npm package: @mervin-dalmet/mcp-client
Description: ## Table of Contents
Author: Mervin Dalmet
Latest version: 1.0.2

# MCP TypeScript SDK ![NPM Version](https://img.shields.io/npm/v/%40modelcontextprotocol%2Fsdk) ![MIT licensed](https://img.shields.io/npm/l/%40modelcontextprotocol%2Fsdk)

## Table of Contents

- [Overview](#overview)
- [Installation](#installation)
- [Quickstart](#quick-start)
- [What is MCP?](#what-is-mcp)
- [Core Concepts](#core-concepts)
  - [Server](#server)
  - [Resources](#resources)
  - [Tools](#tools)
  - [Prompts](#prompts)
  - [Completions](#completions)
  - [Sampling](#sampling)
- [Running Your Server](#running-your-server)
  - [stdio](#stdio)
  - [Streamable HTTP](#streamable-http)
  - [Testing and Debugging](#testing-and-debugging)
- [Examples](#examples)
  - [Echo Server](#echo-server)
  - [SQLite Explorer](#sqlite-explorer)
- [Advanced Usage](#advanced-usage)
  - [Dynamic Servers](#dynamic-servers)
  - [Low-Level Server](#low-level-server)
  - [Writing MCP Clients](#writing-mcp-clients)
  - [Proxy Authorization Requests Upstream](#proxy-authorization-requests-upstream)
  - [Backwards Compatibility](#backwards-compatibility)
- [Documentation](#documentation)
- [Contributing](#contributing)
- [License](#license)

## 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

```bash
npm install @modelcontextprotocol/sdk
```

> ⚠️ MCP requires Node v18.x up to work fine.

## Quick Start

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

```typescript
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)](https://modelcontextprotocol.io) 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:

```typescript
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:

```typescript
// 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:

```typescript
// 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'
      }
    ]
  })
);
```

#### ResourceLinks

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:

```typescript
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](#resources) and [prompt completions](#prompts).

#### Client Usage

```typescript
// 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`

```typescript
// 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:

```typescript
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.

```typescript
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.log("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:

```typescript
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](https://modelcontextprotocol.io/specification/2025-03-26/basic/transports#session-management).

```typescript
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. 
> 
> For example, in Node.js you can configure it like this:
> 
> ```ts
> app.use(
>   cors({
>     origin: ['https://your-remote-domain.com, https://your-other-remote-domain.com'],
>     exposedHeaders: ['mcp-session-id'],
>     allowedHeaders: ['Content-Type', 'mcp-session-id'],
>   })
> );
> ```

#### Without Session Management (Stateless)

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

```typescript
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:

```typescript
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](https://github.com/modelcontextprotocol/inspector). See its README for more information.

## Examples

### Echo Server

A simple server demonstrating resources, tools, and prompts:

```typescript
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:

```typescript
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:

```ts
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, string) }]
  })
);
// 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({
        paramSchema: { 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);
```

### Low-Level Server

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

```typescript
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:

```typescript
// 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

```typescript
// This is a placeholder - implement based on your UI framework
async function getInputFromUser(message: string, schema: any): Promise<{
  action: "accept" | "reject" | "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:

```typescript
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:

```typescript
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 tranport can maintain [backwards compatibility](https://modelcontextprotocol.io/specification/2025-03-26/basic/transports#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:

```typescript
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:

```typescript
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

- [Model Context Protocol documentation](https://modelcontextprotocol.io)
- [MCP Specification](https://spec.modelcontextprotocol.io)
- [Example Servers](https://github.com/modelcontextprotocol/servers)

## 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](LICENSE) file for details.