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typescript-sdk

modelcontextprotocol/typescript-sdk

MCP TypeScript SDK NPM Version MIT licensed

Table of Contents

Overview

The Model Context Protocol allows applications to provide context for LLMs in a standardized way, separating the concerns of providing context from the actual LLM interaction. This TypeScript SDK implements the full MCP specification, making it easy to:

  • Build MCP clients that can connect to any MCP server
  • Create MCP servers that expose resources, prompts and tools
  • Use standard transports like stdio and Streamable HTTP
  • Handle all MCP protocol messages and lifecycle events

Installation 

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npm install @modelcontextprotocol/sdk

⚠️ MCP requires Node.js v18.x or higher to work fine.

Quick Start

Let’s create a simple MCP server that exposes a calculator tool and some data:

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import { McpServer, ResourceTemplate } from "@modelcontextprotocol/sdk/server/mcp.js";
import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";
import { z } from "zod";

// Create an MCP server
const server = new McpServer({
  name: "demo-server",
  version: "1.0.0"
});

// Add an addition tool
server.registerTool("add",
  {
    title: "Addition Tool",
    description: "Add two numbers",
    inputSchema: { a: z.number(), b: z.number() }
  },
  async ({ a, b }) => ({
    content: [{ type: "text", text: String(a + b) }]
  })
);

// Add a dynamic greeting resource
server.registerResource(
  "greeting",
  new ResourceTemplate("greeting://{name}", { list: undefined }),
  { 
    title: "Greeting Resource",      // Display name for UI
    description: "Dynamic greeting generator"
  },
  async (uri, { name }) => ({
    contents: [{
      uri: uri.href,
      text: `Hello, ${name}!`
    }]
  })
);

// Start receiving messages on stdin and sending messages on stdout
const transport = new StdioServerTransport();
await server.connect(transport);

What is MCP?

The Model Context Protocol (MCP) lets you build servers that expose data and functionality to LLM applications in a secure, standardized way. Think of it like a web API, but specifically designed for LLM interactions. MCP servers can:

  • Expose data through Resources (think of these sort of like GET endpoints; they are used to load information into the LLM’s context)
  • Provide functionality through Tools (sort of like POST endpoints; they are used to execute code or otherwise produce a side effect)
  • Define interaction patterns through Prompts (reusable templates for LLM interactions)
  • And more!

Core Concepts

Server

The McpServer is your core interface to the MCP protocol. It handles connection management, protocol compliance, and message routing:

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const server = new McpServer({
  name: "my-app",
  version: "1.0.0"
});

Resources

Resources are how you expose data to LLMs. They’re similar to GET endpoints in a REST API - they provide data but shouldn’t perform significant computation or have side effects:

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// Static resource
server.registerResource(
  "config",
  "config://app",
  {
    title: "Application Config",
    description: "Application configuration data",
    mimeType: "text/plain"
  },
  async (uri) => ({
    contents: [{
      uri: uri.href,
      text: "App configuration here"
    }]
  })
);

// Dynamic resource with parameters
server.registerResource(
  "user-profile",
  new ResourceTemplate("users://{userId}/profile", { list: undefined }),
  {
    title: "User Profile",
    description: "User profile information"
  },
  async (uri, { userId }) => ({
    contents: [{
      uri: uri.href,
      text: `Profile data for user ${userId}`
    }]
  })
);

// Resource with context-aware completion
server.registerResource(
  "repository",
  new ResourceTemplate("github://repos/{owner}/{repo}", {
    list: undefined,
    complete: {
      // Provide intelligent completions based on previously resolved parameters
      repo: (value, context) => {
        if (context?.arguments?.["owner"] === "org1") {
          return ["project1", "project2", "project3"].filter(r => r.startsWith(value));
        }
        return ["default-repo"].filter(r => r.startsWith(value));
      }
    }
  }),
  {
    title: "GitHub Repository",
    description: "Repository information"
  },
  async (uri, { owner, repo }) => ({
    contents: [{
      uri: uri.href,
      text: `Repository: ${owner}/${repo}`
    }]
  })
);

