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skill-authoring — skills that fire, fit, and are proven

A skill is a guard on agent behavior, so fix-the-class applies: a skill you never watched fail-then-pass is not a guard. These rules sit on top of the docs skill (whole-document review, never lose a rule) — read that too.

1. The description is the trigger — and the listing is capped

How Claude Code loads skills (official docs, checked 2026-09-10):

• The model sees only each skill's name + description (+ when_to_use) until it invokes the skill. That combined text is truncated at 1,536 chars per skill in the listing (skillListingMaxDescChars changes the cap).
• The whole listing is budget-capped at 1% of the context window (skillListingBudgetFraction, or SLASH_COMMAND_TOOL_CHAR_BUDGET as a fixed char count, raises it). The listing always keeps every skill name; on overflow Claude Code drops descriptions starting with the skills invoked least, and a skillOverrides entry of "name-only" drops one on purpose. A skill without its description can still be invoked but is rarely chosen on its own — every character you add can push another skill's trigger out.
• Measured in the Claude Code 2.1.263 source, beyond the docs: 4 chars/token (8,000 chars at 200k, ~40,000 at 1M); every entry still costs its name + 2 chars; descriptions are admitted greedily by usage score (use count, 7-day half-life). So every duplicate entry (a synced copy listed as aidream:x) burns budget even with no description.
• Skills in .claude/skills of subdirectories below the launch dir load lazily, the first time a file there is read or edited. That is why repo-scoped duplicates (aidream:x, matrx-frontend:x) appear mid-session, each one another listing entry.
• disable-model-invocation: true removes the description from context entirely: the skill becomes user-invoked only (/name), and subagents cannot preload it either.

Rules:

• Recipe: <what it is — one noun phrase>. Use when <trigger>, <trigger>, … . Primary use case first. Target ≤300 chars, hard 500 — our rule, stricter than the docs, which set only the 1,536 cap and say to put the key use case first.
• Triggers are what the agent will actually see: situations, symptoms, file paths/globs, error strings, the user's words.
• Never the procedure. No "Covers / Encodes / Enforces / pipeline", no step lists — an agent handed the procedure in the description executes the summary and skips the body.
• No dates, rulings, approvals, or keyword dumps — those live in the body. Cutting a dump is a MOVE, not a deletion: every trigger you drop from the description gets a row in a relocation table in the body — old trigger → its new home — an index a repo grep still finds. That table is what "don't lose a trigger" actually asks for, and it is what you show the owner.
• At most one NOT for X (use Y), only when a sibling skill is a real near-miss.
• Slash-only skill (reached by /name and by nothing else) → disable-model-invocation: true. First grep -r "<name>" across all skills and agent definitions — no other skill or subagent can reach it afterwards.
• The lint enforces it: python3 common-docs/meta/scripts/skill_descriptions.py lint (--repo <dir>, --workspace) fails a description over 500 chars unless allowlisted and over 1,024 always; skill-description-allowlist.txt only shrinks. 1,024 is a hard ceiling, never a target — the house target is ≤300, and every proven-good rewrite on record landed at 276–300. Writing up to a cap is the failure this rule exists to stop. Wired into okf_lint.py, aidream scripts/check_skill_descriptions.py, and matrx-frontend pnpm check:skill-descriptions.
• A trigger that keeps missing in a footgun area → run the skill-creator plugin's description optimization loop (should-fire + near-miss queries), never lengthen by hand.

2. Size — the body routes over branches

• SKILL.md body ≤500 lines — the file itself, not the directory. Over budget is fixed by relocating, not by squeezing prose.
• Branch test: inline what every run needs; move what only some runs reach (one stage, one provider, one variant, a long API reference) into a sibling file named for the branch, with a pointer that says when to read it: Stage V only → read stage-v.md.
• A routing list is read as complete. A "read only the branch your run reaches" list names every companion, pre-existing ones included; an unlisted file is never opened (matrx-frontend surface-authoring evals E1: 0/3 reps opened the one reference the list omitted and lost its completion gate). Never end a route with "nothing else": pointers inside a branch file (e.g. "run the safe-cutover skill first") still bind, and 2/3 common-docs data-to-kinds reps skipped one citing those words.
• One level deep. SKILL.md → file, never file → file. A reference file over 100 lines opens with a contents list.
• Point at --help or a script instead of restating flags or deterministic steps.
• Splitting relocates; every rule survives. Canonical SKILL.md companion files ship with sync_skills.py automatically (in common-docs they carry OKF frontmatter).
• Every step ends on a checkable completion criterion — "every touched table returns canonical_certify_ok", never "understanding reached".

