Elysia MCP Plugin

A comprehensive ElysiaJS plugin for implementing Model Context Protocol (MCP) servers with HTTP transport support.

Features

• HTTP Transport: Full HTTP-based MCP transport with Streamable HTTP
• Session Management: Stateful session handling via headers
• Type-Safe: Built with TypeScript and Zod validation
• Easy Integration: Simple plugin architecture for Elysia apps
• Comprehensive Support: Tools, Resources, Prompts, and Logging
• Custom Logger Support: Use any logger (pino, winston, bunyan, etc.)
• Error Handling: Proper JSON-RPC 2.0 error responses
• Testing: Full unit test coverage with Bun test runner

Installation

bun add elysia-mcp
# or
npm install elysia-mcp

Starter Template

To quickly get started with a pre-configured Elysia MCP project, you can use our starter template:

# Create a new project from the starter template
bun create https://github.com/kerlos/elysia-mcp-starter my-mcp-project

# Navigate to the project
cd my-mcp-project

# Install dependencies
bun install

# Start development server
bun run dev

The elysia-mcp-starter template includes:

• Pre-configured Elysia setup with MCP plugin
• TypeScript configuration
• Development scripts
• Basic project structure
• Example MCP server implementation

Quick Start

import { Elysia } from 'elysia';
import { mcp } from 'elysia-mcp';
import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';
import { z } from 'zod';

const app = new Elysia()
  .use(
    mcp({
      serverInfo: {
        name: 'my-mcp-server',
        version: '1.0.0',
      },
      capabilities: {
        tools: {},
        resources: {},
        prompts: {},
        logging: {},
      },
      setupServer: async (server: McpServer) => {
        // Register your MCP tools, resources, and prompts here
        server.registerTool(
          'echo',
          {
            description: 'Echoes back the provided text',
            inputSchema: {
              text: z.string().describe('Text to echo back'),
            },
          },
          async (args) => {
            return {
              content: [{ type: 'text', text: `Echo: ${args.text}` }],
            };
          }
        );
      },
    })
  )
  .listen(3000);

Usage

Running the Examples

Basic Example:

# Run the basic example server (port 3000)
bun run example

# Or with development mode (auto-restart)
bun run dev

Multiple Servers Example:

# Run the multiple MCP servers example (port 3000)
bun example:multi

This example demonstrates how to create multiple MCP plugins in a single Elysia application:

• Math Operations Plugin (/math) - Basic arithmetic tools:

  • add - Add two numbers
  • multiply - Multiply two numbers
  • power - Calculate base to the power of exponent

• Text Utilities Plugin (/text) - Text processing tools:

  • uppercase - Convert text to uppercase
  • word_count - Count words in text
  • reverse - Reverse text characters
  • replace - Replace text with global matching

Testing with MCP Inspector

1. Install MCP Inspector:

   npx @modelcontextprotocol/inspector
   

2. Connect to your server:

   • Basic Example: http://localhost:3000/mcp
   • Multiple Servers Example:
     • Math Plugin: http://localhost:3000/math
     • Text Plugin: http://localhost:3000/text

Configuration Options

• serverInfo: Server information
• capabilities: MCP capabilities to advertise
• logger: Custom logger instance (pino, winston, etc.) - see Custom Logger section
• setupServer: Callback to register tools, resources, and prompts
• basePath: Base path for MCP endpoints (default: '/mcp')
• enableJsonResponse: Enable JSON response mode instead of SSE streaming
• stateless: Enable stateless mode (no session management)
• authentication: Authentication handler for protected routes
• eventStore: Event store for resumability support

Session Management

The plugin automatically handles session management via the Mcp-Session-Id header. Each session maintains its own state and can be terminated cleanly.

Modular Handler Architecture

The plugin supports a modular handler architecture that allows you to create specialized endpoints for different MCP capabilities:

import {
  mcp,
  ToolsHandler,
  ResourcesHandler,
  PromptsHandler,
} from 'elysia-mcp';

const app = new Elysia().use(
  mcp({
    /* config */
  })
);

Custom Logger

The plugin supports custom logger instances, allowing you to use any logging library that conforms to the ILogger interface:

interface ILogger {
  info(message: string, ...args: unknown[]): void;
  error(message: string, ...args: unknown[]): void;
  warn(message: string, ...args: unknown[]): void;
  debug(message: string, ...args: unknown[]): void;
}

