Extend Claude Code

Understand when to use CLAUDE.md, Skills, subagents, hooks, MCP, and plugins.

Claude Code combines a model that reasons about your code with built-in tools for file operations, search, execution, and web access. The built-in tools cover most coding tasks. This guide covers the extension layer: features you add to customize what Claude knows, connect it to external services, and automate workflows.

For how the core agentic loop works, see How Claude Code works.

New to Claude Code? Start with CLAUDE.md for project conventions. Add other extensions as you need them.

Overview

Extensions plug into different parts of the agentic loop:

• CLAUDE.md adds persistent context Claude sees every session
• Skills add reusable knowledge and invocable workflows
• MCP connects Claude to external services and tools
• Subagents run their own loops in isolated context, returning summaries
• Agent teams coordinate multiple independent sessions with shared tasks and peer-to-peer messaging
• Hooks run outside the loop entirely as deterministic scripts
• Plugins and marketplaces package and distribute these features

Skills are the most flexible extension. A skill is a markdown file containing knowledge, workflows, or instructions. You can invoke skills with a slash command like /deploy, or Claude can load them automatically when relevant. Skills can run in your current conversation or in an isolated context via subagents.

Match features to your goal

Features range from always-on context that Claude sees every session, to on-demand capabilities you or Claude can invoke, to background automation that runs on specific events. The table below shows what’s available and when each one makes sense.

Feature	What it does	When to use it	Example
CLAUDE.md	Persistent context loaded every conversation	Project conventions, “always do X” rules	”Use pnpm, not npm. Run tests before committing.”
Skill	Instructions, knowledge, and workflows Claude can use	Reusable content, reference docs, repeatable tasks	/review runs your code review checklist; API docs skill with endpoint patterns
Subagent	Isolated execution context that returns summarized results	Context isolation, parallel tasks, specialized workers	Research task that reads many files but returns only key findings
Agent teams	Coordinate multiple independent Claude Code sessions	Parallel research, new feature development, debugging with competing hypotheses	Spawn reviewers to check security, performance, and tests simultaneously
MCP	Connect to external services	External data or actions	Query your database, post to Slack, control a browser
Hook	Deterministic script that runs on events	Predictable automation, no LLM involved	Run ESLint after every file edit

Plugins are the packaging layer. A plugin bundles skills, hooks, subagents, and MCP servers into a single installable unit. Plugin skills are namespaced (like /my-plugin:review) so multiple plugins can coexist. Use plugins when you want to reuse the same setup across multiple repositories or distribute to others via a marketplace.

Compare similar features

Some features can seem similar. Here’s how to tell them apart.

Skill vs Subagent

Skills and subagents solve different problems:

• Skills are reusable content you can load into any context
• Subagents are isolated workers that run separately from your main conversation

Aspect	Skill	Subagent
What it is	Reusable instructions, knowledge, or workflows	Isolated worker with its own context
Key benefit	Share content across contexts	Context isolation. Work happens separately, only summary returns
Best for	Reference material, invocable workflows	Tasks that read many files, parallel work, specialized workers

Skills can be reference or action. Reference skills provide knowledge Claude uses throughout your session (like your API style guide). Action skills tell Claude to do something specific (like /deploy that runs your deployment workflow).

Use a subagent when you need context isolation or when your context window is getting full. The subagent might read dozens of files or run extensive searches, but your main conversation only receives a summary. Since subagent work doesn’t consume your main context, this is also useful when you don’t need the intermediate work to remain visible. Custom subagents can have their own instructions and can preload skills.

They can combine. A subagent can preload specific skills (skills: field). A skill can run in isolated context using context: fork. See Skills for details.

CLAUDE.md vs Skill

Both store instructions, but they load differently and serve different purposes.

Aspect	CLAUDE.md	Skill
Loads	Every session, automatically	On demand
Can include files	Yes, with @path imports	Yes, with @path imports
Can trigger workflows	No	Yes, with /<name>
Best for	”Always do X” rules	Reference material, invocable workflows

Put it in CLAUDE.md if Claude should always know it: coding conventions, build commands, project structure, “never do X” rules.

Put it in a skill if it’s reference material Claude needs sometimes (API docs, style guides) or a workflow you trigger with /<name> (deploy, review, release).