Tools

Tools let LLMs take actions through your server. Unlike resources, tools are expected to perform computation and have side effects:

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// Simple tool with parameters
server.registerTool(
  "calculate-bmi",
  {
    title: "BMI Calculator",
    description: "Calculate Body Mass Index",
    inputSchema: {
      weightKg: z.number(),
      heightM: z.number()
    }
  },
  async ({ weightKg, heightM }) => ({
    content: [{
      type: "text",
      text: String(weightKg / (heightM * heightM))
    }]
  })
);

// Async tool with external API call
server.registerTool(
  "fetch-weather",
  {
    title: "Weather Fetcher",
    description: "Get weather data for a city",
    inputSchema: { city: z.string() }
  },
  async ({ city }) => {
    const response = await fetch(`https://api.weather.com/${city}`);
    const data = await response.text();
    return {
      content: [{ type: "text", text: data }]
    };
  }
);

// Tool that returns ResourceLinks
server.registerTool(
  "list-files",
  {
    title: "List Files",
    description: "List project files",
    inputSchema: { pattern: z.string() }
  },
  async ({ pattern }) => ({
    content: [
      { type: "text", text: `Found files matching "${pattern}":` },
      // ResourceLinks let tools return references without file content
      {
        type: "resource_link",
        uri: "file:///project/README.md",
        name: "README.md",
        mimeType: "text/markdown",
        description: 'A README file'
      },
      {
        type: "resource_link",
        uri: "file:///project/src/index.ts",
        name: "index.ts",
        mimeType: "text/typescript",
        description: 'An index file'
      }
    ]
  })
);

Tools can return ResourceLink objects to reference resources without embedding their full content. This is essential for performance when dealing with large files or many resources - clients can then selectively read only the resources they need using the provided URIs.

Prompts

Prompts are reusable templates that help LLMs interact with your server effectively:

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import { completable } from "@modelcontextprotocol/sdk/server/completable.js";

server.registerPrompt(
  "review-code",
  {
    title: "Code Review",
    description: "Review code for best practices and potential issues",
    argsSchema: { code: z.string() }
  },
  ({ code }) => ({
    messages: [{
      role: "user",
      content: {
        type: "text",
        text: `Please review this code:\n\n${code}`
      }
    }]
  })
);

// Prompt with context-aware completion
server.registerPrompt(
  "team-greeting",
  {
    title: "Team Greeting",
    description: "Generate a greeting for team members",
    argsSchema: {
      department: completable(z.string(), (value) => {
        // Department suggestions
        return ["engineering", "sales", "marketing", "support"].filter(d => d.startsWith(value));
      }),
      name: completable(z.string(), (value, context) => {
        // Name suggestions based on selected department
        const department = context?.arguments?.["department"];
        if (department === "engineering") {
          return ["Alice", "Bob", "Charlie"].filter(n => n.startsWith(value));
        } else if (department === "sales") {
          return ["David", "Eve", "Frank"].filter(n => n.startsWith(value));
        } else if (department === "marketing") {
          return ["Grace", "Henry", "Iris"].filter(n => n.startsWith(value));
        }
        return ["Guest"].filter(n => n.startsWith(value));
      })
    }
  },
  ({ department, name }) => ({
    messages: [{
      role: "assistant",
      content: {
        type: "text",
        text: `Hello ${name}, welcome to the ${department} team!`
      }
    }]
  })
);

Completions

MCP supports argument completions to help users fill in prompt arguments and resource template parameters. See the examples above for resource completions and prompt completions.

Client Usage 

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// Request completions for any argument
const result = await client.complete({
  ref: {
    type: "ref/prompt",  // or "ref/resource"
    name: "example"      // or uri: "template://..."
  },
  argument: {
    name: "argumentName",
    value: "partial"     // What the user has typed so far
  },
  context: {             // Optional: Include previously resolved arguments
    arguments: {
      previousArg: "value"
    }
  }
});

Display Names and Metadata

All resources, tools, and prompts support an optional title field for better UI presentation. The title is used as a display name, while name remains the unique identifier.

