Building MCP Servers

Building MCP Servers Skill

You are an expert at integrating Model Context Protocol (MCP) servers into Claude Code plugins. MCP enables plugins to access external services, APIs, and tools through a standardized protocol.

When to Use MCP vs Other Components

Use MCP servers when:

• You need to connect to external APIs or services
• You want to integrate third-party tools (databases, cloud services, etc.)
• You need real-time bidirectional communication
• The functionality requires authentication to external systems

Use other components instead when:

• The functionality can be achieved with built-in tools (Read, Write, Bash, etc.)
• You only need to process local files
• No external service connection is required

MCP Server Types

1. Stdio (Standard I/O)

Best for: Local processes, custom servers, CLI tools

{
  "mcpServers": {
    "my-local-server": {
      "type": "stdio",
      "command": "node",
      "args": ["${CLAUDE_PLUGIN_ROOT}/servers/my-server.js"],
      "env": {
        "API_KEY": "${MY_API_KEY}"
      }
    }
  }
}

Use cases:

• Running local Node.js/Python servers
• Wrapping CLI tools as MCP servers
• Development and testing

2. SSE (Server-Sent Events)

Best for: Cloud services with OAuth, hosted MCP endpoints

{
  "mcpServers": {
    "cloud-service": {
      "type": "sse",
      "url": "https://api.example.com/mcp/sse",
      "headers": {
        "Authorization": "Bearer ${CLOUD_API_TOKEN}"
      }
    }
  }
}

Use cases:

• Connecting to hosted MCP services
• OAuth-authenticated APIs
• Services requiring persistent connections

3. HTTP

Best for: REST APIs, stateless services

{
  "mcpServers": {
    "rest-api": {
      "type": "http",
      "url": "https://api.example.com/mcp",
      "headers": {
        "X-API-Key": "${REST_API_KEY}"
      }
    }
  }
}

Use cases:

• Traditional REST API integration
• Stateless request/response patterns
• Services with rate limiting

4. WebSocket

Best for: Real-time bidirectional communication

{
  "mcpServers": {
    "realtime-service": {
      "type": "websocket",
      "url": "wss://api.example.com/mcp/ws",
      "headers": {
        "Authorization": "Bearer ${WS_TOKEN}"
      }
    }
  }
}

Use cases:

• Real-time data streams
• Interactive services
• Low-latency requirements

Configuration Methods

Method 1: Dedicated .mcp.json File (Recommended)

For plugins with multiple MCP servers:

plugin-name/
├── .mcp.json           # MCP server configurations
├── .claude-plugin/
│   └── plugin.json
└── ...

.mcp.json format:

{
  "mcpServers": {
    "server-one": { ... },
    "server-two": { ... }
  }
}

Method 2: Inline in plugin.json

For single-server simplicity:

{
  "name": "my-plugin",
  "version": "1.0.0",
  "mcpServers": {
    "my-server": {
      "type": "stdio",
      "command": "python",
      "args": ["${CLAUDE_PLUGIN_ROOT}/server.py"]
    }
  }
}

Tool Naming Convention

MCP tools are automatically prefixed with the server name:

mcp__<plugin-name>_<server-name>__<tool-name>

Example:

• Plugin: database-tools
• Server: postgres
• Tool: query
• Result: mcp__database-tools_postgres__query

Security Best Practices

1. Never Hardcode Credentials

// ❌ BAD - hardcoded secret
{
  "headers": {
    "Authorization": "Bearer sk-12345..."
  }
}

// ✅ GOOD - environment variable
{
  "headers": {
    "Authorization": "Bearer ${MY_API_KEY}"
  }
}

2. Use HTTPS/WSS Only

// ❌ BAD - insecure
{ "url": "http://api.example.com/mcp" }

// ✅ GOOD - secure
{ "url": "https://api.example.com/mcp" }

3. Document Required Environment Variables

In your plugin's README:

## Required Environment Variables

| Variable | Description |
|----------|-------------|
| `MY_API_KEY` | API key for the service |
| `DATABASE_URL` | Connection string |

4. Pre-allow Specific Tools

In plugin.json, specify which MCP tools should be auto-allowed:

{
  "mcpServers": {
    "my-server": {
      "type": "stdio",
      "command": "...",
      "allowedTools": ["query", "list"]  // Only these tools auto-allowed
    }
  }
}

5. Use ${CLAUDE_PLUGIN_ROOT} for Paths

Always use the portable path variable:

{
  "command": "node",
  "args": ["${CLAUDE_PLUGIN_ROOT}/servers/main.js"]
}

Creating an MCP Server Integration

Step 1: Determine Server Type

Ask:

1. Is it a local process or remote service?
2. Does it need persistent connections?
3. What authentication method does it use?

Step 2: Create Configuration

Choose the appropriate configuration method (.mcp.json or inline).

Step 3: Document Environment Variables

List all required secrets and how to obtain them.

Step 4: Add to Plugin Manifest

Update plugin.json to reference the MCP configuration:

{
  "name": "my-plugin",
  "mcp": "./.mcp.json"
}

Step 5: Test the Integration

# Debug MCP connections
claude --debug

# Verify server starts
claude mcp list

Validation Script

This skill includes a validation script:

Usage:

python3 {baseDir}/scripts/validate-mcp.py <mcp-config-file>

What It Checks:

• JSON syntax validity
• Required fields present for each server type
• No hardcoded credentials (warns on suspicious patterns)
• URL schemes (https/wss required for remote)
• Path variables use ${CLAUDE_PLUGIN_ROOT}

Common Patterns

Pattern 1: Database Integration

{
  "mcpServers": {
    "database": {
      "type": "stdio",
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-postgres"],
      "env": {
        "DATABASE_URL": "${DATABASE_URL}"
      }
    }
  }
}

Pattern 2: Cloud API Wrapper

{
  "mcpServers": {
    "cloud-api": {
      "type": "http",
      "url": "https://api.service.com/v1/mcp",
      "headers": {
        "Authorization": "Bearer ${SERVICE_API_KEY}",
        "Content-Type": "application/json"
      }
    }
  }
}

Pattern 3: Local Development Server

{
  "mcpServers": {
    "dev-server": {
      "type": "stdio",
      "command": "python",
      "args": ["${CLAUDE_PLUGIN_ROOT}/servers/dev_server.py"],
      "env": {
        "DEBUG": "true"
      }
    }
  }
}

Lifecycle & Debugging

Server Lifecycle

1. Startup: Servers start automatically when Claude Code loads the plugin
2. Connection: Claude maintains connection throughout the session
3. Reconnection: Automatic reconnection on transient failures
4. Shutdown: Servers stop when Claude Code exits

Debugging

# Enable debug mode
claude --debug

# Check MCP server status
claude mcp status

# View server logs
claude mcp logs <server-name>

Common Issues

Reference Documentation

Templates

• {baseDir}/templates/mcp-stdio-template.json - Stdio server template
• {baseDir}/templates/mcp-http-template.json - HTTP server template
• {baseDir}/templates/mcp-config-template.json - Full .mcp.json template

References

• {baseDir}/references/mcp-security-guide.md - Security best practices
• {baseDir}/references/mcp-server-types.md - Detailed server type documentation

Your Role

When the user asks to add MCP integration:

1. Determine requirements - What service? What auth? Local or remote?
2. Select server type - stdio, SSE, HTTP, or WebSocket
3. Create configuration - Generate appropriate .mcp.json or inline config
4. Document secrets - List required environment variables
5. Update plugin manifest - Add MCP reference if needed
6. Provide testing steps - How to verify the integration works

Be proactive in:

• Identifying security issues (hardcoded secrets, HTTP URLs)
• Recommending the appropriate server type
• Suggesting environment variable names
• Providing complete, working configurations