3. Match the form to the failure

Observed failure	Write	Never write
Knows the rule, skips it under pressure	Prohibition + named replacement + §4 counters	Soft advice ("prefer", "consider")
Complies, but output has the wrong shape	A recipe: what the output IS, its parts, in order	A list of don'ts — they get negotiated
Omits a required element	A REQUIRED slot in the template it fills	A prose reminder near the template
Behavior depends on a condition	A conditional on an observable predicate	A rule plus an exemption clause

No nuance clauses ("unless it matters") — they reopen the negotiation. A real exception is its own conditional.

4. Discipline skills — close the loopholes you saw

A discipline skill guards a rule that costs the agent time and that it already knows (verify live, never revert a gate, never silence a type error).

• ## Rationalizations — a two-column table, Excuse (verbatim) | Reality (one line). Rows only from observed transcripts or real incidents; an imagined row is bloat.
• ## Red flags — 3–7 thoughts that precede the violation ("this case is different", "the gate is blocking my release").
• Put the about-to-violate symptom in the description: Use when … or when tempted to ….

5. Prove it

Required for a new skill, an edit made because an agent misbehaved, any discipline skill, and any description rewrite. Exempt: typo, path, and pointer fixes.

1. Scenario from reality. Use the prompt or transcript that actually failed (incident, feedback item, review-queue row). Only with no incident, synthesize one with real repo paths, ≥2 real pressures (time, sunk cost, "the user said skip it", a blocking gate), and a forced choice.
2. RED — baseline. Dispatch fresh subagents (lane named; standard unless the consuming lane is known) with the scenario and without the new text (for an edit: the current skill). 3 reps. Record choices and rationalizations verbatim. No failure → nothing to fix; do not write the guidance.
3. GREEN. Write the minimum that answers the recorded failures. Same scenario, lane, 3 reps, with the skill. Pass = all 3 comply and cite the skill.
4. REFACTOR. A new rationalization → a §4 row + red flag → rerun. Reps that disagree with each other mean the wording is not binding — change the form (§3) before adding words.
5. Trigger check (description changed): 3 should-fire and 3 near-miss prompts to fresh subagents with the skill installed; record fire / no-fire.
6. Record the proof in <skill>/evals.md: scenario, lane, date, RED result, GREEN result, rationalizations harvested. It is the regression test — the next editor reruns it. The author never grades a run it performed itself. Every recorded rep cites its agent id or transcript path — a verdict nobody can re-check against the transcript is a claim, not proof; when a rep's transcript genuinely cannot be recovered, its row says "transcript not recoverable" rather than looking auditable. (Two records shipped without ids on 2026-09-10 and had to be reconstructed from runner transcripts two days later.)

Before you save — checklist

• Description follows §1: noun phrase + Use when, ≤300 chars (500 hard), no procedure, no dates; skill_descriptions.py lint passes.
• Slash-only? disable-model-invocation: true after the cross-skill grep.
• Body ≤500 lines; branch-only material disclosed one level deep with when-to-read pointers; any routing list names every companion file.
• Every rep in evals.md cites an agent id / transcript path, or says it is not recoverable.
• Split a SKILL.md? Every original line survives somewhere in the directory (re-grep or an oracle), an agent that did not split it verifies routing, and a §5 scenario run proves no behavior was lost.
• Guidance form matches the observed failure (§3).
• Discipline skill: rationalization rows and red flags come from observed failures.
• §5 run done (or exempt) and evals.md updated.
• docs skill §2 and §8 passed. Canonical SKILL.md? Edit the one under common-docs/skills/ — never a synced copy — run sync_skills.py, commit every touched repo.