Examples

Complete, production-ready examples demonstrating various features of the LeanMCP SDK.

Basic MCP Server

A minimal MCP server with a simple tool.

import { createHTTPServer, MCPServer, Tool, SchemaConstraint } from "@leanmcp/core";

// Define input schema
class CalculateInput {
  @SchemaConstraint({ description: 'First number' })
  a!: number;
  
  @SchemaConstraint({ description: 'Second number' })
  b!: number;
  
  @SchemaConstraint({ 
    description: 'Operation',
    enum: ['add', 'subtract', 'multiply', 'divide']
  })
  operation!: string;
}

// Service with tools
export class CalculatorService {
  @Tool({ 
    description: 'Perform arithmetic operations',
    inputClass: CalculateInput
  })
  async calculate(input: CalculateInput) {
    let result: number;
    
    switch (input.operation) {
      case 'add':
        result = input.a + input.b;
        break;
      case 'subtract':
        result = input.a - input.b;
        break;
      case 'multiply':
        result = input.a * input.b;
        break;
      case 'divide':
        if (input.b === 0) throw new Error('Division by zero');
        result = input.a / input.b;
        break;
      default:
        throw new Error('Invalid operation');
    }
    
    return { result };
  }
}

// Create and start server
const serverFactory = () => {
  const server = new MCPServer({
    name: "calculator-server",
    version: "1.0.0",
    logging: true
  });
  
  server.registerService(new CalculatorService());
  return server.getServer();
};

await createHTTPServer(serverFactory, { port: 3000, cors: true });

Weather Service

MCP server that fetches weather data from an external API.

import { Tool, SchemaConstraint, Optional } from "@leanmcp/core";
import { retry, formatResponse } from "@leanmcp/utils";

class GetWeatherInput {
  @SchemaConstraint({ 
    description: 'City name',
    minLength: 1
  })
  city!: string;
  
  @Optional()
  @SchemaConstraint({ 
    description: 'Country code (ISO 3166)',
    pattern: '^[A-Z]{2}$'
  })
  country?: string;
  
  @Optional()
  @SchemaConstraint({ 
    description: 'Temperature unit',
    enum: ['celsius', 'fahrenheit'],
    default: 'celsius'
  })
  unit?: string;
}

export class WeatherService {
  private apiKey = process.env.WEATHER_API_KEY!;
  
  @Tool({ 
    description: 'Get current weather for a city',
    inputClass: GetWeatherInput
  })
  async getWeather(input: GetWeatherInput) {
    try {
      const location = input.country 
        ? `${input.city},${input.country}`
        : input.city;
      
      const units = input.unit === 'fahrenheit' ? 'imperial' : 'metric';
      
      // Fetch with retry logic
      const data = await retry(
        async () => {
          const response = await fetch(
            `https://api.openweathermap.org/data/2.5/weather?q=${location}&units=${units}&appid=${this.apiKey}`
          );
          
          if (!response.ok) {
            throw new Error(`Weather API error: ${response.statusText}`);
          }
          
          return response.json();
        },
        { maxRetries: 3, delayMs: 1000 }
      );
      
      const temp = Math.round(data.main.temp);
      const unit = input.unit === 'fahrenheit' ? '°F' : '°C';
      
      return {
        content: [{
          type: "text",
          text: `Weather in ${data.name}: ${data.weather[0].description}, ${temp}${unit}`
        }]
      };
      
    } catch (error) {
      return {
        content: [{
          type: "text",
          text: `Failed to fetch weather: ${error.message}`
        }],
        isError: true
      };
    }
  }
}

Authenticated Service with AWS Cognito

MCP server with AWS Cognito authentication for protected operations.

import { Tool, SchemaConstraint, Optional } from "@leanmcp/core";
import { Authenticated, AuthProvider } from "@leanmcp/auth";

// Initialize auth provider
const authProvider = new AuthProvider('cognito', {
  region: process.env.AWS_REGION,
  userPoolId: process.env.COGNITO_USER_POOL_ID,
  clientId: process.env.COGNITO_CLIENT_ID
});
await authProvider.init();

// Input schemas
class CreatePostInput {
  @SchemaConstraint({ description: 'Post title', minLength: 1, maxLength: 200 })
  title!: string;
  
  @SchemaConstraint({ description: 'Post content', minLength: 1 })
  content!: string;
  
