Copilot skills explained: Agent Builder, Copilot Studio and Cowork compared

Most firms’ first agent ends up with one long block of instructions. The letter rules go in, then the filing rules, then the exceptions someone remembered on a Friday afternoon. Every addition makes the agent a little worse at following the rest, and in some places you hit a hard limit on how much you can write.

Skills are Microsoft’s answer. Instead of one ever-growing set of instructions, you package each job as its own skill and the agent picks up the right one when that job comes up. They have arrived in three places at once: Agent Builder, Copilot Studio and Copilot Cowork.

Here is what a skill is, how each of the three handles them, how to package one properly and the honest answer on whether they save you money.

What a skill is

A skill is a folder with one required file, SKILL.md, plus anything else the job needs.

SKILL.md has two parts:

• A name and a description at the top. The description says what the skill does and when to use it. The agent reads this to decide whether the skill fits the request in front of it.
• The instructions underneath. Written in plain Markdown: the steps, what the answer should look like and how to handle the exceptions.

Alongside it you can add reference documents, templates and, in some places, scripts that run a fixed calculation or check the same way every time.

The closest comparison is an office procedures manual. Nobody reads the whole manual before every task. They know what’s in it from the contents page and take down the right procedure when the job needs it. Skills work the same way. The agent sees only the names and descriptions until a request matches, and only then loads the full instructions and files. Microsoft calls this progressive disclosure.

Here is how skills sit alongside the other parts of an agent:

A note on the word itself. “Skills” means other things in Microsoft land too. People Skills is the skills data on staff profile cards, and Microsoft’s own older agent guidance used “skills” for task descriptions written straight into an agent’s instructions. Copilot in SharePoint also has site skills, which we covered in August. This post is about the packaged SKILL.md kind, which follows an open format also used by GitHub Copilot and Anthropic’s Claude.

Why skills are worth a firm’s attention

• The same job, done the same way. A skill turns a process into something the agent follows step by step, rather than something it improvises from a paragraph buried in its instructions.
• Room to be specific. Agents built in Agent Builder have an 8,000-character limit on their instructions. Each skill can hold up to 20,000 characters of its own, and Microsoft says splitting guidance into skills lets you write longer, more specific guidance without the agent getting worse at following it.
• Scripts for the bits that must be exact. Where a skill includes a script, the calculation or check runs the same way every time instead of the model working it out afresh. Microsoft makes the point that this brings more repeatability.
• Write once, use again. In Copilot Studio you can add the same skill to several agents. In Cowork you can share a skill with named colleagues and push updates to them. And because the format is shared, a skill built for one place can usually be uploaded to another.

Where skills live in Microsoft Copilot

Agent Builder (Frontier preview)

Agent Builder is the simple agent maker inside Copilot Chat and Teams. Our comparison with Copilot Studio explains when it’s enough. Skills there are in preview and only available to organisations enrolled in Microsoft’s Frontier programme.

You can describe a skill in plain English and let Agent Builder create it, or upload a package you’ve built. Uploads must be a complete .zip file. Uploading SKILL.md on its own isn’t supported. Each agent can have up to eight skills.

Preview catches worth knowing:

• Skills aren’t available in tenants that use Microsoft Purview Information Barriers.
• An agent can’t have both skills and embedded files yet.
• You can’t reuse one skill across several agents at this stage.

Scripts run in a locked-down sandbox with no internet access and no way to install extra software. They also can’t call your connectors directly. The agent can still use its connectors as normal; the script just can’t reach them.

Copilot Studio

In Copilot Studio, skills only work on agents built on the GitHub Copilot harness. Which harness your agent should be built on explains what that means and why it matters.

There are three ways to add a skill from the Skills section of the agent’s Build tab:

• Upload a SKILL.md file, or a .zip package with its supporting files.
• Create from blank by entering a name, description and instructions.
• Generate with AI, where you describe the skill, answer Copilot’s questions and it writes the skill for you. You can download what it produces to edit or reuse.

Skill names must use lowercase letters, numbers and hyphens only. Test in the Preview tab. If a skill doesn’t appear after you save, it has usually failed a check on its header: a missing name or description, a description that’s too long or a colon that needs quotation marks around it.

