MCP architecture: host, client, and server

🔴 MUST KNOW · Stable idea: a standard integration boundary between AI applications and external capabilities.

Overview

• MCP is a protocol for connecting AI applications to external tools and context.
• The host is the application using the model; a client is its protocol connection to a server; a server exposes supported capabilities.
• The model normally proposes operations, while host and server code perform the actual protocol and business work.

Why this matters

Without a shared protocol, every AI host needs custom integration code for every external service.

An everyday example

A common plug lets different devices connect to compatible sockets while each building still controls its power supply.

A technical example

A support host connects through an MCP client to an insurance server exposing claim tools.

How it works

1. Configure trusted server
2. Establish authenticated connection
3. Exchange supported capabilities
4. Discover allowed tools/context
5. Map model requests to protocol calls
6. Validate results.

Architecture and flow

See the diagram below.

The insurance assistant example

Several approved internal assistants reuse the same policy and claim tools through one server.

When to use it

Shared integrations across hosts, discoverable tool ecosystems, and reusable enterprise capability boundaries.

When to avoid it

A single Node backend calling its own service may only need an ordinary function or REST client.

Interview answer

“MCP standardizes how AI hosts discover and use external tools and context. It complements model tool calling and backend APIs rather than replacing their security.”

Benefits, trade-offs, and production considerations

• MCP transports and negotiation details change; use compatible SDK/protocol versions.
• Stdio fits local process integrations; documented HTTP transports fit remote services.
• The server's credentials and downstream permissions remain your responsibility.
• MCP architecture.

flowchart LR
  U[User] --> H[AI host: app and model orchestration]
  H --> C[MCP client]
  C --> S[Trusted MCP server]
  S --> A[Authorization and validation]
  A --> B[Insurance business services]
  B --> D[Database and APIs]

• The host owns user experience and tool approval policy.
• The client implements the protocol.
• The server publishes capability contracts and enforces execution rules.
• Business services preserve existing domain logic.
• Databases remain behind those services.