Agent, skill, loop, or MCP: what is the difference?

Four terms get used interchangeably and mean different things. They are layers, not alternatives, and most confusion comes from comparing one layer to another.

An agent is the thing that acts. It takes a goal, chooses steps, uses tools, and checks its own progress. Claude Code, Cursor and Codex are agents, or more precisely harnesses around a model that supply the tool access and the permissions. The agent is the runtime; everything below configures it.

A skill teaches an agent how to do one kind of job well. It is a file, typically a SKILL.md, holding the process, the domain knowledge, and the conventions for that job, and the agent loads it when the work matches. A skill is knowledge that persists across tasks. It has no finish line of its own, because it is not a task. Anthropic’s skills format is the common convention, and the ones here follow it.

A loop is one job with a checkable finish line. It has a goal prompt, a verifier prompt, and a stopping condition, which is what lets it run unattended: “done” becomes a fact rather than the agent’s opinion. A skill makes an agent capable; a loop gives it a bounded job and a definition of finished. You can run a loop that uses a skill, and usually do.

An MCP is how an agent reaches something outside itself: a database, a CRM, an API, a search index. It is a connection, not a capability and not a task. When a loop says it needs an MCP, it is naming an external system it cannot work without.

The practical version:

The agent runs. The skill teaches it how. The loop tells it what and when to stop. The MCP connects it to the thing it needs.

Most marketing work that fails with agents fails because a loop was missing, not a skill or an MCP. The agent was capable and connected, and nobody had defined what finished meant, so it stopped when its output looked complete. That is the most common failure and the cheapest to fix.

VibeMarketers publishes one verified marketing agent system every week, with the installable files.