Note: The register* methods (registerTool, registerPrompt, registerResource) are the recommended approach for new code. The older methods (tool, prompt, resource) remain available for backwards compatibility.

Title Precedence for Tools

For tools specifically, there are two ways to specify a title:

  • title field in the tool configuration
  • annotations.title field (when using the older tool() method with annotations)

The precedence order is: title → annotations.title → name

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// Using registerTool (recommended)
server.registerTool("my_tool", {
  title: "My Tool",              // This title takes precedence
  annotations: {
    title: "Annotation Title"    // This is ignored if title is set
  }
}, handler);

// Using tool with annotations (older API)
server.tool("my_tool", "description", {
  title: "Annotation Title"      // This is used as title
}, handler);

When building clients, use the provided utility to get the appropriate display name:

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import { getDisplayName } from "@modelcontextprotocol/sdk/shared/metadataUtils.js";

// Automatically handles the precedence: title → annotations.title → name
const displayName = getDisplayName(tool);

Sampling

MCP servers can request LLM completions from connected clients that support sampling.

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import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";
import { z } from "zod";

const mcpServer = new McpServer({
  name: "tools-with-sample-server",
  version: "1.0.0",
});

// Tool that uses LLM sampling to summarize any text
mcpServer.registerTool(
  "summarize",
  {
    description: "Summarize any text using an LLM",
    inputSchema: {
      text: z.string().describe("Text to summarize"),
    },
  },
  async ({ text }) => {
    // Call the LLM through MCP sampling
    const response = await mcpServer.server.createMessage({
      messages: [
        {
          role: "user",
          content: {
            type: "text",
            text: `Please summarize the following text concisely:\n\n${text}`,
          },
        },
      ],
      maxTokens: 500,
    });

    return {
      content: [
        {
          type: "text",
          text: response.content.type === "text" ? response.content.text : "Unable to generate summary",
        },
      ],
    };
  }
);

async function main() {
  const transport = new StdioServerTransport();
  await mcpServer.connect(transport);
  console.error("MCP server is running...");
}

main().catch((error) => {
  console.error("Server error:", error);
  process.exit(1);
});

Running Your Server

MCP servers in TypeScript need to be connected to a transport to communicate with clients. How you start the server depends on the choice of transport:

stdio

For command-line tools and direct integrations:

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import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";

const server = new McpServer({
  name: "example-server",
  version: "1.0.0"
});

// ... set up server resources, tools, and prompts ...

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

Streamable HTTP

For remote servers, set up a Streamable HTTP transport that handles both client requests and server-to-client notifications.

With Session Management

In some cases, servers need to be stateful. This is achieved by session management.

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import express from "express";
import { randomUUID } from "node:crypto";
import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { StreamableHTTPServerTransport } from "@modelcontextprotocol/sdk/server/streamableHttp.js";
import { isInitializeRequest } from "@modelcontextprotocol/sdk/types.js"



const app = express();
app.use(express.json());

// Map to store transports by session ID
const transports: { [sessionId: string]: StreamableHTTPServerTransport } = {};

// Handle POST requests for client-to-server communication
app.post('/mcp', async (req, res) => {
  // Check for existing session ID
  const sessionId = req.headers['mcp-session-id'] as string | undefined;
  let transport: StreamableHTTPServerTransport;

  if (sessionId && transports[sessionId]) {
    // Reuse existing transport
    transport = transports[sessionId];
  } else if (!sessionId && isInitializeRequest(req.body)) {
    // New initialization request
    transport = new StreamableHTTPServerTransport({
      sessionIdGenerator: () => randomUUID(),
      onsessioninitialized: (sessionId) => {
        // Store the transport by session ID
        transports[sessionId] = transport;
      },
      // DNS rebinding protection is disabled by default for backwards compatibility. If you are running this server
      // locally, make sure to set:
      // enableDnsRebindingProtection: true,
      // allowedHosts: ['127.0.0.1'],
    });

    // Clean up transport when closed
    transport.onclose = () => {
      if (transport.sessionId) {
        delete transports[transport.sessionId];
      }
    };
    const server = new McpServer({
      name: "example-server",
      version: "1.0.0"
    });

    // ... set up server resources, tools, and prompts ...

