Agent Skills vs MCP Servers: What's the Difference?
A skill is instructions: a SKILL.md the agent reads and follows, sometimes with scripts beside it. An MCP server is a running program that gives the agent tools to call, such as querying a database or opening a ticket. Skills tell the agent how to do a job; MCP servers give it new things it can do.
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What is the difference, side by side?
A skill is a file the agent reads. An MCP server is a process the agent talks to.
| Skill | MCP server | |
|---|---|---|
| What it is | Markdown instructions, optionally with scripts and reference files. | A program that exposes tools and data over the Model Context Protocol. |
| How the agent uses it | Reads it when a task matches its description, then follows the steps. | Calls its tools with arguments and gets results back. |
| Where it lives | In the repository, such as .claude/skills/<slug>/. | Runs as a local process or a remote service, set up in the agent's MCP config. |
| Good for | Procedures, conventions, checklists: how your team does a job. | Reaching systems the agent cannot reach on its own: an API with credentials, a database, a ticket tracker. |
| Cost to add | Write a file. Nothing to run or host. | Install, configure and keep a server running. |
What is an MCP server?
A program that speaks the Model Context Protocol, an open standard for connecting AI agents to tools and data. Claude Code, Cursor and other agents can connect to one and call its tools.
Each tool has a name, a description and typed arguments. The agent sees the list, decides when to call one, and reads the result. The server does the actual work: it holds the credentials, runs the query, calls the API.
When should I write a skill instead?
When the agent already has the access it needs and is missing know-how: the steps, the order, the conventions your team follows.
- “How we write a database migration here” is a skill. The agent can already edit files and run commands.
- “How we cut a release” is a skill, even if one step calls a script that ships with it.
- A skill costs little until it is used: the agent reads its description up front and the full text only when a task matches.
When do I need an MCP server?
When the agent needs to reach something it cannot from the terminal and files alone, or when an action should go through fixed, typed calls instead of free-form commands.
- Reading production metrics, querying a ticket tracker, or posting to a chat channel with a stored token.
- Any action you want limited to a few known operations, with the credentials kept out of the agent's context.
Can skills and MCP servers work together?
Yes, and they often do. A skill can tell the agent which MCP tools to call, in what order, and what to check in the results.
Deftro uses both. The skills are SKILL.md files you install with npx @deftro/cli install. @deftro/mcp is an MCP server that lets any MCP client search for skills and install them. Both run with the agent's access, so read either before you add it: what to look for.
More guides
What are agent skills?
What an agent skill is, what goes in a SKILL.md, and how Claude Code, Cursor, Codex and Antigravity load them. With examples you can install.Install Claude Code skills
Where Claude Code looks for skills, how to install one with a single command, and how to keep a team on the same reviewed version.Reviewing agent skills
A skill runs with your agent's access. What can go wrong, what to read before installing one, and how a team reviews each version first.