Copilot Cowork

In Cowork, skills sit under Customize on the Skills tab, split into your own skills and the built-in ones Microsoft ships. Select Add and Cowork walks you through writing a skill in conversation, or choose Upload skill to import a file.

Cowork accepts a single .md file, or a .zip or .skill archive with SKILL.md at its root. Uploaded skills are saved to your OneDrive. You can share a skill with specific people in your organisation, then select Re-share after you change it so their copy updates.

Cowork will also convert a Claude plugin package when you upload it, including the skills and connectors bundled inside. Useful if your firm already has skills built for Claude.

Side by side

What skills cost, and where they save Copilot Credits

There’s no charge for a skill as such. What costs money is the work an agent does, and where that work is billed by usage, a skill can make it cheaper. It comes down to three things:

1. Less to read every time. Usage-based billing in Cowork and on the GitHub Copilot harness depends on how much the model processes. A process written into an agent’s main instructions gets processed on every request, whether it’s needed or not. As a skill, only its name and description are in view until the job comes up.
2. Less working out. A script that runs a fixed check replaces the model reasoning its way through the same check each time.
3. Fewer attempts. An agent that gets it right first time doesn’t need the follow-up messages, each of which costs credits.

How that plays out on each surface:

Copilot Cowork: a real saving. Cowork bills each task in Copilot Credits based on usage, and skill calls count towards it. Microsoft points out that your custom instructions are included in every task, so a long process kept there rides along on every unrelated job. Moving it into a skill means it’s only loaded for the tasks that need it. Ten habits that keep Cowork costs under control has more ways to keep spend down.

Copilot Studio: a saving, with a catch. Agents on the GitHub Copilot harness are billed in Copilot Credits for everything they do, including building, testing and evaluating, and a Microsoft 365 Copilot licence doesn’t cover them. Developer and trial environments moved to the same usage billing on 1 September 2026, so even experimenting costs credits. Skills make a harness agent cheaper than the same agent with everything crammed into its instructions. They don’t make it cheaper than an agent on the standard harness, which licensed users can use in Microsoft 365 channels at no extra charge. What the GitHub Copilot harness changes and what it costs goes into the detail.

Agent Builder: little to save. Users with a Microsoft 365 Copilot licence don’t pay extra to use agents built in Agent Builder. The reasons to use skills there are better answers and more room for detail, not a smaller bill.

One honest warning: a skill isn’t automatically cheaper. One that loads a 200-page reference document or runs a heavy script on every request can cost more than it saves. Microsoft doesn’t publish a typical saving, and we’d be wary of anyone who quotes you a percentage. The quickest way to check your own case is to run the same job both ways in Cowork and compare the /cost command’s approximate total for the task. It’s an estimate rather than a bill, and it will differ with the model, the effort setting and the length of the job.

How to build and package a skill

1. Write SKILL.md

Here is a simple example for reviewing client letters:

---
name: client-letter-review
description: Reviews a draft client letter against the firm's house style and required wording. Use when someone asks to check, tidy or sign off a client letter before it is sent.
---

# Client letter review

1. Read the draft letter the user provides.
2. Check it against house-style.md for tone, spelling, date format and sign-off.
3. Check that every paragraph in required-wording.md appears unchanged.
4. List the fixes needed, most important first.
5. Give the corrected letter in full underneath.
6. Never send or file the letter. A person signs it off.

What makes a skill work:

• The description does the deciding. Say what the skill does and when it should be used, in one line. Too vague and the agent won’t pick it up. Too broad and it’ll pick it up for the wrong jobs.
• Keep the name simple. Lowercase letters, numbers and hyphens. Copilot Studio requires it and it avoids problems everywhere else.
• Watch for colons. If the description contains a colon, wrap the whole description in quotation marks or the header won’t load.
• Write steps, not essays. Number the steps, say what the output should look like and cover the obvious exceptions.
• Name your files. If the skill uses a reference document or template, mention it by file name in the instructions so the agent knows when to open it.
• Save it as plain UTF-8 text. Some Windows editors add hidden characters at the start of a file that stop it loading.

2. Add the supporting files

Put SKILL.md and everything it refers to in one folder:

client-letter-review/
├── SKILL.md
├── house-style.md
└── required-wording.md

Reference files can be ordinary documents. Agent Builder, for example, accepts Word, PDF, Excel, PowerPoint, CSV, Markdown and image files. In Agent Builder and Copilot Studio a skill can also include scripts for checks that must run identically every time, and Agent Builder lists Python, JavaScript, TypeScript and shell scripts as supported. Keep client data out of skill files. A skill describes how to do a job; the client’s information should come from the agent’s knowledge and tools.

