Model Context Protocol implementation for TypeScript
npm install @blocklet/mcpThe 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
``bash`
npm install @modelcontextprotocol/sdk
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",
version: "1.0.0"
});
// Add an addition tool
server.tool("add",
{ a: z.number(), b: z.number() },
async ({ a, b }) => ({
content: [{ type: "text", text: String(a + b) }]
})
);
// Add a dynamic greeting resource
server.resource(
"greeting",
new ResourceTemplate("greeting://{name}", { list: undefined }),
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);
`
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!
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 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.resource(
"config",
"config://app",
async (uri) => ({
contents: [{
uri: uri.href,
text: "App configuration here"
}]
})
);
// Dynamic resource with parameters
server.resource(
"user-profile",
new ResourceTemplate("users://{userId}/profile", { list: undefined }),
async (uri, { userId }) => ({
contents: [{
uri: uri.href,
text: Profile data for user ${userId}
}]
})
);
Tools let LLMs take actions through your server. Unlike resources, tools are expected to perform computation and have side effects:
`typescript
// Simple tool with parameters
server.tool(
"calculate-bmi",
{
weightKg: z.number(),
heightM: z.number()
},
async ({ weightKg, heightM }) => ({
content: [{
type: "text",
text: String(weightKg / (heightM * heightM))
}]
})
);
// Async tool with external API call
server.tool(
"fetch-weather",
{ city: z.string() },
async ({ city }) => {
const response = await fetch(https://api.weather.com/${city});`
const data = await response.text();
return {
content: [{ type: "text", text: data }]
};
}
);
Prompts are reusable templates that help LLMs interact with your server effectively:
`typescriptPlease review this code:\n\n${code}
server.prompt(
"review-code",
{ code: z.string() },
({ code }) => ({
messages: [{
role: "user",
content: {
type: "text",
text:
}
}]
})
);
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:
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);
`
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.
`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
const eventStore = new InMemoryEventStore();
transport = new StreamableHTTPServerTransport({
sessionIdGenerator: () => randomUUID(),
onsessioninitialized: (sessionId) => {
// Store the transport by session ID
transports[sessionId] = transport;
}
});
// 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);
`
#### 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,
});
await server.connect(transport);
await transport.handleRequest(req, res, req.body);
res.on('close', () => {
console.log('Request closed');
transport.close();
server.close();
});
} 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,
});
}
}
});
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
}));
});
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;
app.listen(PORT, () => {
console.log(MCP Stateless Streamable HTTP Server listening on port ${PORT});
});
`
This stateless approach is useful for:
- Simple API wrappers
- RESTful scenarios where each request is independent
- Horizontally scaled deployments without shared session state
To test your server, you can use the MCP Inspector. See its README for more information.
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",
version: "1.0.0"
});
server.resource(
"echo",
new ResourceTemplate("echo://{message}", { list: undefined }),
async (uri, { message }) => ({
contents: [{
uri: uri.href,
text: Resource echo: ${message}
}]
})
);
server.tool(
"echo",
{ message: z.string() },
async ({ message }) => ({
content: [{ type: "text", text: Tool echo: ${message} }]
})
);
server.prompt(
"echo",
{ message: z.string() },
({ message }) => ({
messages: [{
role: "user",
content: {
type: "text",
text: Please process this message: ${message}
}
}]
})
);
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
close: promisify(db.close.bind(db))
};
};
server.resource(
"schema",
"schema://main",
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.tool(
"query",
{ 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();
}
}
);
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 notificaions:
`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', vadmin"])},
// Any mutations here will automatically emit listChanged notificationsError: ${err}
async ({ permission }) => {
const { ok, err, previous } = await upgradeAuthAndStoreToken(permission)
if (!ok) return {content: [{ type: "text", text: }]}
// 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);
`
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);
`
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"
}
});
`
You can proxy OAuth requests to an external authorization provider:
`typescript
import express from 'express';
import { ProxyOAuthServerProvider, mcpAuthRouter } from '@modelcontextprotocol/sdk';
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
Clients and servers with StreamableHttp tranport 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:
`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
sse: {} as Record
};
// 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);
} 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.
- Model Context Protocol documentation
- MCP Specification
- Example Servers
Issues and pull requests are welcome on GitHub at https://github.com/modelcontextprotocol/typescript-sdk.
This project is licensed under the MIT License—see the LICENSE file for details.