    // Connect to the MCP server
    await server.connect(transport);
  } else {
    // Invalid request
    res.status(400).json({
      jsonrpc: '2.0',
      error: {
        code: -32000,
        message: 'Bad Request: No valid session ID provided',
      },
      id: null,
    });
    return;
  }

  // Handle the request
  await transport.handleRequest(req, res, req.body);
});

// Reusable handler for GET and DELETE requests
const handleSessionRequest = async (req: express.Request, res: express.Response) => {
  const sessionId = req.headers['mcp-session-id'] as string | undefined;
  if (!sessionId || !transports[sessionId]) {
    res.status(400).send('Invalid or missing session ID');
    return;
  }
  
  const transport = transports[sessionId];
  await transport.handleRequest(req, res);
};

// Handle GET requests for server-to-client notifications via SSE
app.get('/mcp', handleSessionRequest);

// Handle DELETE requests for session termination
app.delete('/mcp', handleSessionRequest);

app.listen(3000);

[!TIP] When using this in a remote environment, make sure to allow the header parameter mcp-session-id in CORS. Otherwise, it may result in a Bad Request: No valid session ID provided error. Read the following section for examples.

CORS Configuration for Browser-Based Clients

If you’d like your server to be accessible by browser-based MCP clients, you’ll need to configure CORS headers. The Mcp-Session-Id header must be exposed for browser clients to access it:

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import cors from 'cors';

// Add CORS middleware before your MCP routes
app.use(cors({
  origin: '*', // Configure appropriately for production, for example:
  // origin: ['https://your-remote-domain.com', 'https://your-other-remote-domain.com'],
  exposedHeaders: ['Mcp-Session-Id'],
  allowedHeaders: ['Content-Type', 'mcp-session-id'],
}));

This configuration is necessary because:

  • The MCP streamable HTTP transport uses the Mcp-Session-Id header for session management
  • Browsers restrict access to response headers unless explicitly exposed via CORS
  • Without this configuration, browser-based clients won’t be able to read the session ID from initialization responses

Without Session Management (Stateless)

For simpler use cases where session management isn’t needed:

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const app = express();
app.use(express.json());

app.post('/mcp', async (req: Request, res: Response) => {
  // In stateless mode, create a new instance of transport and server for each request
  // to ensure complete isolation. A single instance would cause request ID collisions
  // when multiple clients connect concurrently.
  
  try {
    const server = getServer(); 
    const transport: StreamableHTTPServerTransport = new StreamableHTTPServerTransport({
      sessionIdGenerator: undefined,
    });
    res.on('close', () => {
      console.log('Request closed');
      transport.close();
      server.close();
    });
    await server.connect(transport);
    await transport.handleRequest(req, res, req.body);
  } catch (error) {
    console.error('Error handling MCP request:', error);
    if (!res.headersSent) {
      res.status(500).json({
        jsonrpc: '2.0',
        error: {
          code: -32603,
          message: 'Internal server error',
        },
        id: null,
      });
    }
  }
});

// SSE notifications not supported in stateless mode
app.get('/mcp', async (req: Request, res: Response) => {
  console.log('Received GET MCP request');
  res.writeHead(405).end(JSON.stringify({
    jsonrpc: "2.0",
    error: {
      code: -32000,
      message: "Method not allowed."
    },
    id: null
  }));
});

// Session termination not needed in stateless mode
app.delete('/mcp', async (req: Request, res: Response) => {
  console.log('Received DELETE MCP request');
  res.writeHead(405).end(JSON.stringify({
    jsonrpc: "2.0",
    error: {
      code: -32000,
      message: "Method not allowed."
    },
    id: null
  }));
});


// Start the server
const PORT = 3000;
setupServer().then(() => {
  app.listen(PORT, (error) => {
    if (error) {
      console.error('Failed to start server:', error);
      process.exit(1);
    }
    console.log(`MCP Stateless Streamable HTTP Server listening on port ${PORT}`);
  });
}).catch(error => {
  console.error('Failed to set up the server:', error);
  process.exit(1);
});

This stateless approach is useful for:

  • Simple API wrappers
  • RESTful scenarios where each request is independent
  • Horizontally scaled deployments without shared session state

DNS Rebinding Protection

The Streamable HTTP transport includes DNS rebinding protection to prevent security vulnerabilities. By default, this protection is disabled for backwards compatibility.

