipped with procedural knowledge no model fully holds.

A skill typically provides one or more of: a multi-step workflow, instructions for a specific file format or API, domain knowledge (schemas, policies, conventions), or bundled resources (scripts, references, assets) that save the agent from reinventing them each time.

Core principles

Concise is key

The context window is a shared resource: a skill competes for it with the system prompt, the conversation, other skills' metadata, and the actual request. Assume the agent is already capable — add only what it doesn't already know. Challenge every line: does the agent really need this, and does it justify its token cost? Prefer a short concrete example over a paragraph of explanation.

Set appropriate degrees of freedom

Match how prescriptive you are to how fragile the task is. Think of the agent walking a path:

• High freedom (prose guidance) — when many approaches work and the right choice depends on context. Describe the goal and the heuristics, not exact steps.
• Medium freedom (pseudocode / parameterized scripts) — when a preferred pattern exists but some variation is fine.
• Low freedom (specific scripts, few parameters) — when the operation is fragile, error-prone, or must be done one exact way. Give guardrails.

A narrow bridge with cliffs needs rails; an open field doesn't. The clearest low-freedom case is fragile numeric work — formulas, unit conversions, aggregations — which should live in a bundled scripts/ file rather than trusting the model's in-head arithmetic.

Writing style

Explain the why, not just the what. Today's models have good theory of mind: when they understand why a step matters, they handle the cases your instructions didn't enumerate. If you catch yourself writing ALWAYS / NEVER in caps or building rigid scaffolding, treat it as a yellow flag — usually you can reframe it as a short explanation of the underlying reason, which is more robust and more humane to the reader.

Write for reuse, not for one example. A skill that only works on the cases you tested is nearly useless; aim for a general procedure. Draft it, then re-read it with fresh eyes and improve.

Phrase instructions in the imperative/infinitive, addressed to the agent that will run the skill ("Extract the table…", "To rotate a page, …") — not as narration about a user.

Communicating with the user

Skill authors range from non-technical to expert. Read context cues and calibrate: terms like "workflow" or "example" are safe; terms like "frontmatter", "schema", or "assertion" may need a one-line gloss unless the user has signaled familiarity. When in doubt, briefly define a term.

Anatomy of a skill

skill-name/
├── SKILL.md            (required)
│   ├── YAML frontmatter   (name, description — required)
│   └── Markdown body      (instructions)
└── (optional)
    ├── scripts/        executable code for deterministic/repeated work
    ├── references/     docs loaded into context as needed (schemas, API docs, domain notes)
    └── assets/         files used in the produced output (templates, images, fonts, boilerplate)

Progressive disclosure — skills load in three levels, so design for the cheapest one that works:

1. Metadata (name + description) — always in context. This is the trigger.
2. SKILL.md body — loaded only once the skill triggers. Keep it lean (aim under ~500 lines).
3. Bundled resources — loaded or executed only when a step needs them (effectively unlimited; scripts can run without being read into context).

Keep the core workflow and selection guidance in SKILL.md; push variant-specific detail into references/, and link to those files with a clear note on when to read each. Keep references one level deep (link them directly from SKILL.md), and give any file over ~100 lines a short table of contents at the top. When a skill spans several domains or variants, organize references by variant (e.g. references/aws.md, references/gcp.md) so the agent reads only the relevant one. Give each fact exactly one home — don't repeat content in both SKILL.md and a reference file, or the two copies drift apart (prefer the reference for anything detailed).

The workflow

Follow these in order, skipping a step only when there's a clear reason it doesn't apply.

1. Understand the skill with concrete examples

Get a clear picture of how the skill will actually be used before writing anything. Gather (or propose, then confirm) concrete example requests: "What should this enable? Can you give a few examples of what a user would say to trigger it? What's the expected output?" Ask the most important questions first rather than overwhelming the user. Conclude once the intended functionality is clear.

