Custom Skills in Cowork: Hands-On with SKILL.md

Post 5 of 14 — The Skills Post

Introduction

The 13 built-in skills are the floor, not the ceiling. Cowork allows each user to create up to 20 custom skills using SKILL.md files. The format is the open Agent Skills standard originally developed by Anthropic and adopted across multiple agent products. This post is the hands-on one: how to enable them, how to write one, and what to know before you ship.

Writing Your First Custom Skill

Where custom skills live. In your OneDrive, under /Documents/Cowork/Skills/. Each skill is a subfolder containing a SKILL.md file:

/Documents/Cowork/Skills/
  weekly-report/
    SKILL.md
  client-brief/
    SKILL.md
  expense-reconciliation/
    SKILL.md

How to enable them. No admin toggle is required at the user level. Cowork discovers your skills automatically at the start of each conversation by scanning that folder. If you add a new skill mid-conversation, start a new task to pick it up.

Anatomy of a SKILL.md file. The format requires YAML frontmatter with two fields — name and description — followed by Markdown instructions:

---
name: weekly-report
description: Compiles a weekly status report from my calendar, email, and Teams activity. Use when I ask for "my weekly report" or "wrap up the week."
---

# Weekly Report

When invoked, do the following:

1. Pull all meetings I attended in the past 7 days from my calendar.
2. Identify action items I committed to, from meeting notes and follow-up emails.
3. List open items still awaiting my response.
4. Summarise key decisions made in Teams channels I own.

## Output

Produce a Word document titled "Weekly Report — [date range]" with these sections:
- This week (what I did)
- Open items (what I owe)
- Decisions made
- Next week's priorities

Save to /Documents/Reports/Weekly/ and notify me when ready.

## Guidelines

- Do not include private 1:1 meeting content unless I explicitly ask.
- Use a direct, factual tone.
- Cap each section at 6 bullets.

Why the description matters. Cowork uses the description field to decide when to invoke the skill. A vague description (“helps with reports”) will be ignored or used wrongly. A specific description (“Use when I ask for my weekly report or ‘wrap up the week’”) triggers reliably.

Progressive disclosure. A skill can reference other files in its folder (reference.md, examples.md, scripts), and Cowork loads those only when relevant. Keep the core SKILL.md lean.

Limits and warnings.

Maximum 20 custom skills per user.
Custom skills are not validated by Microsoft. Review their outputs carefully — especially anything that sends external email or modifies shared content.
Skills cannot bypass approvals. Sensitive actions still trigger the approval prompt.
Skills cannot grant Cowork access to data the user does not have.

Practical first skills to build:

A weekly-report skill (above).
A client-brief skill that pulls account history before a call.
An expense-reconciliation skill against a known SharePoint folder.
A board update skill enforcing your house style.

Where the standard comes from. The SKILL.md format is the open Agent Skills standard, with examples and a template at github.com/anthropics/skills and a specification at agentskills.io. Cowork implements this standard for your custom skills.

Takeaway

Custom skills are what you use to move Cowork from a generic assistant to an organizational teammate. Start with one skill, write a sharp description, test it, then iterate. Twenty slots are more than you think — and fewer than you will eventually want.

Custom Skills in Cowork: Hands-On with SKILL.md