title: "Agent Development Kit" description: "5-layer model - understand the complete architecture" section: "Core" readTime: "15 min" badge: "POPULAR"

Agent Development Kit: The 5-Layer Model

"Claude Code isn't a coding assistant. It's an agent platform."

Most people are using 10% of it.

The key is understanding the 5-layer composition model that powers everything.

Layer 1: CLAUDE.md — Memory Layer

Your agent's constitution.

Think of it as an employee onboarding document, not a novel. Keep it under 200 lines.

## Tech Stack
- Node.js 20
- TypeScript
- React 19
 
## Conventions
- Kebab-case filenames
- camelCase variables
- ESLint strict mode
 
## Architecture
- /src/app (Next.js App Router)
- /src/components (React)
- /src/lib (utilities)
 
## Security
- No .env in git
- API keys environment-only
- Rate limiting on all endpoints

What goes in CLAUDE.md:

• Tech stack (not installation instructions)
• Coding conventions (not tutorials)
• Architecture overview (not detailed design)
• Hard constraints (banned patterns, security rules)
• Key file locations (not every file)

What belongs in Skills instead:

• Multi-step workflows
• Complex procedures
• Decision trees
• Testing strategies
• Deployment procedures

Layer 2: Skills — Knowledge Layer

Extend your agent with reusable knowledge modules.

Skills use the Agent Skills open standard (agentskills.io).

Skill Anatomy

---
name: "Database Migration"
description: "Safe schema migrations with rollback"
user-invocable: true
disable-model-invocation: false
allowed-tools: ["bash", "file"]
model: "sonnet"
effort: "high"
context: "fork"
hooks:
  - event: "post-tool-execution"
    pattern: "ALTER TABLE"
    action: "validate"
paths:
  - "db/migrations/**"
  - "prisma/schema.prisma"
---
 
# Database Migration Skill
 
Use this skill to create safe migrations...
 
## Usage
- /db-migrate "add users table"
- /db-migrate --rollback
 
## Behind the Scenes
\`\`\`bash
# Dynamic context injection
echo "Schema: $(cat prisma/schema.prisma | head -20)"
\`\`\`

Skill Frontmatter Fields

Dynamic Context: Code Injection

$ARGUMENTS[0]          # First parameter
${CLAUDE_SESSION_ID}   # Current session UUID
${CLAUDE_SKILL_DIR}    # Skill directory path
 
!`git log -1 --oneline`  # Shell output injection

Skill Scope Priority

1. Enterprise (global across org)
2. Personal (~/.claude/skills/)
3. Project (.claude/skills/)
4. Plugin (npm packages)

Layer 3: Hooks — Guardrail Layer

Hooks intercept Claude Code at specific points:

{
  "hooks": [
    {
      "event": "PreToolUse",
      "pattern": "bash.*rm.*-rf",
      "action": "ask"
    },
    {
      "event": "PostToolExecution",
      "pattern": "git.*commit",
      "action": "notify"
    }
  ]
}

Hook Events:

• PreToolUse — Before shell command
• PostToolExecution — After command runs
• PreCommit — Git commit safety
• PreFileDeletion — File deletion warning
• MissingFile — File not found
• Notification — Slack/webhook alerts

Layer 4: Subagents — Delegation Layer

Isolate context. Run agents in parallel.

---
context: fork    # Runs in isolated subagent
---
 
This skill always runs in a fresh context
with its own memory and filesystem scope.

Built-in Agent Types:

• Explore agent (search codebases, summarize)
• Plan agent (break tasks into steps)
• General-purpose agent (default)

Parallel Execution:

/batch \
  "Implement auth module" \
  "Set up database migrations" \
  "Build API endpoints"

Running 3 agents simultaneously = 3x token cost but saves wall-clock time.

Layer 5: Plugins — Distribution Layer

Bundle and share skills as npm packages.

{
  "name": "@acme/claude-skills",
  "version": "1.0.0",
  "skills": {
    "db-migrate": "./skills/db-migrate.md",
    "deploy": "./skills/deploy.md"
  }
}

Install:

claude plugin add @acme/claude-skills

Bundled Skills (Pre-Loaded)

Putting It Together: A Real Example

Project Structure:
├── CLAUDE.md                    # Layer 1: Memory
├── .claude/
│   ├── skills/
│   │   ├── db-migrate.md       # Layer 2: Skills
│   │   └── deploy.md
│   ├── settings.json           # Layer 3: Hooks
│   └── .mcp.json              # MCP config
├── .git/
└── src/

User runs: claude "Add user authentication"

Agent process:

1. Reads CLAUDE.md (constraints, tech stack)
2. Checks hooks (pre-tool safety rules)
3. Searches for relevant Skills
4. Creates plan (ask for approval)
5. Forks subagent if using isolation
6. Executes with fallback to Layer 2, 3 constraints
7. Logs via Hooks (Layer 3)

Composition Benefits

✅ Reusable knowledge (Skills)
✅ Safety guardrails (Hooks)
✅ Scalability (Plugins)
✅ Isolation (Subagents)
✅ Governance (CLAUDE.md)

Next: Explore Architecture for runtime details.