The architecture: host, client, server, transport

MCP has four roles, and keeping them straight makes everything else click.

Host — the AI application the user actually interacts with (Claude Desktop, Claude Code, an IDE). The host coordinates everything and, crucially, is where user consent lives.

Client — a connector inside the host that maintains exactly one connection to one server. If a host talks to three servers, it runs three clients. This 1:1 pairing keeps server boundaries clean.

Server — the program you write. It exposes capabilities (tools, resources, prompts). A server runs locally (launched by the host, talking over standard input/output) or remotely (an HTTP endpoint serving many clients).

Transport — how bytes move. Two options today: stdio for local servers, and Streamable HTTP for remote ones. (The old "HTTP+SSE" transport is deprecated; SSE now only exists as an optional response stream inside Streamable HTTP.)

Under that, MCP separates a data layer (the JSON-RPC messages: discovering capabilities, calling tools, sending notifications) from the transport layer (stdio or HTTP, plus auth). You mostly work at the data layer; the SDK handles transport.

The single biggest change in the current spec: MCP is now stateless. The old initialize handshake is gone. Instead, every request self-describes — it carries the protocol version, the client's capabilities, and client info in a metadata field, and the server echoes its own info back on every result. There's a server/discover call a client may make up front to learn what a server supports, but it's optional because each request already negotiates itself. Protocol-level sessions (the old Mcp-Session-Id) are also gone; if a server needs to remember something across calls, it mints an explicit handle (like a cart ID) and receives it back as a normal argument.

Why does statelessness matter to you as a builder? It makes remote servers far easier to scale — no sticky sessions, any instance can handle any request — which is exactly what enterprises hit walls on with the older design.