3. Zip it the right way

Zip the contents of the folder, not the folder itself. Open the folder, select everything inside it, right-click and compress it to a ZIP file.

If you right-click the folder and compress that instead, SKILL.md ends up one level down inside the archive. Cowork expects it at the root, and Microsoft’s Agent Builder example shows it there too. This is the most common reason an upload fails.

4. Test it before anyone relies on it

Try requests that should trigger the skill and a few that shouldn’t. Copilot Studio has the Preview tab and Agent Builder has the Try it tab. In Cowork, Microsoft suggests testing a new skill in a fresh conversation. When you change a skill in Copilot Studio, use Replace to upload the new version. In Cowork, Re-share sends the update to the colleagues you shared it with.

The shortcut: let Copilot write the first draft. Copilot Studio’s Generate with AI, Agent Builder’s describe option and Cowork’s guided Add all produce a working skill from a conversation. In Copilot Studio and Cowork you can then download the result, tidy the wording and keep a master copy somewhere the firm controls.

What to sort before skills go live

• Only use skills from sources you trust. A skill is a set of instructions to the AI. One from an unknown source could tell your agent to do things you’d never approve. Cowork reminds you of this the first time you upload one.
• Give each skill an owner and a master copy. Skills drift once people start editing and sharing them. Decide who owns each one and where the approved version lives. Why every agent needs an owner applies to skills too.
• Know where the files are kept. Agent Builder stores skill files in storage scoped to your tenant and keeps their sensitivity labels. Cowork saves skills to the user’s OneDrive.
• Set spending limits before you scale. Copilot Studio agents can have monthly credit limits with a hard stop, set in the Power Platform admin centre. Cowork limits are set per user or group in the Microsoft 365 admin centre. Three numbers to check before your first agent goes live covers the basics.

The short version

Skills let you teach a Copilot agent one job at a time, done your firm’s way, while keeping its main instructions short. The same SKILL.md format now works in Agent Builder, Copilot Studio and Cowork, with different upload rules in each. They can cut Copilot Credit use in Cowork and on Copilot Studio’s GitHub Copilot harness, but they won’t make a metered agent free, and in Agent Builder the gain is quality rather than cost. Writing and testing skills against your own processes is part of the three-hour version of our Copilot agents course and the Advanced Copilot Studio day.

If you want your team’s processes turned into skills an agent can follow, or want to know whether skills would lower what you spend on Copilot Credits, book a free 30-minute call. No pitch, just a straight conversation about what would work for you.

Sources checked

Last checked: 3 October 2026.

• Microsoft Learn, “Custom skills in declarative agents (preview)”, accessed 3 October 2026.
• Microsoft Learn, “Add custom skills to your declarative agent in Agent Builder (preview)”, accessed 3 October 2026.
• Microsoft Learn, “Skills overview for agents” (Copilot Studio), accessed 3 October 2026.
• Microsoft Learn, “Create a skill for an agent” (Copilot Studio), accessed 3 October 2026.
• Microsoft Learn, “Add an existing skill to an agent” (Copilot Studio), accessed 3 October 2026.
• Microsoft Learn, “Manage and delete skills in an agent” (Copilot Studio), accessed 3 October 2026.
• Microsoft Learn, “Customize Copilot Cowork”, accessed 3 October 2026.
• Microsoft Learn, “Manage Copilot Cowork for your organization”, accessed 3 October 2026.
• Microsoft Learn, “Credit usage for Microsoft Copilot Cowork tasks”, accessed 3 October 2026.
• Microsoft Learn, “Overview of usage-based billing” (Copilot Studio), accessed 3 October 2026.
• Microsoft Learn, “Manage costs for agents powered by the GitHub Copilot harness”, accessed 3 October 2026.
• Microsoft Learn, “Govern Copilot Credit consumption for agents powered by the GitHub Copilot harness”, accessed 3 October 2026.
• Microsoft Learn, “Plan licensing and cost” (Microsoft 365 Copilot extensibility), accessed 3 October 2026.