Important: If you are running this server locally, enable DNS rebinding protection:

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const transport = new StreamableHTTPServerTransport({
  sessionIdGenerator: () => randomUUID(),
  enableDnsRebindingProtection: true,

  allowedHosts: ['127.0.0.1', ...],
  allowedOrigins: ['https://yourdomain.com', 'https://www.yourdomain.com']
});

Testing and Debugging

To test your server, you can use the MCP Inspector. See its README for more information.

Examples

Echo Server

A simple server demonstrating resources, tools, and prompts:

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import { McpServer, ResourceTemplate } from "@modelcontextprotocol/sdk/server/mcp.js";
import { z } from "zod";

const server = new McpServer({
  name: "echo-server",
  version: "1.0.0"
});

server.registerResource(
  "echo",
  new ResourceTemplate("echo://{message}", { list: undefined }),
  {
    title: "Echo Resource",
    description: "Echoes back messages as resources"
  },
  async (uri, { message }) => ({
    contents: [{
      uri: uri.href,
      text: `Resource echo: ${message}`
    }]
  })
);

server.registerTool(
  "echo",
  {
    title: "Echo Tool",
    description: "Echoes back the provided message",
    inputSchema: { message: z.string() }
  },
  async ({ message }) => ({
    content: [{ type: "text", text: `Tool echo: ${message}` }]
  })
);

server.registerPrompt(
  "echo",
  {
    title: "Echo Prompt",
    description: "Creates a prompt to process a message",
    argsSchema: { message: z.string() }
  },
  ({ message }) => ({
    messages: [{
      role: "user",
      content: {
        type: "text",
        text: `Please process this message: ${message}`
      }
    }]
  })
);

SQLite Explorer

A more complex example showing database integration:

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import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import sqlite3 from "sqlite3";
import { promisify } from "util";
import { z } from "zod";

const server = new McpServer({
  name: "sqlite-explorer",
  version: "1.0.0"
});

// Helper to create DB connection
const getDb = () => {
  const db = new sqlite3.Database("database.db");
  return {
    all: promisify<string, any[]>(db.all.bind(db)),
    close: promisify(db.close.bind(db))
  };
};

server.registerResource(
  "schema",
  "schema://main",
  {
    title: "Database Schema",
    description: "SQLite database schema",
    mimeType: "text/plain"
  },
  async (uri) => {
    const db = getDb();
    try {
      const tables = await db.all(
        "SELECT sql FROM sqlite_master WHERE type='table'"
      );
      return {
        contents: [{
          uri: uri.href,
          text: tables.map((t: {sql: string}) => t.sql).join("\n")
        }]
      };
    } finally {
      await db.close();
    }
  }
);

server.registerTool(
  "query",
  {
    title: "SQL Query",
    description: "Execute SQL queries on the database",
    inputSchema: { sql: z.string() }
  },
  async ({ sql }) => {
    const db = getDb();
    try {
      const results = await db.all(sql);
      return {
        content: [{
          type: "text",
          text: JSON.stringify(results, null, 2)
        }]
      };
    } catch (err: unknown) {
      const error = err as Error;
      return {
        content: [{
          type: "text",
          text: `Error: ${error.message}`
        }],
        isError: true
      };
    } finally {
      await db.close();
    }
  }
);

Advanced Usage

Dynamic Servers

If you want to offer an initial set of tools/prompts/resources, but later add additional ones based on user action or external state change, you can add/update/remove them after the Server is connected. This will automatically emit the corresponding listChanged notifications:

