6.4.2. Skills: Prebuilt and Custom
💡 First Principle: A skill is a playbook for one type of task. Cowork doesn't try to know everything at once. It loads the playbook it needs (email, scheduling, a Word document) when a step calls for it. Custom skills let you write your own playbook, so Cowork does a recurring task your way every time.
Built-in skills. Cowork ships with 14 built-in skills: Word, Excel, PowerPoint, PDF, Email, Scheduling, Calendar Management, Meetings, Daily Briefing, Enterprise Search, Communications, Deep Research, Adaptive Cards, and App. The App skill builds lightweight interactive apps from a description and is a preview feature that requires the Frontier program. Each skill matches a type of task. Email composes, replies, forwards and sends; Calendar Management creates events and adds Teams links; Meetings prepares meeting intelligence; Communications drafts stakeholder communications. Cowork loads skills as needed, and loaded skills appear as chips in the side panel, so you can see which playbooks are in use. Image generation is also handled by a built-in image-generation skill.
Custom skills. When the built-in behavior isn't quite right for a recurring task (your team's weekly status format, your customer-escalation checklist), you can create a custom skill:
- Customize > Skills > Add > Create new (guided), or
- ask Cowork to create one in chat, or
- add a
SKILL.mdfile yourself in OneDrive at/Documents/Cowork/skills/<skill-name>/SKILL.md.
A SKILL.md has a short YAML header with a name and a description, followed by Markdown instructions. The description must say when Cowork should use the skill. If it's too long the skill never loads, with no trigger phrases it never fires, and if it's too broad it fires when it shouldn't. Cowork evaluates skills automatically on a 0–100 score (trigger clarity, instruction specificity, scope boundaries, robustness).
Rules to remember:
- Cowork discovers custom skills at the start of each session, so test a changed skill in a new session.
- Limits: up to 50 custom skills; each
SKILL.mdup to 1 MB. - Share a skill with Only you or Specific users in your organization, and select Re-share after you change it.
- Custom skills aren't supported on mobile.
- Microsoft doesn't validate custom skills created by users. A skill runs as instructions to the AI, so only upload skills from sources you trust, and review their outputs carefully.
Plugins are different again: they come from the Microsoft 365 App Store and add skills and connectors (for example for Dynamics 365 Sales, Jira, Salesforce, ServiceNow or Workday). Plugins your admin deploys can't be removed by users.
Your own standing instructions live on the same Customize page, which has three tabs: Preferences (your Cowork custom instructions, applied to every task and limited to 20 KB), Plugins and Skills. They are separate from your Copilot Chat custom instructions (4.5).
| Choose… | When… |
|---|---|
| A built-in skill | The task is a standard type (email, scheduling, a Word document, research) and Microsoft's default behavior is fine |
| A custom skill | You repeat a task that needs your own format, steps or rules every time |
| A plugin | You need Cowork to reach a third-party system, such as a CRM or ticketing tool |
⚠️ Exam Trap: A Cowork custom skill isn't a custom agent. It's a SKILL.md instruction file in OneDrive, not something built in Copilot Studio or Agent Builder. And if you edit a skill mid-session and Cowork ignores the change, nothing is broken: skills are discovered when a session starts, so start a new session.
Reflection Question: Every Friday, a team lead asks Cowork for a status summary in the team's fixed five-heading format, and every week she has to correct the headings. What should she create, where does it live, and what must its description contain for Cowork to use it?
