MCP

The host process acts as the container and coordinator. It creates and manages multiple client instances, controls client connection permissions and lifecycle, enforces security policies and user consent requirements, and coordinates LLM integration and sampling.

Official

Each client is created by the host and maintains a stateful, isolated 1:1 session with a specific server. It handles protocol negotiation, capability exchange, bidirectional message routing, and subscription management.

Servers provide specialized context and capabilities via three primitives: Resources (context data), Prompts (templated workflows), and Tools (executable functions). Servers operate independently with focused responsibilities and can be local processes or remote services.

Official

Server-side primitives define the capabilities a server can offer: Resources provide structured data for the model's context window; Prompts are templated messages and workflow instructions; Tools are executable functions the model can invoke to retrieve information or perform actions.

Client-side primitives define capabilities the client exposes to servers: Roots give servers access to filesystem or URI boundaries on the client side; Sampling allows servers to request LLM completions, enabling agentic and recursive behaviors.

MCP uses JSON-RPC 2.0 as its base message format. The transport layer is pluggable: initial versions used stdio streams; later versions added HTTP with Server-Sent Events (SSE). The protocol is stateful within a session.

Official

During session initialization, clients and servers explicitly declare their supported features. This capability-based negotiation determines which protocol features and primitives are available for the session, ensuring forward and backward compatibility.