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Tools and the Model Context Protocol  ›  Lesson

MCP — Hosts, Clients and Servers

MCP Hosts 15 minThe Model Context Protocol is an open standard in which a host application runs one client per connection to servers that expose tools, resources and prompts.

MCP — Hosts, Clients and Servers

The railway company builds a timetable integration for Tripmate. Then a coding assistant wants the same timetable, then a chat app, then a calendar app. Without a shared standard, every app–service pair needs its own custom glue. The Model Context Protocol (MCP) is an open standard that lets a service describe its tools once and have any compatible AI application use them.

The rule

The host is the app, a client is one connection, a server offers capabilities.

MCP has three roles:

  • Host — the AI application the user actually works in: a chat app, a code editor, or your own agent such as Tripmate. It manages the conversation with the model and decides what the model is allowed to use.
  • Client — a connector inside the host. Each client holds one connection to one server. A host talking to three servers runs three clients.
  • Server — a program that exposes capabilities over the protocol. The main kinds are tools (actions the model can call), resources (data the app can read, like a file or a record), and prompts (reusable templates).
Host: Tripmate app ─┬─ client ──→ server: railway timetable (tools: search_trains)
                    ├─ client ──→ server: calendar (resources: events; tools: add_event)
                    └─ client ──→ server: files (resources: trip documents)

Messages between client and server follow a fixed request–response format (MCP uses JSON-RPC). When a client connects, it asks the server what it offers; the host can then pass those tool definitions to the model just like tools it defined itself.

The payoff is arithmetic. With 4 apps and 5 services, custom glue means up to 4 × 5 = 20 integrations. With a shared protocol, each app supports MCP once and each service ships one server: 4 + 5 = 9 pieces.

The shortcut

To place any piece in MCP, ask "Does it hold the user and the model, hold a connection, or hold the capabilities?" User and model → host. One connection → client. Capabilities → server.

Asha asks Tripmate to add the train to her calendarStep 1 of 4

  1. Host decides

    Tripmate (the host) gives the model the calendar server's add_event tool alongside its own.

A common misunderstanding is that MCP makes tools safe or smart. It only standardises the connection. The host still decides which servers to trust, and a tool from a server is exactly as risky as what it can change.

Drills

Answer fast — trust the rule.

quick drill

In MCP, what is a code editor with a built-in AI assistant?

quick drill

A host connects to three MCP servers. How many clients does it run?

quick drill

Which is a capability an MCP server exposes?

quick drill

With 3 AI apps and 6 services, how many pieces does a shared protocol need, compared with custom glue for every pair?

Remember

  • Host is the app, each client is one connection, a server offers tools, resources and prompts.
  • Shortcut: holds the user and model → host; one connection → client; capabilities → server.
  • Trap: assuming MCP makes a tool trustworthy — it standardises the plug, not the safety.

Practice — level up

0/1 solved

Write your answer, then compare it with a rubric and a model answer.

ExplainName the MCP roles

A code editor with an AI assistant connects to a GitHub server and a database server. Explain which part is the host, how many clients there are, what the servers provide, and why this beats writing custom integrations.