Module 2: The What - MCP Architecture Overview
Map out Host, Client, and Server responsibilities in Model Context Protocol layers, routed using standard JSON-RPC communications.
Day 3
MCP Host-Client-Server Architecture
Why this matters
Host-Client-Server: Host/client/server separation keeps security boundaries and lets you swap servers without rewriting the host.
MCP splits responsibilities across three roles:
- Host — user-facing app (Claude Desktop, Cursor) that runs the model.
- Client — protocol agent inside the host; routes JSON-RPC to servers.
- Server — lightweight process exposing tools/resources (Git, DB, filesystem).
MCP Host-Client-Server Layout
Request flow
User asks a question → model chooses a tool → host client sends tools/call to the server → server executes → result returns as structured content for the model.
Common mistakes
- Confusing host with server (the host runs the client; the server is a separate process).
- Hard-coding API keys inside tool implementations instead of env vars.
- Using SSE locally when stdio is simpler and more secure.
Interview checkpoints
- Q: What is host-client-server in MCP? A: One-sentence definition + when it runs in the lifecycle.
- Q: One production pitfall? A: Name transport, auth, or schema mismatch.
Practice
- Basic: Sketch Host-Client-Server on a whiteboard.
- Intermediate: Find it in a real Claude Desktop or Cursor config.
- Advanced: Break it on purpose and document the error message.
Recap
- You can explain host-client-server clearly.
- You know host vs client vs server roles.
- You see how this connects to the next part.
Next: JSON-RPC Layer
Day 4
The MCP Protocol Layer
Why this matters
JSON-RPC Layer: JSON-RPC is the wire format — every tool call is a structured request/response, not ad-hoc HTTP.
On the wire, MCP uses JSON-RPC 2.0. Messages are framed over stdio (local subprocess) or HTTP + SSE (remote).
{
"jsonrpc": "2.0",
"id": 1,
"method": "tools/call",
"params": {
"name": "add_values",
"arguments": { "x": 2, "y": 3 }
}
}Why JSON-RPC
- Language-agnostic — Python, TypeScript, Go servers are all valid.
- Explicit request IDs for async hosts.
- Methods like
tools/list,resources/readare well-defined in the spec.
Common mistakes
- Confusing host with server (the host runs the client; the server is a separate process).
- Hard-coding API keys inside tool implementations instead of env vars.
- Using SSE locally when stdio is simpler and more secure.
Interview checkpoints
- Q: What is json-rpc layer in MCP? A: One-sentence definition + when it runs in the lifecycle.
- Q: One production pitfall? A: Name transport, auth, or schema mismatch.
Practice
- Basic: Sketch JSON-RPC Layer on a whiteboard.
- Intermediate: Find it in a real Claude Desktop or Cursor config.
- Advanced: Break it on purpose and document the error message.
Recap
- You can explain json-rpc layer clearly.
- You know host vs client vs server roles.
- You see how this connects to the next part.
Next: MCP Resources