The map of extensions: when each one is useful

This lesson is still being tested: it improves with learners' feedback. If something is unclear, it is the lesson that needs fixing, not you.

By the end of this lesson, you will be able to

tell apart the main families of Claude Code extensions: instructions, skill, connector (MCP), hook, subagent, plugin;
start from a real need to choose the simplest one that fits;
know what to check before installing anything.

Why it matters to you

Every day you come across “incredible skills” and “agents that do everything”. Yet most needs are solved with the simplest extension, and some extensions give a lot of power to strangers. This map helps you choose without getting lost or putting yourself at risk.

Before you start


You need	Claude Code, permissions (module 4) and CLAUDE.md (module 5)
Duration	30 minutes
Cost	Included in your subscription for this lesson

Understand it in 3 minutes

From the simplest to the most powerful (the course’s own ranking, close to the order in which most teams add these extensions, according to the official documentation checked on 24/10/2026):

Extension	What it is	Example of a need	What it can do
Instructions (CLAUDE.md)	Your instructions, read again at every session	“Keep your answers short, ask before changing anything”	Guide the behavior
Skill	A folder with a SKILL.md file: instructions for a specific task, loaded only when they are needed, or called with /name	“Always write my meeting notes the same way”	Guide a task; some also run commands or allow tools
Connector (MCP server)	A bridge to an outside tool or outside data	“Read my lessons on the course website”, “Access my calendar”	Read, and sometimes act, in another service
Subagent	A specialized assistant that Claude launches for a task, with its own conversation	“Do a search without cluttering my conversation”	Work separately, then report back
Hook	A command that fires automatically at a precise moment (for example before an action)	“Always block this action”, “Let me know when Claude is waiting”	Act at a moment fixed in advance, without the AI choosing to do it
Plugin	A package that can contain several of the items above, installed from a catalog	“Install a whole set of tools at once”	Everything its items contain

The course’s rule: take the simplest extension that is enough. An instruction in CLAUDE.md before a skill, a skill before a connector, and a hook only if you understand exactly what it runs.

Before installing an extension that does not come from you, three questions:

Who made it, and where does it come from?
What can it do: read, write, send, run commands, access which accounts?
How much does it cost, to install and to use?

Do it together

Example: this course’s companion

The course’s Claude Code companion is itself a plugin that contains two items:

a skill named “suivre” (French for “follow”): the instructor’s guidelines;
an MCP connector: the address of the course server, which only reads lessons.

It contains no hook and no added program: its skill is launched by typing /formation-ia:suivre. This is the kind of check you will do for any extension. Its “Continue in Claude Code” page says what it does, what it does not do and what it sends.

Ask Claude Code to explain an extension to you

In your working folder, in Manual mode (Claude Code reads without asking in this folder; it usually asks for your approval to read elsewhere, except with some read-only commands, and to edit a file or run a command that does something other than read, unless you already allowed that action):

Explain to me in 10 lines the difference between a skill, an MCP connector, a hook and a plugin in Claude Code, with a concrete example for each, suited to a beginner.

Compare its answer with the table in this lesson. If there is a difference, the official documentation is right.

Your turn

Your decision sheet. Take 3 real needs (for example from your “My requests” note or your to-do list) and, for each one, fill in a line:

My need	The simplest extension	Why not a more powerful one

Check your work

It worked if:

each of your 3 needs has a chosen extension, with a reason;
at least one need is solved simply with CLAUDE.md or a skill;
you can name the 3 questions to ask before installing an extension.

Common mistakes:

installing a plugin “just to see” without looking at what it contains;
using a hook for something an instruction is enough to get;
thinking an extension is free because installing it is: some connectors go through paid services.

Keep it and pick it up later

Keep your decision sheet: the need that calls for a skill will be the exercise of the next lesson.

Going further

optional

The official documentation, in English: code.claude.com/docs/en/skills, /mcp, /hooks-guide, /sub-agents, /plugins/marketplace-reference.

New words

5 words explained

Word	What it means
Extension	What you add to Claude Code to give it instructions, access or automations
Skill	Instructions for a task, loaded when they are needed
Connector, MCP server	A bridge to an outside tool or outside data
Hook	A command that fires by itself at a precise moment
Plugin	A package of extensions installed from a catalog