Claude Code Basics

Introduction

Claude Code 2.1.3+ unified skills and slash commands into a single system. This guide explains the core concepts you need to know.

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Core Concepts

Concept	What It Is	How to Use
Skills	Reusable instructions in SKILL.md	/skill-name or auto-invoke via keywords
Plugins	Packages with skills, agents, hooks	claude plugin install sw@specweave
Agents	Isolated subagents with own context	Task tool or context: fork in skill

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Skills and Commands (Now Unified)

Since Claude Code 2.1.3, skills and slash commands are the same thing. Both file formats create the same /name command:

• .claude/commands/review.md → /review
• .claude/skills/review/SKILL.md → /review

Skill Locations

# Project-level (your repo)
.claude/skills/my-skill/SKILL.md     → /my-skill
.claude/commands/my-cmd.md           → /my-cmd

# Plugin skills (namespaced)
plugins/specweave/skills/pm/SKILL.md → /sw:pm (auto-activates on keywords)
plugins/specweave/commands/do.md     → /sw:do (explicit command)

Invocation Control (Frontmatter)

Frontmatter	User Invoke?	Claude Invoke?	Use Case
(default)	Yes	Yes	Most skills
disable-model-invocation: true	Yes	No	Workflows with side effects
user-invocable: false	No	Yes	Background knowledge

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Plugins

Plugins bundle related skills, agents, and hooks into installable packages.

Installing Plugins

claude plugin install sw@specweave          # Core SpecWeave
claude plugin install sw-frontend@specweave # Frontend expertise
claude plugin list                          # Show installed

Plugin Structure

plugins/specweave/
├── .claude-plugin/
│   └── plugin.json           # Plugin manifest
├── commands/                 # User-invocable workflows
│   ├── do.md                # → /sw:do
│   ├── done.md              # → /sw:done
│   └── status.md            # → /sw:status
├── skills/                   # Auto-activating expertise
│   ├── architect/SKILL.md   # → activates on "architecture"
│   ├── pm/SKILL.md          # → activates on "product", "MVP"
│   └── tech-lead/SKILL.md   # → activates on "code review"
└── agents/                   # Isolated subagents
    └── diagrams/AGENT.md    # Spawned via Task tool

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Agents (Subagents)

Agents are isolated AI instances with their own context window, system prompt, and tool restrictions.

Skills vs Agents

Aspect	Skills	Agents
Context	Same context (or context: fork)	Own isolated context
Invocation	/skill-name or auto-keywords	Task tool spawn
Model	Inherits (or specify model:)	Can override model
Tools	All available	Can restrict tools

When to Use What

Scenario	Use
Domain expertise (architecture, security)	Skills (auto-activate)
Complex isolated tasks	Agents (Task tool)
Background knowledge	Skills with user-invocable: false

Skill with Forked Context

A skill can run as an isolated subagent by adding context: fork:

---
context: fork
model: opus
---

This gives the skill its own context window while still being invocable as /increment.

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Invoking Skills

Per official Anthropic documentation:

"Claude uses skills when relevant, or you can invoke one directly with /skill-name."

Two Invocation Mechanisms

1. Auto-Activation (Primary): Skills auto-activate when their description keywords match your request
2. Explicit Invocation (Fallback): Use /skill-name or Skill tool when auto-activation doesn't trigger

Explicit Invocation (User Types Command)

/sw:do                              # Execute tasks
/sw:increment "auth feature"        # Plan new increment
/sw:status                          # Show status

Auto-Activation (Claude Detects Keywords)

Just describe what you need - Claude loads the relevant skill:

"Design the authentication architecture"  # → architect skill
"Help me plan this product feature"       # → PM skill
"Review my code for security issues"      # → security skill

Fallback: Skill Tool (When Auto-Activation Fails)

If a skill didn't auto-activate, Claude can explicitly invoke it:

// Skill didn't auto-load? Invoke explicitly:
Skill({ skill: "sw-frontend:frontend-architect", args: "dashboard" })

Via Task Tool (Subagents)

For complex tasks requiring isolated context:

// Use valid subagent types: Explore, Plan, general-purpose
Task({
  subagent_type: "Explore",
  prompt: "Analyze the frontend codebase structure and patterns",
  description: "Frontend codebase exploration"
})

Note: Task tool accepts these subagent types: Explore, Plan, general-purpose. For skills, use the Skill tool instead.

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Creating Your Own Skills

Basic Skill

Create .claude/skills/my-skill/SKILL.md:

---
name: my-skill
description: Does something useful. Activates for: keyword1, keyword2
---

# My Skill

Instructions for what this skill does...

Skill with Model Override

---
name: deep-analysis
description: Deep code analysis
model: opus
context: fork
allowed-tools: Read, Grep, Glob
---

Command (Explicit Only)

Create .claude/commands/deploy.md:

---
name: deploy
description: Deploy to production
disable-model-invocation: true
---

# Deploy Command

Only runs when user types /deploy explicitly.

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Summary

1. Skills = Commands (since 2.1.3) - same system, different activation
2. Plugins bundle skills, agents, and hooks
3. Agents are isolated subagents with own context
4. context: fork makes a skill run as a subagent
5. Use frontmatter to control who can invoke (disable-model-invocation, user-invocable)