# MCP Server Architecture
## What is an MCP Server?
A **Model Context Protocol (MCP) server** is a Node.js process that implements the MCP specification to extend Claude's capabilities with custom tools and data sources.
## How It Works
### Core Components
1. **Node.js Process**: Your MCP server runs as a separate Node.js process
2. **MCP Protocol**: Follows the Model Context Protocol specification for communication
3. **JSON-RPC Communication**: Uses JSON-RPC 2.0 over stdio (standard input/output)
4. **Tool Registration**: Exposes tools that Claude can discover and execute
### Communication Flow
```
Claude Client (Code/Desktop) ←─────────→ MCP Server (Node.js)
│ │
│ 1. Launch subprocess │
│ ─────────────────────────────→ │
│ │
│ 2. "tools/list" │
│ ─────────────────────────────→ │
│ Tool definitions │
│ ←───────────────────────────── │
│ │
│ 3. "tools/call" │
│ ─────────────────────────────→ │
│ Tool execution result │
│ ←───────────────────────────── │
```
### Integration with Claude
#### Claude Code
- Reads MCP configuration from `~/.claude/mcp.json`
- Launches your server: `node dist/index.js`
- Communicates via stdio transport
#### Claude Desktop
- Reads MCP configuration from platform-specific config file
- Same communication protocol as Claude Code
- Works identically across both clients
### JSON-RPC Protocol
Your server handles these key methods:
**tools/list** - Claude asks what tools are available:
```json
{
"jsonrpc": "2.0",
"id": 1,
"method": "tools/list",
"params": {}
}
```
**tools/call** - Claude executes a specific tool:
```json
{
"jsonrpc": "2.0",
"id": 2,
"method": "tools/call",
"params": {
"name": "hello_world",
"arguments": {"name": "Claude"}
}
}
```
### Server Lifecycle
1. **Startup**: Claude launches your Node.js process
2. **Handshake**: MCP protocol initialization
3. **Tool Discovery**: Claude calls `tools/list` to see available tools
4. **Tool Execution**: Claude calls `tools/call` as needed during conversation
5. **Shutdown**: Process terminates when Claude session ends
## Technical Architecture
### Our WordPress Docs MCP Server
```
src/index.ts
├── Server setup with @modelcontextprotocol/sdk
├── StdioServerTransport for communication
├── ListToolsRequestSchema handler (tool definitions)
└── CallToolRequestSchema handler (tool execution)
```
### Key Dependencies
- **@modelcontextprotocol/sdk**: Official MCP SDK for Node.js
- **TypeScript**: Type safety and modern JavaScript features
- **stdio transport**: Communication channel with Claude
This architecture allows you to create powerful extensions to Claude that can access external APIs, process data, and provide domain-specific functionality - all while maintaining security through the sandboxed subprocess model.
