Let’s use a game analogy.
In games like Dungeons & Dragons, skills are written on parchment scrolls. Likewise, a Skill is a skill, and the parchment scroll is the SKILL document.
Sometimes, for a skill—such as certain spells—to be cast, specific tools are needed. Or, if you have a specific tool, like a crystal ball, the spell becomes more powerful. That tool is MCP.
Think about it: the simplest Skill is actually just a prompt, involving no tool calls at all (no use of tools). Slightly more complex, more powerful Skills require calling MCP. The difference between these Skills is like the difference between beginner magic and advanced magic.
An Agent is a character in the game, like a wizard. Generally speaking, the more spells a wizard knows, the stronger they are. Correspondingly, the more Skills attached to an Agent, the more things it can naturally do.
Multiple wizards can form a magic circle and unleash powerful force. Multiple Agents can form a Workflow and complete more complex tasks.