Documentation Index
Fetch the complete documentation index at: https://code.claude.com/docs/llms.txt Use this file to discover all available pages before exploring further.
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, then add other extensions as specific triggers come up.
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
- Code intelligence connects Claude to a language server for symbol-level navigation and live type errors
- 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 fire on lifecycle events and can run a script, HTTP request, prompt, or subagent
- 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 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 | /deploy runs your deployment 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 |
| Code intelligence | Language-server navigation and diagnostics | Typed languages, large codebases where grep is slow or imprecise | Jump to a symbol's definition instead of reading the whole file |
| MCP | Connect to external services | External data or actions | Query your database, post to Slack, control a browser |
| Hook | Script, HTTP request, prompt, or subagent triggered by events | Automation that must run on every matching event | 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.
Build your setup over time
You don't need to configure everything up front. Each feature has a recognizable trigger, and most teams add them in roughly this order:
| Trigger | Add |
|---|---|
| Claude gets a convention or command wrong twice | Add it to CLAUDE.md |
| You keep typing the same prompt to start a task | Save it as a user-invocable skill |
| You paste the same playbook or multi-step procedure into chat for the third time | Capture it as a skill |
| You keep copying data from a browser tab Claude can't see | Connect that system as an MCP server |
| Claude reads many files to find where a symbol is defined or used | Install a code intelligence plugin for your language |
| A side task floods your conversation with output you won't reference again | Route it through a subagent |
| You want something to happen every time without asking | Write a hook |
| A second repository needs the same setup | Package it as a plugin |
The same triggers tell you when to update what you already have. A repeated mistake or a recurring review comment is a CLAUDE.md edit, not a one-off correction in chat. A workflow you keep tweaking by hand is a skill that needs another revision.
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 |
| Context window impact | Adds to your main window | Uses a separate window with its own input and output tokens |
| 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.
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 200 lines. If it's growing, move reference content to skills or split into .claude/rules/ files.
All three store instructions, but they load differently:
| Aspect | CLAUDE.md | .claude/rules/ | Skill |
|---|---|---|---|
| Loads | Every session | Every session, or when matching files are opened | On demand, when invoked or relevant |
| Scope | Whole project | Can be scoped to file paths | Task-specific |
| Best for | Core conventions and build commands | Language-specific or directory-specific guidelines | Reference material, repeatable workflows |
Use CLAUDE.md for instructions every session needs: build commands, test conventions, project architecture.
Use rules to keep CLAUDE.md focused. Rules with paths frontmatter only load when Claude works with matching files, saving context.
Use skills for content Claude only needs sometimes, like API documentation or a deployment checklist you trigger with /<name>.
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 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 purpose-built tools for an external system, with the connection and authentication handled by the server.
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.
A hook fires on a lifecycle event; a skill is loaded into context for Claude to apply.
| Aspect | Hook | Skill |
|---|---|---|
| Runs | A shell command, HTTP request, LLM prompt, or subagent | Instructions Claude reads and follows |
| Triggered by | Lifecycle events such as PostToolUse or SessionStart | You typing /<name>, or Claude matching the description to your task |
| Determinism | Always fires on its event; the trigger is guaranteed | Claude interprets the instructions; outcome can vary |
| Context cost | Zero unless the hook returns output | Description loads each session; full content loads when used |
| Best for | Linting after edits, blocking unsafe commands, logging, notifications | Workflows that need reasoning, reference material, multi-step tasks |
Use a hook when the action must happen the same way every time and doesn't need Claude to think. For example: format on save, reject rm -rf /, post a Slack message when a session ends.
Use a skill when Claude should decide how to apply the steps, or when the content is knowledge rather than a script. For example: a /release checklist, your API style guide, a debugging playbook.
Put guardrails in hooks. An instruction like "never edit .env" in CLAUDE.md or a skill is a request, not a guarantee. A PreToolUse hook that blocks the edit is enforcement. If a rule must hold every time, make it a hook rather than a prompt instruction.
Hook output lands in context. A PostToolUse hook that runs your linter feeds results back as text Claude reads; a /fix-lint skill tells Claude how to resolve them.
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.md files load.
- 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 | /audit 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. For an interactive view of how these features combine in a running session, see Explore the context window.
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 | Tool names; full schemas on demand | Low until a tool is used |
| Code intelligence | After file edits and on demand | Diagnostics after edits; symbol locations on lookup | Low; reduces file reads elsewhere |
| 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. For a skill you didn't write, set skillOverrides in settings to do the same without editing its file.
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.md files load 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). Claude Code includes bundled skills like /code-review, /batch, and /debug that work out of the box. 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 listed in the subagent's skills field are fully preloaded into its context at launch. Subagents can still discover and invoke unlisted project, user, and plugin skills through the Skill tool.
disable-model-invocation: true for skills with side effects. This saves context and ensures only you trigger them.When: Session start.
What loads: Tool names from connected servers. Full JSON schemas stay deferred until Claude needs a specific tool.
Context cost: Tool search is on by default, so idle MCP tools consume minimal context.
/mcp to see connection status and token costs per server. Claude Code reconnects to remote servers automatically if they drop, and you can disconnect servers you're not actively using.When: After file edits, and on demand when Claude navigates code.
What loads: Type errors and warnings after each file edit. Definition, reference, and type information when Claude looks up a symbol.
Context cost: Low. Symbol lookups often replace broad file reads, so net context use can go down.
When: On demand, when you or Claude spawns one for a task.
What loads: Fresh, isolated context containing:
- The agent's own system prompt, not the full Claude Code system prompt
- Full content of skills listed in the agent's
skills:field - CLAUDE.md and git status, except the built-in Explore and Plan agents omit both
- 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 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 execute outside the main conversation.
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
Coordinate multiple sessions working in parallel
Connect Claude to external services
Automate workflows with hooks
Bundle and share feature sets
Host and distribute plugin collections