2. Discover existing skills — update over create

Before scaffolding anything new, check whether a similar skill already exists. A near-duplicate is harmful: when two skills cover the same ground, the agent can't tell which to invoke (triggering gets diluted), and the same knowledge drifts out of sync across both.

Run the discovery helper against the directory where skills live:

python scripts/discover_skills.py "<proposed name or one-line intent>" --skills-dir <skills-dir>

(Fallback without scripts: list the skills directory and skim each SKILL.md's name and description.)

• If a listed skill is the same capability, update it in place — keep its name and folder, extend its SKILL.md / resources to cover the new need, and add the new need's trigger phrases to the description (broaden it, never narrow): a consolidated skill must still trigger on everything its merged parts would have, or the consolidation silently loses coverage. Then continue to step 5.
• If matches are merely adjacent (related but a genuinely different capability), proceed to create a new one.

Use judgment: the score is only a hint. "Same capability, broader scope" → update. "Shares keywords but a different job" → new skill.

3. Plan reusable contents

For each concrete example, ask: how would the agent do this from scratch, and what would help if it had to do it repeatedly? Bundle resources generously — a skill that adds only prose often fails to beat the baseline. This surfaces:

• scripts/ — executable code for anything fragile, multi-step, or done one exact way. The highest-value resource, and the most under-used. In particular, work that has to be exact belongs in a script, not in prose — a calculation or formula, a unit/format/date conversion, a multi-step parse or aggregation: anything where a model doing it in its head drifts or silently picks the wrong convention. A script does it the same way every time. Write it to take the inputs and return the result (e.g. scripts/compute.py --inputs ...), and test it on a worked example with a known answer before shipping.
• references/ — the exact specs the agent (and any script) must agree on: the precise definition or convention a task hinges on (which formula variant, rounding rule, boundary, or edge-case handling), schemas, domain rules. Keep the authoritative version here, not paraphrased in prose.
• assets/ — templates, boilerplate, images, fonts used verbatim in the output.

Handle conventions explicitly. Many exact operations have more than one accepted convention — a formula variant, a rounding or tie-breaking rule, a sort order, a date/boundary convention — and a request often names the one it wants. Don't hardcode a single convention: make it a parameter the script accepts (covering the common variants), catalog the variants in references/ with how to recognize which one a request is asking for, and have the SKILL.md detect the requested convention and pass it to the script. Silently assuming one convention is a leading cause of close-but-wrong answers — the script runs fine, but computes the wrong variant.

If a skill performs an exact operation (a calculation, a transform, a formatted output) and ships no script for it, treat it as a yellow flag — the agent will re-derive it, and can mis-derive it, every run.

4. Scaffold (new skills only)

Create the folder and a SKILL.md skeleton:

python scripts/scaffold_skill.py <skill-name> --path <dir> [--resources scripts,references,assets]

(Fallback without scripts: create the folder and a SKILL.md with name + description frontmatter directly — that's all the scaffolder does.)

5. Write the SKILL.md and resources

Build the resources first (and test any scripts by actually running them — a representative sample is enough when there are many similar ones), then write the body that ties them together. Remember you're writing for another agent instance: include the non-obvious procedural knowledge, and skip what a capable agent already knows.

Frontmatter (name, description):

• name — hyphen-case, lowercase letters/digits/hyphens, ≤64 chars; name the folder to match. Prefer a short, verb-led phrase that names the action (e.g. rotate-pdf, summarize-thread), and namespace by tool when it sharpens triggering (e.g. gh-address-comments).
• description — the primary trigger, and the only thing the model sees when deciding whether to invoke the skill. It is matched against the user's words, not read for understanding, so it must contain the literal phrases a matching request would use. Put every "when to use" cue here (the body loads only after triggering). Models tend to under-trigger, so be pushy and concrete:
  • Enumerate the literal trigger phrases — don't characterize abstractly. List (ideally quoted) the exact terms, operations, file types, and identifiers a matching task would contain. For a PDF skill, write "Use when a request says 'merge PDFs', 'split a PDF', 'rotate pages', or 'extract text from a PDF'" — NOT "use for PDF manipulation tasks". Both are accurate, but only the first matches the words a user actually types; abstract framing badly under-triggers and is the most common reason a skill silently never fires.
  • Cover every capability the skill has. If it does N things (especially after consolidating several into one skill), name trigger phrases for all N — a description that mentions only the first capability triggers only for the first.
  • Add a short negative boundary to stay precise — a "Do NOT use for ..." naming adjacent cases that should not fire (e.g. simple lookups). Descriptions are the one place to set aside "be concise / explain the why": completeness of concrete trigger cues beats elegance.

