Claude Desktop Extension Builder

Build Claude Desktop Extensions using the Model Context Protocol to extend Claude Desktop with custom tools and capabilities.

What It Does

• Creates MCP-compatible server implementations
• Generates valid extension manifest files
• Sets up proper tool definitions and schemas
• Configures stdio transport for local execution
• Enables custom functionality in Claude Desktop

How to Use

Run this command when you want to create a new Desktop Extension:

/claude-desktop-extension

The command will guide you through creating a complete extension with server code and manifest.

Example Structure

A basic DXT includes two main files:

manifest.json

{
  "name": "my-extension",
  "version": "1.0.0",
  "description": "Custom tools for Claude",
  "main": "server.js",
  "mcp": {
    "transport": "stdio"
  }
}

server.js

import { Server } from '@modelcontextprotocol/sdk/server/index.js';
import { StdioServerTransport } from '@modelcontextprotocol/sdk/server/stdio.js';

const server = new Server({
  name: 'my-extension',
  version: '1.0.0'
}, {
  capabilities: {
    tools: {}
  }
});

server.setRequestHandler('tools/list', async () => ({
  tools: [{
    name: 'my_tool',
    description: 'Does something useful',
    inputSchema: {
      type: 'object',
      properties: {
        input: { type: 'string' }
      }
    }
  }]
}));

server.setRequestHandler('tools/call', async (request) => {
  const { name, arguments: args } = request.params;

  if (name === 'my_tool') {
    return {
      content: [{ type: 'text', text: `Processed: ${args.input}` }]
    };
  }

  throw new Error(`Unknown tool: ${name}`);
});

const transport = new StdioServerTransport();
await server.connect(transport);

Use Cases

• File System Tools: Add custom file operations beyond built-in capabilities
• API Integrations: Connect Claude to external services and APIs
• Data Processing: Create specialized data transformation tools
• System Utilities: Access system information or execute commands
• Database Access: Query and modify database contents

Best Practices

• Error Handling: Wrap tool logic in try-catch blocks and return meaningful error messages
• Input Validation: Use JSON Schema to validate tool inputs before processing
• Async Operations: Use async/await for I/O operations and external API calls
• Logging: Add console.error() for debugging (output goes to Claude Desktop logs)
• Tool Naming: Use snake_case for tool names and be descriptive
• Schema Design: Define clear input schemas with required fields and types
• Response Format: Always return structured content arrays
• Testing: Test with sample inputs before deploying to Claude Desktop

Installation

After building your extension:

1. Save files in a dedicated directory
2. Run npm install @modelcontextprotocol/sdk
3. Add to Claude Desktop config:

macOS: ~/Library/Application Support/Claude/claude_desktop_config.json Windows: %APPDATA%/Claude/claude_desktop_config.json

{
  "mcpServers": {
    "my-extension": {
      "command": "node",
      "args": ["/path/to/your/server.js"]
    }
  }
}

4. Restart Claude Desktop

Common Patterns

Reading environment variables:

const apiKey = process.env.API_KEY;
if (!apiKey) {
  throw new Error('API_KEY not configured');
}

Making HTTP requests:

const response = await fetch('https://api.example.com/data');
const data = await response.json();
return { content: [{ type: 'text', text: JSON.stringify(data) }] };

File operations:

import fs from 'fs/promises';
const content = await fs.readFile(args.path, 'utf-8');
return { content: [{ type: 'text', text: content }] };

Troubleshooting

• Extension not appearing: Check Claude Desktop logs for connection errors
• Tool failures: Ensure proper error handling and valid return formats
• Transport issues: Verify stdio transport is configured correctly
• Schema validation: Test input schemas match your tool expectations

Resources

• DXT Documentation: https://github.com/anthropics/dxt
• MCP SDK: https://github.com/modelcontextprotocol
• Examples: https://github.com/anthropics/dxt/tree/main/examples