D3.1’s CLAUDE.md is always-on context. This chapter is the other half of the instruction layer: capabilities the agent loads only when it needs them. A slash command is a stored prompt you trigger by typing /name; a skill is a richer, directory-bundled capability that Claude can also reach for on its own. The exam-relevant facts are that the two have converged, and that a skill’s lazy-loading — and its budget — are what keep it cheap.

Do I know this already? Diagnostic

Answer these confidently and you can skim ahead to Exam essentials; if any is shaky, read closely — each is developed below.

Where does a slash command have to appear in your message, and what becomes of the text after it?
True or false: a skill’s description is always loaded at session start.
How do you pass and reference arguments inside a skill body?
Roughly how much context does a skill cost at startup vs once invoked, and how long does the body persist?
Name the four skill scopes in precedence order.

Check your answers

At the start of the message — text that follows the command name is passed to it as arguments.
False — a disable-model-invocation: true skill carries no description in context at all, and even ordinary descriptions can be dropped under budget pressure (least-invoked first).
$ARGUMENTS expands to all arguments passed, and $ARGUMENTS[N] / $N pick a specific 0-indexed argument; if the body never references them, args are appended as ARGUMENTS: <value>.
A ~100-token description at startup (against a budget defaulting to 1% of the context window); the full body loads on invocation and stays for the rest of the session, carried through compaction within a budget (≤5,000 tokens each, 25,000 combined).
Enterprise > personal > project, plus plugin — namespaced as plugin-name:skill-name, so it never conflicts.

Commands and skills have merged

A slash command controls Claude Code from inside a session, and “a command is only recognized at the start of your message. Text that follows the command name is passed to it as arguments.” [Official] Commands · AnthropicT1-official original Alongside the many built-in commands, some entries are bundled skills: “they use the same mechanism as skills you write yourself: a prompt handed to Claude, which Claude can also invoke automatically when relevant. Everything else is a built-in command whose behavior is coded into the CLI.” [Official] Commands · AnthropicT1-official original

That same mechanism is why the two authoring formats have converged: “Custom commands have been merged into skills. A file at .claude/commands/deploy.md and a skill at .claude/skills/deploy/SKILL.md both create /deploy and work the same way. Your existing .claude/commands/ files keep working.” [Official] Extend Claude with skills · AnthropicT1-official original Old flat-file commands still run; skills are the recommended form for new work because they add directory bundling, frontmatter, and auto-invocation.

Skills are lazy-loaded directories

A skill is “a markdown directory: .claude/skills/<name>/SKILL.md,” with optional supporting files (reference.md, scripts/, and the like) alongside. [Official] Extend Claude with skills · AnthropicT1-official original The reason a skill is cheap is its loading model — unlike CLAUDE.md, which loads every session: “skills load on demand. The agent receives skill descriptions at startup and loads the full content when relevant.” [Official] Extend Claude with skills · AnthropicT1-official original Each description is roughly 100 tokens; the full body materializes only when the skill is invoked, “enters the conversation as a single message and stays there for the rest of the session,” and is not re-read on later turns. [Official] Extend Claude with skills · AnthropicT1-official original

The lazy-load lifecycle in full

The “cheap” story has a budget and a lifecycle the exam can probe at each stage:

Concept ·

Startup (budget). Skill descriptions load into a context budget that defaults to 1% of the model’s context window (skillListingBudgetFraction; per-description cap maxSkillDescriptionChars, default 1,536). On overflow, the least-invoked skills’ descriptions are dropped first — so a rarely-used skill can become invisible. /doctor reports whether the budget is overflowing. [Official] Extend Claude with skills · AnthropicT1-official original
Invocation (arguments). Inside the body, $ARGUMENTS expands to all arguments passed (if you never reference it, the args are appended as ARGUMENTS: <value>), and $ARGUMENTS[N] / $N pick a specific 0-indexed argument ($0 first, $1 second), with shell-style quoting. [Official] Extend Claude with skills · AnthropicT1-official original
Persistence (compaction). Once loaded the body stays for the session, and compaction carries it forward within a budget — the first 5,000 tokens of each most-recently-invoked skill, up to a combined 25,000 tokens post-compaction. [Official] Extend Claude with skills · AnthropicT1-official original

One operational nicety: live change detection — adding, editing, or removing a skill under ~/.claude/skills/, the project’s .claude/skills/, or an --add-dir directory takes effect within the current session, no restart — the one exception being a brand-new top-level skills/ directory that did not exist at launch. [Official] Extend Claude with skills · AnthropicT1-official original

A deploy skill through its lifecycle Worked example

# .claude/skills/deploy/SKILL.md
---
name: deploy
description: Deploy the app to an environment. Use when asked to deploy, ship, or release a build.
argument-hint: "[environment]"
---
Deploy to $0 by running the runbook in scripts/deploy.sh, then verify the health check…

Trace it: at startup, only the ~100-token description loads (counted against the 1%-of-context budget) — the runbook body does not. On /deploy staging, $0 (and $ARGUMENTS) expand to staging, and the full body enters the conversation as one message. It then persists for the session; if a compaction fires, the body is carried forward (up to 5,000 tokens of it, within the 25,000-token combined skill budget). Flip disable-model-invocation: true and the calculus changes: the description is no longer in context at startup at all, so Claude can’t auto-invoke on “ship it” — only an explicit /deploy loads it.

