MCP vs Plugin
A live connection to an outside system, versus a packaged bundle of local capabilities
MCP
A protocol and a live server connection that exposes tools reaching an outside system: a database, an API, a service the agent could not otherwise touch. Defined by the connection, not by packaging.
Plugin
A packaged, installable bundle of skills, hooks, and commands, all local to the harness, distributed and versioned as one unit. Defined by packaging, not by any external connection.
Which one do you actually need
These solve different problems and are not really substitutes. MCP is about reaching something outside the agent's existing capabilities. A Plugin is about packaging and distributing capabilities the harness already runs locally, skills, hooks, commands, as one shareable set. A single Plugin can even include the configuration for connecting to an MCP server, the two can compose rather than compete.