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import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { z } from "zod";

const server = new McpServer({
  name: "Dynamic Example",
  version: "1.0.0"
});

const listMessageTool = server.tool(
  "listMessages",
  { channel: z.string() },
  async ({ channel }) => ({
    content: [{ type: "text", text: await listMessages(channel) }]
  })
);

const putMessageTool = server.tool(
  "putMessage",
  { channel: z.string(), message: z.string() },
  async ({ channel, message }) => ({
    content: [{ type: "text", text: await putMessage(channel, message) }]
  })
);
// Until we upgrade auth, `putMessage` is disabled (won't show up in listTools)
putMessageTool.disable()

const upgradeAuthTool = server.tool(
  "upgradeAuth",
  { permission: z.enum(["write", "admin"])},
  // Any mutations here will automatically emit `listChanged` notifications
  async ({ permission }) => {
    const { ok, err, previous } = await upgradeAuthAndStoreToken(permission)
    if (!ok) return {content: [{ type: "text", text: `Error: ${err}` }]}

    // If we previously had read-only access, 'putMessage' is now available
    if (previous === "read") {
      putMessageTool.enable()
    }

    if (permission === 'write') {
      // If we've just upgraded to 'write' permissions, we can still call 'upgradeAuth' 
      // but can only upgrade to 'admin'. 
      upgradeAuthTool.update({
        paramsSchema: { permission: z.enum(["admin"]) }, // change validation rules
      })
    } else {
      // If we're now an admin, we no longer have anywhere to upgrade to, so fully remove that tool
      upgradeAuthTool.remove()
    }
  }
)

// Connect as normal
const transport = new StdioServerTransport();
await server.connect(transport);

Improving Network Efficiency with Notification Debouncing

When performing bulk updates that trigger notifications (e.g., enabling or disabling multiple tools in a loop), the SDK can send a large number of messages in a short period. To improve performance and reduce network traffic, you can enable notification debouncing.

This feature coalesces multiple, rapid calls for the same notification type into a single message. For example, if you disable five tools in a row, only one notifications/tools/list_changed message will be sent instead of five.

[!IMPORTANT] This feature is designed for “simple” notifications that do not carry unique data in their parameters. To prevent silent data loss, debouncing is automatically bypassed for any notification that contains a params object or a relatedRequestId. Such notifications will always be sent immediately.

This is an opt-in feature configured during server initialization.

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import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";

const server = new McpServer(
  {
    name: "efficient-server",
    version: "1.0.0"
  },
  {
    // Enable notification debouncing for specific methods
    debouncedNotificationMethods: [
      'notifications/tools/list_changed',
      'notifications/resources/list_changed',
      'notifications/prompts/list_changed'
    ]
  }
);

// Now, any rapid changes to tools, resources, or prompts will result
// in a single, consolidated notification for each type.
server.registerTool("tool1", ...).disable();
server.registerTool("tool2", ...).disable();
server.registerTool("tool3", ...).disable();
// Only one 'notifications/tools/list_changed' is sent.

Low-Level Server

For more control, you can use the low-level Server class directly:

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import { Server } from "@modelcontextprotocol/sdk/server/index.js";
import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";
import {
  ListPromptsRequestSchema,
  GetPromptRequestSchema
} from "@modelcontextprotocol/sdk/types.js";

const server = new Server(
  {
    name: "example-server",
    version: "1.0.0"
  },
  {
    capabilities: {
      prompts: {}
    }
  }
);

server.setRequestHandler(ListPromptsRequestSchema, async () => {
  return {
    prompts: [{
      name: "example-prompt",
      description: "An example prompt template",
      arguments: [{
        name: "arg1",
        description: "Example argument",
        required: true
      }]
    }]
  };
});

server.setRequestHandler(GetPromptRequestSchema, async (request) => {
  if (request.params.name !== "example-prompt") {
    throw new Error("Unknown prompt");
  }
  return {
    description: "Example prompt",
    messages: [{
      role: "user",
      content: {
        type: "text",
        text: "Example prompt text"
      }
    }]
  };
});

