# Skill patterns

Course: Claude Code: Pro Level — https://zero2vibecode.com/learn/claude-code-pro
Canonical URL: https://zero2vibecode.com/learn/claude-code-pro/5-2-skill-patterns
Verified against: Claude Code v2.1.292 (2026-10-15)
Last updated: 2026-10-15

A skill can do more than instructions and $ARGUMENTS. It can insert a command's output into itself, keep reference material in neighbouring files, run in a separate subagent, take named arguments and pre-approve the tools it needs.

You already know that a skill is a folder with a `SKILL.md` file: a description in the frontmatter and instructions in the body, plus `$ARGUMENTS` for whatever you typed after the name. Now, five patterns that make a skill stronger — and leaner.

## Five patterns

| Pattern | How you write it | What it does |
|---|---|---|
| Live data | `` !`git diff HEAD` `` in the body | Claude Code runs the command before Claude reads the skill and inserts the output |
| Neighbouring files | files next to `SKILL.md`, linked from it | long reference material Claude reads only when needed |
| Its own context | `context: fork` in the frontmatter | the skill becomes a task for a background subagent |
| Named arguments | `arguments: [focus]`, and `$focus` in the body | each word after the name fills its own field |
| Allowed tools | `allowed-tools: Read, Grep` | these tools run without a prompt during the skill's turn |

## A skill for reviewing changes

Let's put it all together. The file `.claude/skills/changes-review/SKILL.md`:

```markdown
---
name: changes-review
description: Reviews uncommitted changes against the team checklist. Use when asked for a review before committing.
context: fork
arguments: [focus]
allowed-tools: Bash(git diff *)
---

## Changes

!`git diff HEAD`

## Task

Review the changes above against [checklist.md](checklist.md), paying most attention to $focus.
Name each problem with its file and line, and finish with one line: is it ready to commit.
```

And next to it, in the same folder, `checklist.md`:

```markdown
- Every changed function has a test.
- Errors are reported, not swallowed.
- There are no secrets in the code or configs.
```

What happens when you type `/changes-review tests`:

1. Claude Code runs `git diff HEAD` and inserts the diff into the skill's text.
2. The word `tests` goes in place of `$focus`.
3. The skill becomes a task for a subagent. It works in the background, **does not see your conversation**, opens `checklist.md` itself and sends a report when it is done.

Try `/changes-review secrets` — the same skill will look at secrets first.

## Fine points

**A failed command aborts the skill.** If an injected command exits with an error, or your permission rules do not let it through, the skill does not run at all: you will see `Shell command failed` or `Shell command permission check failed`. Run the command by hand and look at the `allowed-tools` line. Outside Auto mode, a command your rules do not allow stops the skill — which is why `allowed-tools: Bash(git diff *)` is not redundant here.

**allowed-tools forbids nothing.** It only allows what it lists without a prompt, for the turn the skill runs in. Other tools stay available — just with the usual prompts.

**Keep SKILL.md short.** Once it runs, its whole text stays in the conversation. The documentation recommends no more than 500 lines, with the details in neighbouring files: Claude opens those only if it needs them.

**Careful.** 
An injected command runs before Claude even sees the skill, and Claude cannot review or cancel it. Only inject commands that read, not change: git diff, git log, git status. And remember the output lands in the context: a huge diff eats it just like a huge file.

**Practice in the terminal.** Create the .claude/skills/changes-review folder and build SKILL.md line by line with echo: the first line with >, the rest with >>. The frontmatter goes between two --- lines, and the body holds the diff injection. Put checklist.md next to it and check everything with cat.