What are Skills?
When you ask Claude to create a Word document, you might notice it does a surprisingly good job — neat headings, proper formatting, a table of contents. That’s not magic. Behind the scenes, Claude is using a skill: a set of detailed instructions that tell it exactly how to handle a specific type of task.
Think of skills as recipes. Without a recipe, a decent cook can make a passable pasta. With a well-tested recipe, that same cook produces something consistently excellent. Skills work the same way for Claude.
In short: Skills are instruction sets that make Claude significantly better at specific tasks. They’re loaded automatically when needed — you don’t have to do anything special.
Built-in skills
Cowork comes with a set of built-in skills that activate automatically when you ask Claude to work with certain file types:
| Skill | What it does | Triggered by |
|---|---|---|
docx | Creates and edits Word documents with professional formatting, headers, page numbers | ”Create a Word document…”, “Edit my .docx file…” |
xlsx | Builds spreadsheets with formulas, conditional formatting, charts | ”Make a spreadsheet…”, “Analyse this Excel file…” |
pptx | Designs presentations with layouts, speaker notes, consistent styling | ”Create a presentation…”, “Make a slide deck…” |
pdf | Reads, merges, splits, and fills in PDF files | ”Summarise this PDF…”, “Merge these PDFs…” |
These skills are always available. When you say “create a presentation about Q1 results”, Claude automatically reads the pptx skill before it starts working — ensuring it follows best practices for slide design, layout, and formatting.
How do built-in skills actually work?
Each skill is essentially a SKILL.md file — a markdown document with detailed instructions. When Claude detects that a task matches a skill (for instance, you mention “spreadsheet” or “.xlsx”), it reads the skill file first, then follows those instructions while completing your task.
This means Claude doesn’t just know how to create an Excel file — it knows how to create a good one: with proper data types, readable formatting, and formulas that actually work.
Plugin skills
Beyond the built-in file skills, Cowork plugins bring dozens of specialised skills for different professional domains:
Data & analysis
data-exploration— Profile a dataset: distributions, nulls, outliersdata-visualization— Create publication-quality charts with Pythoninteractive-dashboard-builder— Build HTML dashboards with filters and chartssql-queries— Write optimised SQL for any dialect
Finance
journal-entry-prep— Prepare journal entries with debits, credits, and documentationreconciliation— Reconcile GL balances against bank statementsvariance-analysis— Break down financial variances with waterfall charts
Marketing
content-creation— Draft blog posts, social media, emails, press releasescampaign-planning— Plan campaigns with channels, calendar, and KPIsseo-audit— Full SEO audit with keyword research and competitor analysis
Legal
contract-review— Review contracts and flag deviations from your playbooknda-triage— Screen NDAs as green/yellow/red
Product management
feature-spec— Write PRDs with user stories and acceptance criteriaroadmap-management— Prioritise features using RICE, MoSCoW, or ICE
These skills activate either automatically (when Claude detects a matching task) or when you explicitly invoke them. For example, asking “audit my website’s SEO” will trigger the seo-audit skill.
Creating your own skills
Here’s where it gets really interesting. You can create custom skills that teach Claude exactly how you want things done.
Why create custom skills?
Imagine you write a weekly client report. Every week, it needs the same structure: a summary, key metrics, action items, and a professional sign-off. Without a skill, you’d need to explain this format every time. With a skill, you say “write the weekly report” and Claude knows exactly what to do.
Other examples:
- A skill for writing emails in your company’s tone of voice
- A skill for formatting data exports the way your team expects
- A skill for generating invoices with your company’s layout
How to create a skill
The easiest way? Just ask Claude:
Create a skill for writing weekly client reports.
The report should always include:
- A 3-line executive summary
- A table with this week's KPIs vs. last week
- A bullet list of completed tasks
- A section "Next week" with planned actions
- Sign off with "The [Company] Team"
Tone: professional but warm. Language: English.
Format: Word document.
Claude will create a skill folder with a SKILL.md file containing all these instructions. From then on, whenever you say “write the weekly report”, Claude follows your template perfectly.
Skill structure
A skill lives in a folder and consists of at minimum a SKILL.md file:
📁 .skills/
📁 weekly-report/
📄 SKILL.md ← the instructions
📄 template.docx ← optional: a reference template
📁 invoice-generator/
📄 SKILL.md
📄 company-logo.png ← optional: assets
The SKILL.md file is simply a markdown document that describes:
- When the skill should be triggered (what keywords or requests)
- What Claude should do step by step
- How the output should look (format, tone, structure)
Global vs. folder-specific skills
Skills can live in two places:
- Global skills (in
~/.claude/skills/) — available in every Cowork session, regardless of which folder you’re working in. Great for skills you use across projects. - Folder skills (in
your-project/.skills/) — only available when you’re working in that specific folder. Perfect for project-specific formats and workflows.
Skills vs. CLAUDE.md — when to use which?
Both skills and the CLAUDE.md file give Claude instructions. But they serve different purposes:
| CLAUDE.md | Skills | |
|---|---|---|
| Scope | General project context | Specific, repeatable tasks |
| Activation | Always active in the project | Triggered by matching requests |
| Best for | ”Always write in Dutch”, “Target audience is…" | "When I say ‘weekly report’, do this…” |
| Structure | Free-form instructions | Structured recipe with trigger, steps, output |
The sweet spot: Use CLAUDE.md for context that always applies (language, tone, audience, project background) and skills for specific deliverables you produce regularly.
A practical example
Your CLAUDE.md might say:
# Project: Acme Corp Newsletter
- Language: English
- Tone: friendly, informal
- Audience: small business owners
- Brand colour: #FF6B35
And your skill newsletter-draft handles the specific format:
# Newsletter Draft Skill
When asked to draft a newsletter:
1. Start with a personal greeting
2. Include 3-4 short articles (max 150 words each)
3. End with a call-to-action
4. Total length: max 600 words
5. Output as .docx with the brand header
Claude combines both: it knows the overall context and the specific format.
Tips for getting the most out of skills
Start simple. Your first skill doesn’t need to cover every edge case. Start with the basics and refine as you use it.
Be specific about output. Tell Claude exactly what format you want: file type, number of sections, word count, tone. The more specific, the more consistent the results.
Include examples. If you have a report or document that’s exactly how you want it, include it as a reference file in the skill folder. Claude learns quickly from examples.
Test and iterate. After creating a skill, run it a few times. If something isn’t right, update the SKILL.md. Skills are living documents — they get better over time.
Share with your team. Skills can be shared as plugins. If you’ve built a skill that works well, package it so your colleagues can use it too.
What’s next?
Now that you understand skills, you’re ready to explore plugins — bundles of skills, connectors, and tools that extend Claude’s capabilities even further. Read the plugin guide to learn how to install and configure them.
This article is part of the DataWoods Lab series on working productively with Claude.