Getting Started with Model Context Protocol
Lesson 2: MCP Architecture – Servers, Clients, Hosts, and Transports
Lesson Objectives
By the end of this lesson, students should be able to:
- Describe the roles of host, client, and server in an MCP deployment
- Explain the two primary transport mechanisms (stdio and HTTP/SSE)
- Trace a tool call from user request to tool execution to Claude response
- Map the MCP architecture to a concrete integration scenario
Lesson Content
The three roles.
Host: The application that contains or manages the Claude integration – Claude Code, Claude Desktop, or a custom application that embeds Claude. The host manages one or more MCP clients.
Client: Lives inside the host. Maintains a connection to one MCP server. Passes tool calls from Claude to the server and returns results. One client per server.
Server: A separate process that exposes capabilities (tools, resources, prompts) via the MCP protocol. The server has access to the real resources – the file system, database, API.
In Claude Code: Claude Code is the host; it manages clients for each connected MCP server. When Claude needs a file system tool, the client connected to the file system server handles the call.
Transport mechanisms.
MCP servers communicate with clients via two transports (verify current transport specification at modelcontextprotocol.io):
stdio (Standard I/O): The server runs as a child process of the host; communication happens via stdin/stdout. Simple, low-overhead, appropriate for local servers running on the same machine. The most common transport for development and personal tools.
HTTP with SSE (Server-Sent Events): The server runs as an independent HTTP service; clients connect via HTTP. Appropriate for servers running on remote infrastructure, shared servers accessed by multiple clients, and production deployments.
Tracing a tool call.
- User asks Claude a question that requires external data
- Claude generates a tool call (function name + parameters)
- Host receives the tool call from Claude
- Host passes it to the appropriate client (matched by tool name)
- Client sends the call to the server via the transport
- Server executes the tool and returns the result
- Client returns the result to the host
- Host adds the result to the conversation and sends back to Claude
- Claude generates the final response using the tool result
Configuration.
MCP servers are configured in the host's configuration file. In Claude Code: .claude/mcp_servers.json (verify current configuration path at docs.anthropic.com/en/docs/claude-code/mcp). Each entry specifies the server's launch command (for stdio) or URL (for HTTP).
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