  @Optional()
  @SchemaConstraint({ description: 'Post tags' })
  tags?: string[];
}

class GetPostsInput {
  @Optional()
  @SchemaConstraint({ description: 'Author user ID' })
  authorId?: string;
  
  @Optional()
  @SchemaConstraint({ description: 'Maximum number of posts', minimum: 1, maximum: 100, default: 10 })
  limit?: number;
}

// Blog service with authentication
export class BlogService {
  private posts: any[] = [];
  
  @Tool({ description: 'Create a new blog post (requires authentication)' })
  @Authenticated(authProvider)
  async createPost(input: CreatePostInput) {
    // Token is validated via _meta.authorization.token
    const post = {
      id: `post-${Date.now()}`,
      title: input.title,
      content: input.content,
      tags: input.tags || [],
      createdAt: new Date().toISOString()
    };
    
    this.posts.push(post);
    
    return {
      success: true,
      post
    };
  }
  
  @Tool({ description: 'Get blog posts (public)' })
  async getPosts(input: GetPostsInput) {
    let filtered = this.posts;
    
    if (input.authorId) {
      filtered = filtered.filter(p => p.authorId === input.authorId);
    }
    
    const limit = input.limit || 10;
    const posts = filtered.slice(0, limit);
    
    return {
      posts,
      total: filtered.length
    };
  }
  
  @Tool({ description: 'Delete a blog post (requires authentication)' })
  @Authenticated(authProvider)
  async deletePost(input: { postId: string }) {
    const postIndex = this.posts.findIndex(p => p.id === input.postId);
    
    if (postIndex === -1) {
      throw new Error('Post not found');
    }
    
    this.posts.splice(postIndex, 1);
    
    return {
      success: true,
      message: 'Post deleted successfully'
    };
  }
}

Service with Elicitation

MCP server using elicitation to collect input from users.

import { Tool, SchemaConstraint, Optional } from "@leanmcp/core";
import { Elicitation } from "@leanmcp/elicitation";

class SlackService {
  @Tool({ description: "Create a new Slack channel" })
  @Elicitation({
    title: "Create Channel",
    description: "Please provide channel details",
    fields: [
      {
        name: "channelName",
        label: "Channel Name",
        type: "text",
        required: true,
        validation: {
          pattern: "^[a-z0-9-]+$",
          errorMessage: "Must be lowercase alphanumeric with hyphens"
        }
      },
      {
        name: "isPrivate",
        label: "Private Channel",
        type: "boolean",
        defaultValue: false
      }
    ]
  })
  async createChannel(args: { channelName: string; isPrivate: boolean }) {
    return {
      success: true,
      channelId: `C${Date.now()}`,
      channelName: args.channelName
    };
  }
}

Database Service with Resources

MCP server providing both tools and resources.

import { Tool, Resource, Prompt, SchemaConstraint } from "@leanmcp/core";

interface User {
  id: string;
  name: string;
  email: string;
  createdAt: string;
}

export class DatabaseService {
  private users: User[] = [
    { id: '1', name: 'John Doe', email: '[email protected]', createdAt: '2024-01-01' },
    { id: '2', name: 'Jane Smith', email: '[email protected]', createdAt: '2024-01-02' }
  ];
  
  // Tool: Query users
  @Tool({ 
    description: 'Search users by name or email',
    inputClass: class {
      @SchemaConstraint({ description: 'Search query' })
      query!: string;
    }
  })
  async searchUsers(input: { query: string }) {
    const query = input.query.toLowerCase();
    const results = this.users.filter(u => 
      u.name.toLowerCase().includes(query) || 
      u.email.toLowerCase().includes(query)
    );
    
    return { users: results, count: results.length };
  }
  
  // Resource: Get all users
  @Resource({ 
    description: 'Get all users in the database',
    mimeType: 'application/json'
  })
  async getAllUsers() {
    return {
      users: this.users,
      total: this.users.length,
      lastUpdated: new Date().toISOString()
    };
  }
  
  // Resource: Get database stats
  @Resource({ 
    description: 'Get database statistics',
    mimeType: 'application/json'
  })
  async getDatabaseStats() {
    return {
      totalUsers: this.users.length,
      oldestUser: this.users[0]?.createdAt,
      newestUser: this.users[this.users.length - 1]?.createdAt
    };
  }
  