Using Pino Logger

import { Elysia } from 'elysia';
import { mcp } from 'elysia-mcp';
import pino from 'pino';

const logger = pino({ level: 'debug' });

const app = new Elysia()
  .use(
    mcp({
      logger, // Pass your custom logger
      serverInfo: {
        name: 'my-mcp-server',
        version: '1.0.0',
      },
      // ... other options
    })
  )
  .listen(3000);

Using Winston Logger

import winston from 'winston';

const logger = winston.createLogger({
  level: 'debug',
  format: winston.format.json(),
  transports: [new winston.transports.Console()],
});

const app = new Elysia()
  .use(
    mcp({
      logger, // Pass your winston logger
      // ... other options
    })
  )
  .listen(3000);

Default Console Logger

If you don't provide a custom logger, the plugin will use a default console logger when enableLogging is set to true:

const app = new Elysia()
  .use(
    mcp({
      enableLogging: true, // Uses default console logger with colors
      // ... other options
    })
  )
  .listen(3000);

Custom Logger Implementation

You can also implement your own logger:

import type { ILogger } from 'elysia-mcp';

class MyCustomLogger implements ILogger {
  info(message: string, ...args: unknown[]): void {
    // Your custom implementation
  }
  
  error(message: string, ...args: unknown[]): void {
    // Your custom implementation
  }
  
  warn(message: string, ...args: unknown[]): void {
    // Your custom implementation
  }
  
  debug(message: string, ...args: unknown[]): void {
    // Your custom implementation
  }
}

const logger = new MyCustomLogger();

const app = new Elysia()
  .use(
    mcp({
      logger,
      // ... other options
    })
  )
  .listen(3000);

API Reference

Tools

Register tools using the MCP Server instance:

server.registerTool(
  'tool-name',
  {
    description: 'Tool description',
    inputSchema: {
      param: z.string().describe('Parameter description'),
    },
  },
  async (args) => {
    // Tool implementation
    return {
      content: [{ type: 'text', text: 'Tool result' }],
    };
  }
);

Resources

Register resources for file or data access:

server.registerResource(
  'resource-name',
  'resource://uri',
  {
    title: 'Resource Name',
    description: 'Resource description',
  },
  async () => {
    return {
      contents: [
        {
          uri: 'resource://uri',
          mimeType: 'text/plain',
          text: 'Resource content',
        },
      ],
    };
  }
);

Prompts

Register reusable prompt templates following MCP best practices:

server.registerPrompt(
  'prompt-name',
  {
    title: 'Prompt Title',
    description: 'Prompt description',
    argsSchema: {
      param: z.string().describe('Parameter description'),
    },
  },
  async (args) => {
    return {
      description: 'Generated prompt',
      messages: [
        {
          role: 'user',
          content: {
            type: 'text',
            text: `Generated prompt with ${args.param}`,
          },
        },
      ],
    };
  }
);

Testing

Run the comprehensive test suite:

bun test

License

MIT - see LICENSE file for details.

Contributing

Contributions are welcome! Please feel free to submit a Pull Request.

Related

• Model Context Protocol
• ElysiaJS
• MCP TypeScript SDK

Plugin Configuration

Plugin Options

interface MCPPluginOptions {
  /**
   * Base path for MCP endpoints (default: '/mcp')
   */
  basePath?: string;

  /**
   * Server information
   */
  serverInfo?: {
    name: string;
    version: string;
  };

  /**
   * MCP server capabilities
   */
  capabilities?: ServerCapabilities;

  /**
   * @deprecated Use logger option instead
   * Enable or disable logging
   */
  enableLogging?: boolean;

  /**
   * Custom logger instance (pino, winston, etc.)
   * If not provided and enableLogging is true, will use default console logger
   */
  logger?: ILogger;

  /**
   * Enable JSON response mode instead of SSE streaming
   */
  enableJsonResponse?: boolean;

  /**
   * Authentication handler
   */
  authentication?: (
    context: McpContext
  ) => Promise<{ authInfo?: AuthInfo; response?: unknown }>;

  /**
   * Setup function to configure the MCP server with tools, resources, and prompts
   */
  setupServer?: (server: McpServer) => void | Promise<void>;

  /**
   * Enable stateless mode (no session management)
   */
  stateless?: boolean;

  /**
   * Provide a custom MCP server instance
   */
  mcpServer?: McpServer;

  /**
   * Event store for resumability support
   */
  eventStore?: EventStore;
}

Architecture

flowchart LR
    A["HTTP Client"] --> B["Elysia HTTP Handler"]
    B --> C["MCP Plugin"]
    C --> D["McpServer<br/>(Singleton)"]