Rule of thumb: Keep CLAUDE.md under ~500 lines. If it’s growing, move reference content to skills.

Subagent vs Agent team

Both parallelize work, but they’re architecturally different:

• Subagents run inside your session and report results back to your main context
• Agent teams are independent Claude Code sessions that communicate with each other

Aspect	Subagent	Agent team
Context	Own context window; results return to the caller	Own context window; fully independent
Communication	Reports results back to the main agent only	Teammates message each other directly
Coordination	Main agent manages all work	Shared task list with self-coordination
Best for	Focused tasks where only the result matters	Complex work requiring discussion and collaboration
Token cost	Lower: results summarized back to main context	Higher: each teammate is a separate Claude instance

Use a subagent when you need a quick, focused worker: research a question, verify a claim, review a file. The subagent does the work and returns a summary. Your main conversation stays clean.

Use an agent team when teammates need to share findings, challenge each other, and coordinate independently. Agent teams are best for research with competing hypotheses, parallel code review, and new feature development where each teammate owns a separate piece.

Transition point: If you’re running parallel subagents but hitting context limits, or if your subagents need to communicate with each other, agent teams are the natural next step.

Agent teams are experimental and disabled by default. See agent teams for setup and current limitations.

MCP vs Skill

MCP connects Claude to external services. Skills extend what Claude knows, including how to use those services effectively.

Aspect	MCP	Skill
What it is	Protocol for connecting to external services	Knowledge, workflows, and reference material
Provides	Tools and data access	Knowledge, workflows, reference material
Examples	Slack integration, database queries, browser control	Code review checklist, deploy workflow, API style guide

These solve different problems and work well together:

MCP gives Claude the ability to interact with external systems. Without MCP, Claude can’t query your database or post to Slack.

Skills give Claude knowledge about how to use those tools effectively, plus workflows you can trigger with /<name>. A skill might include your team’s database schema and query patterns, or a /post-to-slack workflow with your team’s message formatting rules.

Example: An MCP server connects Claude to your database. A skill teaches Claude your data model, common query patterns, and which tables to use for different tasks.

Understand how features layer

Features can be defined at multiple levels: user-wide, per-project, via plugins, or through managed policies. You can also nest CLAUDE.md files in subdirectories or place skills in specific packages of a monorepo. When the same feature exists at multiple levels, here’s how they layer:

• CLAUDE.md files are additive: all levels contribute content to Claude’s context simultaneously. Files from your working directory and above load at launch; subdirectories load as you work in them. When instructions conflict, Claude uses judgment to reconcile them, with more specific instructions typically taking precedence. See how Claude looks up memories.
• Skills and subagents override by name: when the same name exists at multiple levels, one definition wins based on priority (managed > user > project for skills; managed > CLI flag > project > user > plugin for subagents). Plugin skills are namespaced to avoid conflicts. See skill discovery and subagent scope.
• MCP servers override by name: local > project > user. See MCP scope.
• Hooks merge: all registered hooks fire for their matching events regardless of source. See hooks.

Combine features

Each extension solves a different problem: CLAUDE.md handles always-on context, skills handle on-demand knowledge and workflows, MCP handles external connections, subagents handle isolation, and hooks handle automation. Real setups combine them based on your workflow.

For example, you might use CLAUDE.md for project conventions, a skill for your deployment workflow, MCP to connect to your database, and a hook to run linting after every edit. Each feature handles what it’s best at.

Pattern	How it works	Example
Skill + MCP	MCP provides the connection; a skill teaches Claude how to use it well	MCP connects to your database, a skill documents your schema and query patterns
Skill + Subagent	A skill spawns subagents for parallel work	/review skill kicks off security, performance, and style subagents that work in isolated context
CLAUDE.md + Skills	CLAUDE.md holds always-on rules; skills hold reference material loaded on demand	CLAUDE.md says “follow our API conventions,” a skill contains the full API style guide
Hook + MCP	A hook triggers external actions through MCP	Post-edit hook sends a Slack notification when Claude modifies critical files

Understand context costs

Every feature you add consumes some of Claude’s context. Too much can fill up your context window, but it can also add noise that makes Claude less effective; skills may not trigger correctly, or Claude may lose track of your conventions. Understanding these trade-offs helps you build an effective setup.

