Phase 8Module 2 of 25

MCP Architecture

When a connection fails, you need to know which layer broke: the app, the connector, or the server process.

A restaurant: the dining room is the host, the waiter is the client, the kitchen is the server.

Visual Workflows

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Overview

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First connection

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Nothing works until initialize succeeds and the client learns what the server can do.

Key Takeaways

1.Three layers: Host (the app), Client (the connector), Server (the capabilities) One client talks to one server — add more servers, add more clients.
2.Startup handshake: initialize, then list tools, resources, and prompts. JSON-RPC carries every request and response on the chosen transport.
3.The host owns the user session. Each client is a 1:1 session with one server.
4.The server answers list_tools, call_tool, list_resources, and read_resource.

Learn elsewhere

→stdio vs HTTP — covered in Transport
→How to implement handlers — covered in MCP Server

Real Example

Scenario

Cursor (host) starts a client that spawns a Postgres MCP server. After initialize, the agent sees query and schema tools.

What you would do

In Model Context Protocol, apply MCP Architecture to this scenario: Cursor (host) starts a client that spawns a Postgres MCP server. Identify the inputs, run the technique, validate the output, and note one thing you would monitor in production.

Commands

Commands to Remember

Host = app the human uses
Client = one connection to one server
Server = exposes tools, resources, prompts
initialize must succeed before any tool call

Cheat Sheet

Quick recap

quick ref

•Host / Client / Server
•One client per server connection
•initialize → list → operate
•JSON-RPC is the message format

Common Mistakes

✕Calling the host the server
✕Sharing one client across many servers
✕Calling tools before initialize finishes