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.
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
- 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 |
| 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.
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. The subagent might read dozens of files or run extensive searches, but your main conversation only receives a summary. 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.
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.
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.
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.
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.
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.
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.
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.
When: On trigger. Hooks can run before or after tool executions, at session start, before compaction, and at other lifecycle events. 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.
Learn more
Each feature has its own guide with setup instructions, examples, and configuration options.
Store project context, conventions, and instructions
Give Claude domain expertise and reusable workflows
Offload work to isolated context
Connect Claude to external services
Run scripts on Claude Code events
Bundle and share feature sets
Host and distribute plugin collections
To find navigation and other pages in this documentation, fetch the llms.txt file at: https://code.claude.com/docs/llms.txt