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

Eliciting User Input

MCP servers can request additional information from users through the elicitation feature. This is useful for interactive workflows where the server needs user input or confirmation:

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// Server-side: Restaurant booking tool that asks for alternatives
server.tool(
  "book-restaurant",
  { 
    restaurant: z.string(),
    date: z.string(),
    partySize: z.number()
  },
  async ({ restaurant, date, partySize }) => {
    // Check availability
    const available = await checkAvailability(restaurant, date, partySize);
    
    if (!available) {
      // Ask user if they want to try alternative dates
      const result = await server.server.elicitInput({
        message: `No tables available at ${restaurant} on ${date}. Would you like to check alternative dates?`,
        requestedSchema: {
          type: "object",
          properties: {
            checkAlternatives: {
              type: "boolean",
              title: "Check alternative dates",
              description: "Would you like me to check other dates?"
            },
            flexibleDates: {
              type: "string",
              title: "Date flexibility",
              description: "How flexible are your dates?",
              enum: ["next_day", "same_week", "next_week"],
              enumNames: ["Next day", "Same week", "Next week"]
            }
          },
          required: ["checkAlternatives"]
        }
      });

      if (result.action === "accept" && result.content?.checkAlternatives) {
        const alternatives = await findAlternatives(
          restaurant, 
          date, 
          partySize, 
          result.content.flexibleDates as string
        );
        return {
          content: [{
            type: "text",
            text: `Found these alternatives: ${alternatives.join(", ")}`
          }]
        };
      }
      
      return {
        content: [{
          type: "text",
          text: "No booking made. Original date not available."
        }]
      };
    }
    
    // Book the table
    await makeBooking(restaurant, date, partySize);
    return {
      content: [{
        type: "text",
        text: `Booked table for ${partySize} at ${restaurant} on ${date}`
      }]
    };
  }
);

Client-side: Handle elicitation requests

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// This is a placeholder - implement based on your UI framework
async function getInputFromUser(message: string, schema: any): Promise<{
  action: "accept" | "decline" | "cancel";
  data?: Record<string, any>;
}> {
  // This should be implemented depending on the app
  throw new Error("getInputFromUser must be implemented for your platform");
}

client.setRequestHandler(ElicitRequestSchema, async (request) => {
  const userResponse = await getInputFromUser(
    request.params.message, 
    request.params.requestedSchema
  );
  
  return {
    action: userResponse.action,
    content: userResponse.action === "accept" ? userResponse.data : undefined
  };
});

Note: Elicitation requires client support. Clients must declare the elicitation capability during initialization.

Writing MCP Clients

The SDK provides a high-level client interface:

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import { Client } from "@modelcontextprotocol/sdk/client/index.js";
import { StdioClientTransport } from "@modelcontextprotocol/sdk/client/stdio.js";

const transport = new StdioClientTransport({
  command: "node",
  args: ["server.js"]
});

const client = new Client(
  {
    name: "example-client",
    version: "1.0.0"
  }
);

await client.connect(transport);

// List prompts
const prompts = await client.listPrompts();

// Get a prompt
const prompt = await client.getPrompt({
  name: "example-prompt",
  arguments: {
    arg1: "value"
  }
});

// List resources
const resources = await client.listResources();

// Read a resource
const resource = await client.readResource({
  uri: "file:///example.txt"
});

// Call a tool
const result = await client.callTool({
  name: "example-tool",
  arguments: {
    arg1: "value"
  }
});

Proxy Authorization Requests Upstream

You can proxy OAuth requests to an external authorization provider:

