Skills

s01 → s02 → s03 → s04 → s05 → s06 → s07 → s08 → s09 → ... → s20

"Load when needed, don't stuff the prompt" — Inject via tool_result, not system prompt.

Harness Layer: Knowledge — load on demand, don't fill the context.

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The Problem

Your project has a React component spec, a SQL style guide, and an API design doc. You want the Agent to follow these specs automatically. The most straightforward idea — stuff them all into the system prompt:

SYSTEM = (
    f"You are a coding agent. "
    + open("docs/react-style.md").read()       # 2000 lines
    + open("docs/sql-style.md").read()         # 1500 lines
    + open("docs/api-design.md").read()        # 3000 lines
)

6500 lines of system prompt. The Agent carries these docs on every LLM call — whether it's changing a CSS color or fixing a SQL query. 99% of the content is irrelevant to the current task, burning tokens for nothing.

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The Solution

The minimal hook structure, todo_write, and sub-Agent from the previous chapter are preserved. This chapter focuses on the new load_skill tool. At startup, inject the skill catalog into the SYSTEM prompt; at runtime, register one more tool to load full content, spending tokens only when used.

Two-level design:

The dispatch mechanism is unchanged, load_skill auto-dispatches via TOOL_HANDLERS[block.name].

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How It Works

skills/ directory, one subdirectory per skill, each containing a SKILL.md file:

skills/
  agent-builder/SKILL.md
  code-review/SKILL.md
  mcp-builder/SKILL.md
  pdf/SKILL.md

Level 1: Inject catalog at startup: the harness calls _scan_skills() at startup to scan the skills/ directory, parsing each SKILL.md's YAML frontmatter (name, description) into a SKILL_REGISTRY dictionary. list_skills() generates the catalog from the registry, injected into the SYSTEM prompt. The Agent sees "which skills I have available" every turn, with no extra API calls:

SKILL_REGISTRY: dict[str, dict] = {}

def _scan_skills():
    if not SKILLS_DIR.exists():
        return
    for d in sorted(SKILLS_DIR.iterdir()):
        if not d.is_dir():
            continue
        manifest = d / "SKILL.md"
        if manifest.exists():
            raw = manifest.read_text()
            meta, body = _parse_frontmatter(raw)
            name = meta.get("name", d.name)
            desc = meta.get("description", raw.split("\n")[0].lstrip("#").strip())
            SKILL_REGISTRY[name] = {"name": name, "description": desc, "content": raw}

_scan_skills()  # runs once at startup

def list_skills() -> str:
    return "\n".join(f"- **{s['name']}**: {s['description']}" for s in SKILL_REGISTRY.values())

def build_system() -> str:
    catalog = list_skills()
    return (
        f"You are a coding agent at {WORKDIR}. "
        f"Skills available:\n{catalog}\n"
        "Use load_skill to get full details when needed."
    )

SYSTEM = build_system()

Level 2: load_skill: the Agent decides "I need the SQL style guide" and calls load_skill("sql-style"). Lookup goes through the registry, not file paths, eliminating path traversal risk. The SKILL.md content is injected via tool_result, and can include later access to referenced references/, scripts/, or assets/ through the existing file and bash tools.

def load_skill(name: str) -> str:
    skill = SKILL_REGISTRY.get(name)
    if not skill:
        return f"Skill not found: {name}"
    return skill["content"]

The key distinction: skill content is not part of the system prompt. It enters the current messages as a tool result. Subsequent calls carry it along with the history until context compaction, truncation, or session end. This naturally connects to s08's compact: on-demand loading solves "don't carry what you shouldn't", compact solves "how to drop what you should."

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Changes from s06

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Try It

cd learn-claude-code
python s07_skill_loading/code.py

Try these prompts:

1. What skills are available?
2. Load the code-review skill and follow its instructions
3. I need to do a code review -- load the relevant skill first

What to watch for: Does the Agent know available skills from the SYSTEM catalog? Does [HOOK] load_skill appear when full instructions are needed? Does the answer use the loaded skill's instructions?

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What's Next

On-demand loading solved "don't carry what you shouldn't." But another problem looms: after the Agent works for 30 minutes, the messages list fills up with intermediate process. Old tool_results, stale file contents, occupying context but adding no value.

→ s08 Context Compact: A four-layer compaction strategy. Cheap layers run first, expensive layers run last.

Dive into CC Source Code

The following is based on analysis of CC source code loadSkillsDir.ts, SkillTool.ts, bundledSkills.ts, commands.ts.

1. Skill Sources: Not Just One skills/ Directory

The teaching version assumes all skills live in a skills/ directory. CC loads from multiple sources spread across multiple files: loadSkillsDir.ts handles user/project/--add-dir directories and legacy commands (.claude/commands/); bundledSkills.ts handles built-in skills; SkillTool.ts handles MCP remote skills; commands.ts handles command aggregation. Types include managed/policy skills, user skills (~/.claude/skills/), project skills (.claude/skills/), --add-dir skills, legacy commands, dynamic skills, conditional skills (with paths frontmatter, activated by file path), bundled skills, plugin skills, MCP skills.

2. SKILL.md Frontmatter — Common Fields

CC's SKILL.md YAML frontmatter is parsed by parseSkillFrontmatterFields() in loadSkillsDir.ts. Common fields include:

Field	Purpose
name / description	Display name and description
when_to_use	Guides the model on when to invoke
allowed-tools	Auto-allow list of tools available to the skill
context	inline (default) or fork (run as sub-Agent)
model	Model override (haiku/sonnet/opus/inherit)
hooks	Skill-level hook configuration
paths	Glob patterns for conditional activation
user-invocable	Users can invoke via /name

The complete field list changes across versions; above are the core fields relevant to the teaching version.

3. Precise Implementation of Two-Level Loading

1. Catalog (at startup): getSkillDirCommands() scans directory → registers as Command objects containing only metadata. getSkillListingAttachments() formats the skill list as attachments, budgeted at ~1% of the context window (cap 8000 characters).
2. Load (on invocation): Model calls Skill tool (input fields are skill + optional args; teaching version uses name) → getPromptForCommand() expands full SKILL.md content → SkillTool returns a tool_result with display text "Launching skill: {name}", while the actual skill content is injected via newMessages. The teaching version merges both into "injected via tool_result" as a simplification; the loaded SKILL.md can still guide later access to referenced resources through existing file/bash tools.

The Teaching Version's Simplification Is Intentional

• Multiple files and sources → 1 skills/ directory: sufficient to demonstrate the core concept of two-level loading
• Multiple frontmatter fields → only parse name/description: reduces parsing complexity
• Forked skills (context: 'fork') → omitted: the teaching version only expands inline skill loading
• Skill tool input skill+args → teaching version uses name: avoids extra argument parsing complexity