Capability: skill package

The artifact is a whole skill directory — SKILL.md plus references/, scripts/ and assets/ — and all of it is editable: materialize() exposes every file as a component, apply() can rewrite or CREATE one (a new bundled script included), and validate() checks the result against the skill-creator authoring rules (first-party sources in references/concepts.md).

What you can change (highest leverage first)

Pick the lever that fixes the biggest failure cluster; depth is in the references.

1. The description / trigger — the only text loaded before the skill fires, so the single highest-leverage edit. Third person; state what it does AND when to use it; use the keywords a user would actually say. Lean slightly pushy for under-trigger, tighten the boundary and name near-miss cases for over-trigger, and front-load the key use case (hosts truncate the listing — 1,536 chars on Claude Code by default). Ex: "Formats data" → "Exports records to CSV. Use when the user asks to export or download a table." Playbook + the measurable loop: references/description-optimization.md.
2. A skipped step → a bundled script (the determinism lever). Prose is only likely to be followed; code that runs is repeatable. When the traces show the agent skipping a step, re-deriving the same helper, or doing a deterministic transform by hand, write it into scripts/ and make the body invoke it by command line. Write real, working code — never ... or a docstring-only stub — give it a --self-check entry point (validate() runs it, so a broken script is caught before any rollout is paid for), and say execute, don't read: a script's source never enters the agent's context, only its output. Also the cost/latency lever here (#684): when the agent re-invokes the same tool/step N times across a multi-step sequence (agent-optimize's detect_loop_patterns.py names the exact tool + repeat count from the champion's rollouts), bundle the WHOLE sequence into one script entrypoint instead of N separate agent-invoked steps — one call replaces N round-trips.
3. The body — improve clarity and altitude, delete dead weight, fix the instruction the agent misreads. The body loads on every trigger and stays in context all session — a recurring cost — so keep it ≤500 lines (enforced), imperative, and explain a rule's why briefly instead of piling on ALL-CAPS MUSTs.
4. References — move mutually-exclusive or rarely-co-used detail into references/*.md. Keep them one level deep (a ref must not point at another ref — the agent may read only part of it), link each directly from SKILL.md with a pointer saying what it holds and when to load it, and give a long ref (>300 lines) a table of contents at the very top, above any orientation prose — the check is positional because a TOC the head-reader never reaches is not a TOC. Multiple variants/domains → one ref per variant (references/aws.md, gcp.md, …) plus a selection body, so only one is read.
5. Assets — assets/ holds files the skill emits (templates, icons, fonts), not context the agent reads. Edit one only when the skill's output depends on it.

Every edit must leave a valid skill. validate() fails a candidate on: no SKILL.md; name missing/>64 chars/not [a-z0-9-]/containing an XML tag; description empty/>1024 chars/containing an XML tag; a body over 500 lines; a broken references|scripts|assets/… link; a bundled script that does not compile or whose --self-check fails. It warns on the softer authoring smells (POV drift, ALL-CAPS in the description, orphan or nested references, a missing TOC, a stub script, a script with no self-check, network/subprocess use in new code). A skill is executable context: keep bundled code auditable and free of surprises.

Adapting to the reader's capability tier

Scale body density to WHO follows it at runtime (see the THE READER block in your instructions, if present). A mid/weak reader needs more worked steps, explicit ordering, and examples in the body — and benefits most from lever 2, since code it executes cannot be skipped the way a rule can. A frontier reader follows a compact, principle-first body and is slowed by over-specification. The tier changes how explicit the retained body is, not how long it may be.

Trigger rate is a second objective

Task reward is the gate signal, and cap-evolve owns that machinery — do not add a private eval loop here. But triggering is invisible to task reward when the skill never fires, so measure it separately with scripts/trigger_eval.py on a held-out set of should-trigger / should-NOT-trigger prompts (with near-miss negatives) and keep the description that wins on the held-out half.

How to run

python scripts/check.py                            # self-test (must pass)
python scripts/run.py --path <skill_dir>           # candidate + validity report
python scripts/token_report.py --path <skill_dir>  # budget + script inventory
python scripts/trigger_eval.py --eval-set <json> --skill <dir> --judge-cmd '<cmd>'

Handlers in scripts/abstract.py: materialize(dir) → every file as a component · apply(dir, edits) → {changed, refused}, contained to the package and gated by the action policy (policy.json: frontmatter|body|reference|script|asset|add|remove, so a run can allow prose but forbid new code) · validate(dir) → {ok, problems, warnings, scripts}.

References

• references/concepts.md — the authoring model and the validity rules, with first-party sources. Load for grounding.
• references/description-optimization.md — the trigger-tuning playbook. Load when fixing under/over-trigger.
• references/anti-patterns.md — skill smells and the why. Load when a draft "feels off" or to review an edit.