These four terms get confused easily. Here's the distinction, with an analogy.

An analogy: a small IT team

• The main agent is you, the lead. You get a request and decide who handles it.
• A subagent is a specialized colleague you hand a precise task to. They don't know your whole history with the request — you give them just the brief they need. They come back with a result, not their full train of thought.
• A skill is a documented procedure any colleague can pull up and apply when the situation matches, without you re-explaining it every time.
• An MCP is access to the team's real tools — the VPN to the firewall, the project tracker account. Without it, an agent can reason and write code, but can't touch anything real.

Technical definitions

Agent (main). The session you drive directly — the only level with the full conversation history, and the one deciding how work is split.

Subagent. An isolated Claude instance spun up for one bounded task: isolated context (doesn't see prior conversation or files already read), a precise mission, only the tools it's been given, and it returns a final result rather than its intermediate steps. This lets work run in parallel and keeps the main session's context from being cluttered with details that only matter to the sub-task.

Skill. A file (often with supporting scripts or templates) that Claude loads automatically when a task matches its description — an extension of capability, usable by any agent, main or sub.

MCP (Model Context Protocol). The standardized protocol connecting Claude to external tools and services (databases, APIs, apps). It's what turns reasoning into an actual, real action on a system.

Why the distinction matters

A common misconception: assuming a "project manager" subagent can direct the others in real time like a human manager. It can't — each subagent is isolated and never talks directly to the others. That's exactly why the documentation-and-tracking agent in this setup only documents and tracks; the decision of who does what always stays with the main agent.