Body — instructions for using the skill, written per the Core principles above.

6. Validate

python scripts/validate_skill.py <path/to/skill-folder>

It checks the frontmatter format, required fields, and naming rules. (Fallback without scripts: confirm by hand that SKILL.md opens with a --- block, and that name is hyphen-case ≤64 chars and description is ≤1024 chars with no angle brackets.) Fix anything it reports.

7. Self-reflect and optimize

This is the step that turns a first draft into a good skill, and it replaces any external "optimization loop": do it inline, yourself — no subagents, no separate processes, nothing a harness might skip. After drafting, re-read your own skill with fresh eyes and the deliberate mindset of a skeptical reviewer who has never seen it, then revise. Work through these lenses, and explain to yourself why each change helps rather than rubber-stamping:

1. Triggering (fresh eyes) — usually the highest-leverage fix. Reading only the description, write down 3 realistic requests that should invoke this skill and 2 near-misses that should not. Then check two things: (a) does the description contain the literal words those should-trigger requests use, not just an abstract description of them? If a request says "rotate the PDF 90 degrees" but the description only says "document tasks", it will miss — add the exact phrase. (b) Does it name trigger cues for every capability in the body? Walk the body's sections and confirm each has a matching cue in the description. Rewrite any abstract framing into concrete, quoted enumerations, and add/tighten a "Do NOT use for ..." line if a near-miss would fire. Re-check.
2. Self-containment. Read the body as a fresh agent with no prior context. Is every instruction actionable? Any unexplained assumption, dangling reference to a file that isn't there, or step where you'd stall? Fix it.
3. Concision and altitude. Cut anything a capable agent already knows or that doesn't earn its tokens. Replace any all-caps MUST/NEVER with a short explanation of the underlying reason. Delete rules that only make sense for the one example you had in mind.
4. Progressive disclosure. Is SKILL.md carrying detail that belongs in a references/ file? Move it, and leave a clear pointer about when to read it. Is the body over ~500 lines?
5. Degrees of freedom. Does the specificity match each task's fragility — guardrails where the operation is fragile, room to maneuver where many paths are valid?
6. Dry run — and for anything numeric, actually compute it. Mentally execute the skill end-to-end on one realistic request; note wherever it stalls, loops, or produces the wrong shape of output. If the skill computes a number, don't eyeball the prose math — run its bundled script (or the formula) on a worked example with known inputs and confirm the result is exactly right. Prose math that "looks correct" is the most common silent failure; a skill that describes a multi-step calculation without a script is the signal to write one and verify it here.

Apply the revisions. If the pass turned up substantial issues, do it once more.

8. Iterate on real usage

Use the skill on real tasks and watch where it struggles or wastes effort. If several runs all independently write the same helper script or take the same multi-step detour, that's a strong signal to bundle that script (or fold that guidance) into the skill so future runs don't reinvent it. Update SKILL.md or its resources and repeat.

What not to include

A skill should contain only what an agent needs to do the job. Don't add auxiliary docs about the skill's own creation — no README.md, INSTALLATION.md, CHANGELOG.md, or notes on your process and testing. They add clutter and confusion without helping the agent execute.

Principle of no surprise

Skills must not contain malware, exploit code, or anything that could compromise security, and a skill's behavior should not surprise the user given its description. Don't build skills designed to mislead or to enable unauthorized access or data exfiltration. (Benign role-play or persona skills are fine.)