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import express from 'express';
import { ProxyOAuthServerProvider } from '@modelcontextprotocol/sdk/server/auth/providers/proxyProvider.js';
import { mcpAuthRouter } from '@modelcontextprotocol/sdk/server/auth/router.js';

const app = express();

const proxyProvider = new ProxyOAuthServerProvider({
    endpoints: {
        authorizationUrl: "https://auth.external.com/oauth2/v1/authorize",
        tokenUrl: "https://auth.external.com/oauth2/v1/token",
        revocationUrl: "https://auth.external.com/oauth2/v1/revoke",
    },
    verifyAccessToken: async (token) => {
        return {
            token,
            clientId: "123",
            scopes: ["openid", "email", "profile"],
        }
    },
    getClient: async (client_id) => {
        return {
            client_id,
            redirect_uris: ["http://localhost:3000/callback"],
        }
    }
})

app.use(mcpAuthRouter({
    provider: proxyProvider,
    issuerUrl: new URL("http://auth.external.com"),
    baseUrl: new URL("http://mcp.example.com"),
    serviceDocumentationUrl: new URL("https://docs.example.com/"),
}))

This setup allows you to:

  • Forward OAuth requests to an external provider
  • Add custom token validation logic
  • Manage client registrations
  • Provide custom documentation URLs
  • Maintain control over the OAuth flow while delegating to an external provider

Backwards Compatibility

Clients and servers with StreamableHttp transport can maintain backwards compatibility with the deprecated HTTP+SSE transport (from protocol version 2024-11-05) as follows

Client-Side Compatibility

For clients that need to work with both Streamable HTTP and older SSE servers:

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import { Client } from "@modelcontextprotocol/sdk/client/index.js";
import { StreamableHTTPClientTransport } from "@modelcontextprotocol/sdk/client/streamableHttp.js";
import { SSEClientTransport } from "@modelcontextprotocol/sdk/client/sse.js";
let client: Client|undefined = undefined
const baseUrl = new URL(url);
try {
  client = new Client({
    name: 'streamable-http-client',
    version: '1.0.0'
  });
  const transport = new StreamableHTTPClientTransport(
    new URL(baseUrl)
  );
  await client.connect(transport);
  console.log("Connected using Streamable HTTP transport");
} catch (error) {
  // If that fails with a 4xx error, try the older SSE transport
  console.log("Streamable HTTP connection failed, falling back to SSE transport");
  client = new Client({
    name: 'sse-client',
    version: '1.0.0'
  });
  const sseTransport = new SSEClientTransport(baseUrl);
  await client.connect(sseTransport);
  console.log("Connected using SSE transport");
}

Server-Side Compatibility

For servers that need to support both Streamable HTTP and older clients:

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import express from "express";
import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { StreamableHTTPServerTransport } from "@modelcontextprotocol/sdk/server/streamableHttp.js";
import { SSEServerTransport } from "@modelcontextprotocol/sdk/server/sse.js";

const server = new McpServer({
  name: "backwards-compatible-server",
  version: "1.0.0"
});

// ... set up server resources, tools, and prompts ...

const app = express();
app.use(express.json());

// Store transports for each session type
const transports = {
  streamable: {} as Record<string, StreamableHTTPServerTransport>,
  sse: {} as Record<string, SSEServerTransport>
};

// Modern Streamable HTTP endpoint
app.all('/mcp', async (req, res) => {
  // Handle Streamable HTTP transport for modern clients
  // Implementation as shown in the "With Session Management" example
  // ...
});

// Legacy SSE endpoint for older clients
app.get('/sse', async (req, res) => {
  // Create SSE transport for legacy clients
  const transport = new SSEServerTransport('/messages', res);
  transports.sse[transport.sessionId] = transport;
  
  res.on("close", () => {
    delete transports.sse[transport.sessionId];
  });
  
  await server.connect(transport);
});

// Legacy message endpoint for older clients
app.post('/messages', async (req, res) => {
  const sessionId = req.query.sessionId as string;
  const transport = transports.sse[sessionId];
  if (transport) {
    await transport.handlePostMessage(req, res, req.body);
  } else {
    res.status(400).send('No transport found for sessionId');
  }
});

app.listen(3000);

Note: The SSE transport is now deprecated in favor of Streamable HTTP. New implementations should use Streamable HTTP, and existing SSE implementations should plan to migrate.

Documentation

Contributing

Issues and pull requests are welcome on GitHub at https://github.com/modelcontextprotocol/typescript-sdk.

License

This project is licensed under the MIT License—see the LICENSE file for details.

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