SKILL.md frontmatter and the four scopes

The SKILL.md frontmatter is where a skill declares its behavior. Among the fields: name (display name in skill listings, defaults to the directory name — the directory name, not this field, sets the /command you type, except for a plugin-root SKILL.md), description (drives auto-invocation; description + when_to_use capped at 1,536 chars by default), argument-hint, disable-model-invocation, user-invocable, allowed-tools (CLI-only), model, effort, context, and paths (glob patterns that limit when the skill activates). [Official] Extend Claude with skills · AnthropicT1-official original

Skills resolve across four scopes by precedence — “enterprise > personal > project; plugin skills use a plugin-name:skill-name namespace and never conflict”: Enterprise (managed settings, all users), Personal (~/.claude/skills/, all your projects), Project (.claude/skills/, auto-discovered from cwd up to repo root), and Plugin (namespaced). [Official] Extend Claude with skills · AnthropicT1-official original

Who can invoke it

Two frontmatter switches decide who may call a skill. user-invocable: false hides it from the / menu so only Claude can invoke it; disable-model-invocation: true does the inverse — only the user can trigger it via /, Claude cannot auto-invoke, its description is kept out of context, and it also blocks subagent preloading. [Official] Extend Claude with skills · AnthropicT1-official original Between them you can build a skill that is purely automatic, purely manual, or both.

Practice

Exercise solutions

Solution ↑ Exercise

B. A skill lazy-loads: only its ~100-token description sits in context at startup, and the full 2,000-token body loads on invocation — and Claude can auto-invoke it when a deploy is relevant. A is the D3.1 budget mistake: a CLAUDE.md is loaded every session, so the whole 2,000 tokens would be spent on every unrelated turn. C is wrong on the “only way” claim — commands have merged into skills, so .claude/commands/deploy.md and .claude/skills/deploy/SKILL.md both create /deploy; the skill form is recommended. D defeats the point of a reusable capability. The skill is the form that is both cheap (lazy) and discoverable (auto-invocable).

Solution ↑ Exercise

disable-model-invocation: true keeps the skill’s description out of startup context entirely — so unlike an ordinary skill (whose ~100-token description Claude sees and can match against), the model is never told this skill exists. That is exactly the point for a risky release action: it forces a human to type /force-release, because Claude cannot auto-invoke something it cannot see. The teammate got the intended safety behavior; “Claude doesn’t know it exists” is the feature, not a bug. (If they wanted Claude to know about it but still gate execution, that is a permission/ask concern, not this flag.)

Solution ↑ Exercise

(a) Only the ~100-token description enters context at session start, counted against the skill-listing budget (default 1% of the model’s context window); the ~3,000-token body does not load yet. (b) The body loads on invocation, enters as a single message, and stays for the rest of the session (it is not re-read each turn). If a compaction fires, the body is carried forward within a budget — up to the first 5,000 tokens of each most-recently-invoked skill, capped at a combined 25,000 tokens post-compaction.

Exam essentials

Commands merged into skills — .claude/commands/x.md and .claude/skills/x/SKILL.md both create /x; old commands keep working, skills are recommended (directory bundling, frontmatter, auto-invocation). A command is recognized only at message start; text after it is arguments.
Skills lazy-load on a budget — a ~100-token description at session start (budget defaults to 1% of the context window; on overflow least-invoked descriptions drop first; /doctor shows it), the full body on invocation; the body persists and compaction carries it forward (≤5,000 tokens each, 25,000 combined).
Arguments — $ARGUMENTS (all args), $ARGUMENTS[N] / $N (0-indexed) substitute into the body; absent references append ARGUMENTS: <value>.
The description is the retrieval interface — but not unconditional — disable-model-invocation: true keeps the description out of context (user-only via /), and budget overflow can drop one. user-invocable: false = Claude-only.
Four scopes — enterprise > personal > project; plugin skills are namespaced (plugin-name:skill-name) and never conflict. Live change detection: editing a skill takes effect mid-session without restart (except a brand-new top-level skills/ dir).

Commands and skills have merged

Skills are lazy-loaded directories

The lazy-load lifecycle in full

SKILL.md frontmatter and the four scopes

Who can invoke it

Practice

Exercise solutions

Exam essentials

Further reading