Don't Panic — Model Context Protocol (MCP)

Model Context Protocol, or MCP, is the agreed way for an AI application to talk to outside systems. It does not make the model wiser, and it does not replace an API. It gives the model a standard route to live context and actions, which is considerably less mystical than it sounds and considerably more useful than copying information between tabs.

Before MCP, every AI application needed its own special adapter for every database, file store, calendar, or development tool. That is the M times N arrangement: every new application meets every new system and they all exchange introductions. MCP changes the arithmetic to M plus N: build a server for the system, and compatible applications can use that server. The protocol is the shared plug shape, not the device at either end.

The cast has three members. A host is the AI application. Inside it, a client keeps one connection to one server. The server offers the useful things: tools for actions, resources for read-only context, and prompts for reusable interaction templates. One host can have several clients, which is why an assistant can consult files, a database, and an API without turning into a small box of incompatible cables.

The useful distinction is between a tool and a resource. A tool can query a database, send a message, or alter the outside world. A resource supplies data without that side effect. That sounds like vocabulary homework until a tool has permission to do something expensive. Then it becomes the difference between looking through a window and being handed the keys.

For a local server, STDIO transport carries protocol traffic through standard input and output. For a remote service, Streamable HTTP transport uses HTTP and can use authentication. Under either transport, JSON-RPC messages carry discovery, requests, responses, and notifications. A client first learns what a server offers, then lists and calls the tools it actually needs.

Read the Introduction for the architecture and the places MCP fits. Use Slides for the map of hosts, clients, servers, transports, and primitives. Keep the Cheatsheet nearby when names such as tools/list and tools/call start appearing in configuration or code. The Practice Reference is for making a server connection work. Field Notes is for the part that tends to arrive after the first successful connection: deciding what the server should be allowed to do.