Context cost by feature

Each feature has a different loading strategy and context cost:

Feature	When it loads	What loads	Context cost
CLAUDE.md	Session start	Full content	Every request
Skills	Session start + when used	Descriptions at start, full content when used	Low (descriptions every request)*
MCP servers	Session start	All tool definitions and schemas	Every request
Subagents	When spawned	Fresh context with specified skills	Isolated from main session
Hooks	On trigger	Nothing (runs externally)	Zero, unless hook returns additional context

*By default, skill descriptions load at session start so Claude can decide when to use them. Set disable-model-invocation: true in a skill’s frontmatter to hide it from Claude entirely until you invoke it manually. This reduces context cost to zero for skills you only trigger yourself.

Understand how features load

Each feature loads at different points in your session. The tabs below explain when each one loads and what goes into context.

CLAUDE.md

When: Session start

What loads: Full content of all CLAUDE.md files (managed, user, and project levels).

Inheritance: Claude reads CLAUDE.md files from your working directory up to the root, and discovers nested ones in subdirectories as it accesses those files. See How Claude looks up memories for details.

Keep CLAUDE.md under ~500 lines. Move reference material to skills, which load on-demand.

Skills

Skills are extra capabilities in Claude’s toolkit. They can be reference material (like an API style guide) or invocable workflows you trigger with /<name> (like /deploy). Some are built-in; you can also create your own. Claude uses skills when appropriate, or you can invoke one directly.

When: Depends on the skill’s configuration. By default, descriptions load at session start and full content loads when used. For user-only skills (disable-model-invocation: true), nothing loads until you invoke them.

What loads: For model-invocable skills, Claude sees names and descriptions in every request. When you invoke a skill with /<name> or Claude loads it automatically, the full content loads into your conversation.

How Claude chooses skills: Claude matches your task against skill descriptions to decide which are relevant. If descriptions are vague or overlap, Claude may load the wrong skill or miss one that would help. To tell Claude to use a specific skill, invoke it with /<name>. Skills with disable-model-invocation: true are invisible to Claude until you invoke them.

Context cost: Low until used. User-only skills have zero cost until invoked.

In subagents: Skills work differently in subagents. Instead of on-demand loading, skills passed to a subagent are fully preloaded into its context at launch. Subagents don’t inherit skills from the main session; you must specify them explicitly.

Use disable-model-invocation: true for skills with side effects. This saves context and ensures only you trigger them.

MCP servers

When: Session start.

What loads: All tool definitions and JSON schemas from connected servers.

Context cost: Tool search (enabled by default) loads MCP tools up to 10% of context and defers the rest until needed.

Reliability note: MCP connections can fail silently mid-session. If a server disconnects, its tools disappear without warning. Claude may try to use a tool that no longer exists. If you notice Claude failing to use an MCP tool it previously could access, check the connection with /mcp.

Run /mcp to see token costs per server. Disconnect servers you’re not actively using.

Subagents

When: On demand, when you or Claude spawns one for a task.

What loads: Fresh, isolated context containing:

• The system prompt (shared with parent for cache efficiency)
• Full content of skills listed in the agent’s skills: field
• CLAUDE.md and git status (inherited from parent)
• Whatever context the lead agent passes in the prompt

Context cost: Isolated from main session. Subagents don’t inherit your conversation history or invoked skills.

Use subagents for work that doesn’t need your full conversation context. Their isolation prevents bloating your main session.

Hooks

When: On trigger. Hooks fire at specific lifecycle events like tool execution, session boundaries, prompt submission, permission requests, and compaction. See Hooks for the full list.

What loads: Nothing by default. Hooks run as external scripts.

Context cost: Zero, unless the hook returns output that gets added as messages to your conversation.

Hooks are ideal for side effects (linting, logging) that don’t need to affect Claude’s context.

Learn more

Each feature has its own guide with setup instructions, examples, and configuration options.

CLAUDE.md

Store project context, conventions, and instructions

Skills

Give Claude domain expertise and reusable workflows

Subagents

Offload work to isolated context

Agent teams

Coordinate multiple sessions working in parallel

MCP

Connect Claude to external services

Hooks

Automate workflows with hooks

Plugins

Bundle and share feature sets

Marketplaces

Host and distribute plugin collections