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Anthropic

Claude Code — Anthropic’s coding agent in your terminal

Type claude in a project folder and it starts. It reads your , answers questions about it, edits files and runs commands to test its own work. This page covers installing it and your first run, and links to the course chapters that go deeper.

Advanced — extend it: skills, subagents, hooks, MCP, headless

Teach repeated jobs with skills, hand big jobs to , enforce must-follow rules with hooks, connect outside tools with , and put unattended work into scripts with claude -p.

Agent skills — teach it jobs you repeat

A is a folder with a SKILL.md in it. Normally Claude knows only each skill's name and description; when a job matches the description, it reads the body and follows it. So many skills take up almost no room. The old custom commands (.claude/commands/) have now been merged into skills.

  • .claude/skills/<name>/SKILL.md is for this project, ~/.claude/skills/<name>/SKILL.md for all your projects.
  • Say clearly in description what it does and when to use it, so Claude picks it on its own.
  • You can also call it directly with /<skill name>, and /skills shows the list.
  • Keep scripts and reference documents in the same folder and point to them from SKILL.md. Keep SKILL.md short, under 500 lines.
.claude/skills/release-notes/SKILL.md
---
name: release-notes
description: Write release notes from the git log. Use when the user asks for release notes, a changelog or "what changed since the last tag".
---

# Release notes

1. Find the last tag: `git describe --tags --abbrev=0`
2. Read the commits since then: `git log <tag>..HEAD --oneline`
3. Group them under Added / Fixed / Changed.
4. Write plain sentences a user can understand. Skip internal refactors.

Skills are an open standard — but Claude Code works its own way

Skills were first made by Anthropic, the company behind Claude, and released for free. Today they are an open standard called Agent Skills, managed at agentskills.io. Codex, Cursor, Gemini CLI, GitHub Copilot and many other tools read the same SKILL.md folder.

Be careful, though: the standard only defines what goes inside a skill folder. Where to look for skills and which one wins when names clash are left to each tool, and Claude Code works differently from what agentskills.io recommends. So a skill that works in another tool can behave differently in Claude Code.

  • Folder: agentskills.io recommends .agents/skills/, which many tools share, but Claude Code reads .claude/skills/ (in the project) and ~/.claude/skills/ (personal). It does not read .agents/skills/.
  • Same name: the common rule in the standard is that a project skill beats a personal one. In Claude Code it is the other way round — enterprise beats personal, and personal (~/.claude/skills/) beats project.
  • Parent and child folders: the standard does not say. Claude Code reads the folder you start in and every parent up to the git repository root, and a child folder only when it works there (next section).
  • Frontmatter: the standard has six fields — name, description, license, compatibility, metadata and allowed-tools. Claude Code adds more, such as disable-model-invocation, context: fork and paths. Other tools ignore them, and claude.ai uploads and the Skills API stop with an error.

For a skill you share with other AI tools, write the frontmatter with only the six standard fields. Then it loads unchanged in Claude Code and in the other tools.

Where Claude Code finds skills — your folder and its parents together

If the folder you are in has a .claude/skills folder and a parent folder has one too, Claude Code uses the skills in both together. It only goes up as far as the root.

  • It reads .claude/skills/ in the folder where you start claude and in every parent folder up to the repository root. Start in apps/web/ and the root skills are there too.
  • A .claude/skills/ above the repository root is not read. Put skills for all your projects in ~/.claude/skills/. In a git worktree it goes up only to the worktree root.
  • Skills in a folder below where you started are not read at first. They load the first time Claude reads or edits a file in that folder and stay until the session ends — until then they are not in the / menu. To load them early, run /add-dir with that folder.
  • If the root and apps/web/ both have a skill named deploy, both can be used: /deploy runs the root one and /apps/web:deploy runs the one in the child folder. Claude is told to use the skill of the folder that holds the files it is working on.
  • Plugin skills are separate, called as /<plugin-name>:<skill-name>, so they never clash with yours.
Run claude in apps/web/
~/projects/                    ← above the repository root
├── .claude/skills/            ← not loaded (put shared skills in ~/.claude/skills/)
└── repo/                      ← git repository root
    ├── .claude/skills/        ← loaded
    └── apps/web/              ← run claude here
        ├── .claude/skills/    ← loaded (used together with the root)
        └── src/
            └── .claude/skills/  ← not at start — loads when Claude first reads or edits a file in src/
Same name in two folders (claude started at the repository root)
repo/
├── .claude/skills/deploy/SKILL.md            → /deploy
└── apps/web/.claude/skills/deploy/SKILL.md   → /apps/web:deploy

Run /skills to see which skills are loaded now. With many skills in parent and child folders, the descriptions of the least-used ones get cut first, so check now and then with /skill-doctor.

