What are Claude Code skills?

Tell a skill apart from CLAUDE.md and from a request, and decide which part of your data work repeats enough to become a skill.

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A is a recipe you write for Claude Code once, for a task you repeat. Inside, it's a folder with a SKILL.md file: a short header that says what it's for and, below it, the steps in your own words. When you ask for something that matches that description, Claude Code loads the file and follows your steps. You can also call it directly by typing / followed by the folder's name.

Even though the name sounds technical, a skill is written in plain text, just like CLAUDE.md. If a procedure also needs a template or helper , you can keep them in that same folder, but the heart of the skill is your instructions in plain language. (Skills also exist in the Claude app and in ; here we focus on how they work inside Claude Code.)

With this one you now have three ways to give Claude Code instructions, and each plays a different role in your folder:

Piece	What it holds	When Claude Code reads it	What happens to your
Your request	Today's task or question	The moment you send it	Takes up space only on that turn
CLAUDE.md	What's always true in this folder (rules and data)	At the start of every session	Loads in full in every session
A skill	The steps of a procedure you repeat	When your request matches its description, or when you type `/name`	At startup only its description comes in; the full steps load only when it's used

The difference from CLAUDE.md protects your session's space. Since CLAUDE.md loads in full from the first hello, if you put your month-end close instructions, the rules for joining two tables and the format for three different reports in there, all of it takes up room even when today you just want to ask a quick question. With a skill, Claude Code reads only the description at the start; the steps come in when the task calls for them. That way you can have many skills ready without crowding the session.

What a skill looks like inside

A project's skills live inside your working folder, at the path .claude/skills/ (since .claude starts with a dot, File Explorer or Finder may hide it if you haven't turned on hidden files). Each skill has its own subfolder and, unless you say otherwise, that folder's name is the command you call it with:

exoplanetas/├── CLAUDE.md├── planetas.csv└── .claude/    └── skills/        └── revisa-tabla/            └── SKILL.md

A project folder with CLAUDE.md and a skill called revisa-tabla ("check the table").

The SKILL.md file starts, on its very first line, with a header between two --- lines. The recommended field there is description: it's the text Claude Code reads when the session opens to decide when to load the skill. Below the second --- line you write the procedure's steps. The docs recommend keeping SKILL.md under 500 lines so it stays clear and light when it loads.

You have two ways to use a skill:

Let it load on its own: you make your usual request and, if it matches description, Claude Code opens SKILL.md and applies the steps.
Call it by name: you type /revisa-tabla in the terminal or the desktop app (in the app you can also type / or click + and select Slash commands to pick it from the list). If you have a skill that does something sensitive and you'd rather it only run when you call it with /name, you can add disable-model-invocation: true to the header between the --- lines.

If you create a skill that's useful in every folder on your computer and not just in exoplanetas, save it in your personal folder: ~/.claude/skills/revisa-tabla/SKILL.md. If a personal skill and a project skill have the same name, the personal one wins.

Common mistake: Vague descriptions that never trigger the skill

A skill with description: Helps with data or description: For reports almost never loads on its own, because it doesn't tell Claude Code when to use it. Always write what it does and when to trigger it: for example, Checks a table before counting or averaging anything with it; use it when I ask you to analyze a new table.

What becomes a skill in data work

A task deserves to become a skill when you do it with the same steps every week or every month and you're tired of explaining it from scratch:

checking a freshly downloaded table before calculating anything: how many rows it has, what a row stands for, how many empty cells there are, and a sample of rows;
building the recurring report: which file goes in, which records get dropped, how it's grouped and what format it's delivered in;
two sources that should give the same total, like billing against bank deposits;
preparing an Excel workbook for leadership with a summary sheet and live formulas in view.

Here's how the work splits between CLAUDE.md, a skill and the day's request in a real office case:

Separate standing rules, a skill and the day's request

Worked example · 3 steps

You work in a folder with the file invoices.csv. You know three things:

In invoices.csv each row is one product line (the same invoice can take several rows), amounts are before tax, and canceled invoices have status = "Canceled".
Every Friday you match invoices.csv against bank_deposits.csv: you filter out the canceled ones, add up by payment date, compare the daily totals and save the differences in a new Excel file with formulas.
Today is Tuesday and you just want to know how many distinct invoices were issued yesterday.

What goes in CLAUDE.md, what do you save in a skill, and what do you type in today's request?

Work out the next step yourself, then open it to compare.

On the next page you'll build your first skill for checking tables in the exoplanetas folder. If you have a recurring task at work and want to know whether it's worth turning into a skill, tell your Maestro.

Try it

Think of a task with tables you repeat at work every week or every month. Write two lines on paper or in a notes file: what you'd call the folder inside .claude/skills/ and what you'd put in its description. Check that your description says both what the skill does and in what situation it should load. If you want a second look at your description, paste it to your Maestro.

Then answer this question:

Check yourself: Every month you build the same sales report: a file comes in, you remove the canceled sales, add up by branch and deliver an Excel file with formulas. Where do you put those steps?