Model Context Protocol. A protocol for plugging external tool servers into a harness — how an agent gets tools beyond what the harness ships with. The agent never "calls MCP"; it calls a tool, and the harness happens to have gotten that tool from an MCP server. Also exposes resources (read-only data) and prompts (reusable templates), but tool provision is the primary use.
Usage:
"The agent needs to read tickets from Linear."
"Configure the harness to use the Linear MCP server — it exposes the Linear API as tools the agent can call. Saves you writing custom tool wrappers."