  // Prompt: Generate user report
  @Prompt({ description: 'Generate a user report prompt' })
  userReport(input: { userId: string }) {
    const user = this.users.find(u => u.id === input.userId);
    
    if (!user) {
      throw new Error('User not found');
    }
    
    return {
      messages: [{
        role: "user",
        content: {
          type: "text",
          text: `Generate a detailed report for user: ${user.name} (${user.email}). Include account age and activity summary.`
        }
      }]
    };
  }
}

Multi-Service Server

Complete server with multiple services.

import { createHTTPServer, MCPServer } from "@leanmcp/core";
import { CalculatorService } from "./services/calculator";
import { WeatherService } from "./services/weather";
import { BlogService } from "./services/blog";
import { DatabaseService } from "./services/database";

const serverFactory = () => {
  const server = new MCPServer({
    name: "multi-service-mcp",
    version: "1.0.0",
    logging: true
  });
  
  // Register all services
  server.registerService(new CalculatorService());
  server.registerService(new WeatherService());
  server.registerService(new BlogService());
  server.registerService(new DatabaseService());
  
  return server.getServer();
};

// Start server
const PORT = process.env.PORT || 3000;

await createHTTPServer(serverFactory, {
  port: PORT,
  cors: true,
  logging: true
});

console.log(`🚀 MCP Server running on http://localhost:${PORT}`);

Testing Your Server

Using cURL

# List all tools
curl -X POST http://localhost:3000/mcp \
  -H "Content-Type: application/json" \
  -d '{
    "jsonrpc": "2.0",
    "method": "tools/list",
    "id": 1
  }'

# Call a tool
curl -X POST http://localhost:3000/mcp \
  -H "Content-Type: application/json" \
  -d '{
    "jsonrpc": "2.0",
    "method": "tools/call",
    "params": {
      "name": "calculate",
      "arguments": {
        "a": 10,
        "b": 5,
        "operation": "add"
      }
    },
    "id": 2
  }'

# Call authenticated tool (with _meta)
curl -X POST http://localhost:3000/mcp \
  -H "Content-Type: application/json" \
  -d '{
    "jsonrpc": "2.0",
    "method": "tools/call",
    "params": {
      "name": "createPost",
      "arguments": {
        "title": "My First Post",
        "content": "Hello, World!"
      },
      "_meta": {
        "authorization": {
          "type": "bearer",
          "token": "YOUR_COGNITO_TOKEN"
        }
      }
    },
    "id": 3
  }'

Using TypeScript Client

async function callMCPTool(toolName: string, args: any, token?: string) {
  const params: any = {
    name: toolName,
    arguments: args
  };
  
  // Add authentication if token provided
  if (token) {
    params._meta = {
      authorization: {
        type: "bearer",
        token: token
      }
    };
  }
  
  const response = await fetch('http://localhost:3000/mcp', {
    method: 'POST',
    headers: {
      'Content-Type': 'application/json'
    },
    body: JSON.stringify({
      jsonrpc: '2.0',
      method: 'tools/call',
      params,
      id: Date.now()
    })
  });
  
  return response.json();
}

// Use it
const result = await callMCPTool('calculate', {
  a: 10,
  b: 5,
  operation: 'multiply'
});

console.log(result);

Next Steps

Core Package

Learn more about decorators and schemas

Authentication

Add authentication to your server

CLI Tool

Scaffold projects quickly

GitHub Examples

More examples on GitHub

Overview

Core Concepts

Packages

Best Practices

Resources

Examples

Examples

Basic MCP Server

Weather Service

Authenticated Service with AWS Cognito

Service with Elicitation

Database Service with Resources

Multi-Service Server

Testing Your Server

Using cURL

Using TypeScript Client

Next Steps

Core Package

Authentication

CLI Tool

GitHub Examples

Overview

Core Concepts

Packages

Best Practices

Resources

​Examples

​Basic MCP Server

​Weather Service

​Authenticated Service with AWS Cognito

​Service with Elicitation

​Database Service with Resources

​Multi-Service Server

​Testing Your Server

​Using cURL

​Using TypeScript Client

​Next Steps

Core Package

Authentication

CLI Tool

GitHub Examples

Examples

Basic MCP Server

Weather Service

Authenticated Service with AWS Cognito

Service with Elicitation

Database Service with Resources

Multi-Service Server

Testing Your Server

Using cURL

Using TypeScript Client

Next Steps