Four extensibility surfaces, each solving a different problem. Knowing which is which saves you from reaching for the wrong one.
The four
Skills — reusable playbooks in a SKILL.md file. Claude auto-loads one when relevant, or you invoke /skill-name. The body loads only on use, so long reference material costs nothing until needed. This is how you package a repeatable workflow (“fix a GitHub issue end to end”).
Plugins — a bundle of skills, agents, hooks, and MCP servers, installed from a marketplace. Browse and manage them from the desktop app’s + → Plugins panel, no terminal required. This is how capability is shared and distributed.
MCP servers — the open protocol connecting Claude to outside tools: GitHub, Postgres, Slack, Notion, Sentry. Add them as Connectors in the desktop app with a guided OAuth flow. This is how Claude reaches external data and actions.
Hooks — shell commands that fire on lifecycle events (before a tool runs, after an edit, on stop). Deterministic — they always happen, unlike advice in CLAUDE.md. This is how you enforce rather than ask.
The distinction that matters
A skill is capability Claude chooses to use. A hook is automation the harness guarantees. If you catch yourself writing “always run the linter after editing” in CLAUDE.md and it keeps getting skipped — that’s a hook, not a rule. Just ask Claude: “write a hook that runs eslint after every file edit.”
Likewise, a slash command and a skill overlap (both give you /name), but skills add a supporting-files directory, auto-invocation, frontmatter control, and the ability to run in a forked subagent. Reach for a skill when the workflow is more than one line.
A security note on MCP
Project-scoped MCP servers require a trust approval, and anything a server fetches from the outside world is a potential prompt-injection vector. Add connectors you trust, keep the sensitive ones behind auth, and let auto mode’s input screening handle the rest. Tools are also deferred by default — Claude searches for the ones it needs rather than loading every server’s toolset into context — so you can connect many servers without paying for them upfront.