# Plugins, Skills, and MCP: Giving Your Agent New Abilities Out of the box, Claude Code and Codex can read files, write code, and run commands in a folder. Three add-ons let them do much more: skills teach them how, MCP connects them to other programs, and plugins bundle both into one install. This guide explains each in plain English, when to use which, and how to add them safely. It works the same with or without Zo. Official references: [Claude Code skills](https://code.claude.com/docs/en/skills), [Claude Code MCP](https://code.claude.com/docs/en/mcp), [Claude Code plugins](https://code.claude.com/docs/en/plugins), [Codex skills](https://developers.openai.com/codex/skills), and [Codex MCP](https://developers.openai.com/codex/mcp). ## The three ideas, in one line each - **A skill is a recipe.** A folder with a `SKILL.md` file of instructions (and sometimes scripts) that teaches the agent how to do one job well. The agent loads it only when the job comes up. - **MCP is a cable.** Model Context Protocol is a standard plug that connects the agent to another program, such as Unity, Figma, Notion, a browser, or a database, so it can see and operate that program directly. - **A plugin is a box.** A single install that can contain skills, MCP connections, and commands together. Unity's official plugin, for example, ships skills plus its editor connection. Rule of thumb: if the agent does the job badly, it needs a skill. If the agent cannot reach the thing at all, it needs MCP. If a vendor offers a plugin, start there. ## Skills: teach it how Skills follow an open standard ([agentskills.io](https://agentskills.io)), so one skill folder works in Claude Code, Codex, and other agents that support the standard. Where to put them: - **Claude Code:** `.claude/skills//SKILL.md` inside a project, or `~/.claude/skills//SKILL.md` to have it everywhere. - **Codex:** `.agents/skills//SKILL.md` inside a project. See the Codex skills page above for user-wide locations. The easiest way to make one is to ask: `Turn what we just did into a skill so you can do it the same way next time.` The agent writes the folder for you. Good skills come from jobs you have already done twice. Free starting point: the [Classified skill library](https://github.com/Classified-AI-Core/skill-library) has over 200 screened skills (writing, video, research, design, dev) you can drop into either agent. Browse it on the [Resources page](https://www.classified.llc/resources). ## MCP: connect it to things An MCP server is a small program that exposes another tool to your agent. Some run in the cloud (you add a URL and log in), some run on your computer (you add a command). Adding one in **Claude Code**, from a normal terminal: ``` claude mcp add --transport http ``` or, for a server that runs on your machine: ``` claude mcp add -- ``` Inside a session, type `/mcp` to see what is connected and log in to servers that need it. The [MCP quickstart](https://code.claude.com/docs/en/mcp-quickstart) walks through a first server step by step. Adding one in **Codex**: ``` codex mcp add -- ``` Run `codex mcp list` to see what is configured, and `codex mcp login ` for servers that need a login. Many apps now publish their own MCP server or a one-click setup. Search " MCP" and prefer the vendor's official page. ## Plugins: get both in one install In Claude Code, type `/plugin` inside a session to browse, install, and manage plugins. Vendors publish a "marketplace" you add once, then install from: ``` /plugin marketplace add / ``` ``` /plugin install @ ``` Codex has the same idea with `codex plugin marketplace add` and `codex plugin add`. A real worked example is in [Build Games with an AI Agent](https://www.classified.llc/guides/game-dev-with-ai-agents.md), which installs Unity's official plugin in both. ## Safety: what you are actually installing Skills, MCP servers, and plugins run with your agent's permissions on your computer. Treat them like installing software, because that is what they are. - **Prefer official sources.** The vendor's own docs or repository first, a well-known open-source project second, a random link last. - **Read the skill before you trust it.** A `SKILL.md` is plain text. Ask the agent: `Read this skill and tell me everything it would do, including any commands it runs or sites it contacts.` - **Check before connecting accounts.** An MCP server you log in to can act as you in that app. Give it the narrowest access the app allows. - **Keep secrets out of chat.** If a server needs an API key, ask the agent how to store it in an environment variable or config file, not in the conversation. - **Fewer is better.** Every connected server adds tools the agent has to consider. Remove the ones you are not using. ## Ideas worth trying - **Game dev:** Unity's plugin and editor connection, covered in the game dev guide. - **Docs that stay current:** a documentation MCP server so the agent reads today's library docs instead of guessing from training data. - **Design:** a Figma connection so the agent builds from your real designs. - **Your notes and tasks:** Notion, Linear, or Google Drive, so the agent can read the brief and update the ticket itself. - **Your own routines:** any checklist you paste more than twice belongs in a skill. ## Related guides - [Claude Code and Codex: Setup for Beginners](https://www.classified.llc/guides/claude-code-and-codex-setup.md) - [Prompting and Persona Craft](https://www.classified.llc/guides/prompting-and-persona-craft.md) - [Personas, Scopes, and Guardrails](https://www.classified.llc/guides/personas-scopes-guardrails.md) for the deeper version of "what can this thing reach?"