Subagents — hand off parts of the work

A subagent is a helper with its own , instructions and tools. Give it a big search or review: however many files it reads, only the result comes back to the main conversation, which stays clean.

  • .claude/agents/<name>.md is for the project (commit it to share with the team), ~/.claude/agents/ is just for you. Only name and description are required.
  • Limiting the tools it may use with tools is safer — let a reviewer only read.
  • Call it like "Review this change with the code-reviewer subagent", or Claude hands work to it on its own when the description fits.
  • To make a new subagent, ask Claude to create one or write the file in the folder above yourself. Built-in ones such as Explore and Plan are there from the start.
.claude/agents/code-reviewer.md
---
name: code-reviewer
description: Reviews code changes for bugs, security problems and missing tests. Use after writing or changing code.
tools: Read, Grep, Glob, Bash
model: sonnet
---

You are a careful senior reviewer.
Run `git diff` to see the changes, then report:
1. Bugs (with file and line)
2. Security problems
3. Missing tests
Do not edit files.

Hooks — commands that always run at set moments

CLAUDE.md is a request, so it can sometimes be forgotten, but a hook is a command Claude Code always runs at a set moment. Use hooks to run a formatter after a file edit, block dangerous commands or edits to secret files, and get a notification when work is done.

  • PreToolUse — before a tool is used (can block it), PostToolUse — after a tool is used
  • UserPromptSubmit — when you send a request, Stop — when Claude finishes its answer, SessionStart — when a session starts, Notification — when it needs your attention
  • matcher chooses which tools the hook applies to (for example Edit|Write, Bash).
  • If the hook command ends with exit code 2, the action is blocked and the reason written to stderr is passed to Claude.
  • /hooks shows the hooks in place. The example above tidies every edited file with Prettier (it needs jq).
.claude/settings.json
{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Edit|Write",
        "hooks": [
          {
            "type": "command",
            "command": "jq -r '.tool_input.file_path' | xargs npx prettier --write"
          }
        ]
      }
    ]
  }
}

Hooks run on your computer with your permissions. Add a hook from someone else only after you have read and understood its command.

MCP — connect outside tools

MCP (Model Context Protocol) is an open standard that lets AI tools use outside services (issue trackers, , , document tools). Add an MCP and Claude can use the tools that server provides.

  • claude mcp add --transport http <name> <url> — add a remote server on the internet.
  • claude mcp add <name> -- <command> — add a server that runs on your computer (stdio). Everything after -- goes to the server unchanged.
  • Added with --scope project, it is saved in .mcp.json at the top of the repository and shared with the team (approval is asked on first use).
  • /mcp shows the connection status and signs in (OAuth). claude mcp list shows the list.
Terminal
# A remote server (HTTP)
claude mcp add --transport http notion https://mcp.notion.com/mcp

# A server that runs on your computer (stdio)
claude mcp add playwright -- npx @playwright/mcp@latest

# Share it with the team — saved in .mcp.json
claude mcp add --scope project playwright -- npx @playwright/mcp@latest

claude mcp list

Add only MCP servers you trust. A server that fetches outside text can bring in instructions hidden in that text (prompt injection).

Headless — scripts without the chat screen

claude -p "…" runs once without the chat screen, prints the result and exits. Use Claude as a building block in shell scripts, cron and .

  • Pass input through a pipe (cat error.log | claude -p "…").
  • --output-format json gives the result, session id and cost as JSON; stream-json sends progress line by line.
  • --allowedTools sets the tools allowed for this run, and --max-turns the maximum number of turns.
  • claude -p --continue continues the previous run, or --resume <session id> continues that session.
Terminal
# One question, one answer
claude -p "Explain what src/auth.ts does in 5 lines"

# Pipe input in
cat build-error.log | claude -p "Find the cause and suggest a fix"

# JSON output for scripts (result, session_id, total_cost_usd)
claude -p "List the TODO comments in src/" --output-format json

# Limit what it may do and how long it runs
claude -p "Run the tests and fix failures" \
  --allowedTools "Read,Edit,Bash(npm test *)" --max-turns 10

In scripts, --bare starts fast without reading CLAUDE.md, hooks, plugins or MCP (the official docs say it will become the default for -p). Leave it out if you need the project instructions.

Advanced practice

  1. Turn one job you do often (release notes, commit messages) into a skill in .claude/skills/ and call it with /skill-name.
  2. Make a read-only code-reviewer subagent and have it review your current change.
  3. Use a PostToolUse hook so a formatter or linter runs on every file edit.
  4. Add one MCP server and check the connection with /mcp.
  5. Write a one-line shell script that summarizes an error log with claude -p.

Done all five? Move on to the [Master] tab.

Official documentation

Checked against the official documentation on 2026-10-18. These tools change often — if a command fails, see the official docs.

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