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backwards compatible client
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‎src/examples/client/streamableHttpWithSseFallbackClient.ts‎

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import { Client } from '../../client/index.js';
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import { StreamableHTTPClientTransport } from '../../client/streamableHttp.js';
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import { SSEClientTransport } from '../../client/sse.js';
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import {
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ListToolsRequest,
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ListToolsResultSchema,
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CallToolRequest,
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CallToolResultSchema,
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LoggingMessageNotificationSchema,
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} from '../../types.js';
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/**
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* Simplified Backwards Compatible MCP Client
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*
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* This client demonstrates backward compatibility with both:
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* 1. Modern servers using Streamable HTTP transport (protocol version 2025-03-26)
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* 2. Older servers using HTTP+SSE transport (protocol version 2024-11-05)
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*
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* Following the MCP specification for backwards compatibility:
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* - Attempts to POST an initialize request to the server URL first (modern transport)
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* - If that fails with 4xx status, falls back to GET request for SSE stream (older transport)
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*/
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// Command line args processing
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const args = process.argv.slice(2);
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const serverUrl = args[0] || 'http://localhost:3000/mcp';
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async function main(): Promise<void> {
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console.log('MCP Backwards Compatible Client');
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console.log('===============================');
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console.log(`Connecting to server at: ${serverUrl}`);
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let client: Client;
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let transport: StreamableHTTPClientTransport | SSEClientTransport;
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let transportType: 'streamable-http' | 'sse';
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try {
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// Try connecting with automatic transport detection
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const connection = await connectWithBackwardsCompatibility(serverUrl);
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client = connection.client;
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transport = connection.transport;
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transportType = connection.transportType;
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// Set up notification handler
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client.setNotificationHandler(LoggingMessageNotificationSchema, (notification) => {
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console.log(`Notification: ${notification.params.level} - ${notification.params.data}`);
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});
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// DEMO WORKFLOW:
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// 1. List available tools
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console.log('\n=== Listing Available Tools ===');
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await listTools(client);
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// 2. Call the notification tool
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console.log('\n=== Starting Notification Stream ===');
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await startNotificationTool(client);
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// 3. Wait for all notifications (5 seconds)
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console.log('\n=== Waiting for all notifications ===');
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await new Promise(resolve => setTimeout(resolve, 5000));
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// 4. Disconnect
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console.log('\n=== Disconnecting ===');
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await transport.close();
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console.log('Disconnected from MCP server');
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} catch (error) {
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console.error('Error running client:', error);
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process.exit(1);
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}
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}
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/**
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* Connect to an MCP server with backwards compatibility
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* Following the spec for client backward compatibility
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*/
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async function connectWithBackwardsCompatibility(url: string): Promise<{
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client: Client,
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transport: StreamableHTTPClientTransport | SSEClientTransport,
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transportType: 'streamable-http' | 'sse'
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}> {
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console.log('1. Trying Streamable HTTP transport first...');
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// Step 1: Try Streamable HTTP transport first
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const client = new Client({
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name: 'backwards-compatible-client',
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version: '1.0.0'
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});
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client.onerror = (error) => {
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console.error('Client error:', error);
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};
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const baseUrl = new URL(url);
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try {
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// Create modern transport
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const streamableTransport = new StreamableHTTPClientTransport(baseUrl);
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await client.connect(streamableTransport);
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console.log('Successfully connected using modern Streamable HTTP transport.');
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return {
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client,
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transport: streamableTransport,
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transportType: 'streamable-http'
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};
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} catch (error) {
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// Step 2: If transport fails, try the older SSE transport
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console.log(`StreamableHttp transport connection failed: ${error}`);
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console.log('2. Falling back to deprecated HTTP+SSE transport...');
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try {
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// Create SSE transport pointing to /sse endpoint
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const sseTransport = new SSEClientTransport(baseUrl);
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await client.connect(sseTransport);
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console.log('Successfully connected using deprecated HTTP+SSE transport.');
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return {
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client,
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transport: sseTransport,
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transportType: 'sse'
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};
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} catch (sseError) {
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console.error(`Failed to connect with either transport method:\n1. Streamable HTTP error: ${error}\n2. SSE error: ${sseError}`);
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throw new Error('Could not connect to server with any available transport');
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}
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}
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}
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/**
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* List available tools on the server
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*/
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async function listTools(client: Client): Promise<void> {
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try {
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const toolsRequest: ListToolsRequest = {
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method: 'tools/list',
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params: {}
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};
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const toolsResult = await client.request(toolsRequest, ListToolsResultSchema);
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console.log('Available tools:');
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if (toolsResult.tools.length === 0) {
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console.log(' No tools available');
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} else {
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for (const tool of toolsResult.tools) {
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console.log(` - ${tool.name}: ${tool.description}`);
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}
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}
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} catch (error) {
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console.log(`Tools not supported by this server: ${error}`);
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}
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}
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/**
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* Start a notification stream by calling the notification tool
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*/
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async function startNotificationTool(client: Client): Promise<void> {
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try {
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// Call the notification tool using reasonable defaults
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const request: CallToolRequest = {
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method: 'tools/call',
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params: {
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name: 'start-notification-stream',
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arguments: {
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interval: 1000, // 1 second between notifications
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count: 5 // Send 5 notifications
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}
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}
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};
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console.log('Calling notification tool...');
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const result = await client.request(request, CallToolResultSchema);
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console.log('Tool result:');
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result.content.forEach(item => {
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if (item.type === 'text') {
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console.log(` ${item.text}`);
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} else {
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console.log(` ${item.type} content:`, item);
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}
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});
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} catch (error) {
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console.log(`Error calling notification tool: ${error}`);
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}
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}
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// Start the client
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main().catch((error: unknown) => {
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console.error('Error running MCP client:', error);
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process.exit(1);
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});

‎src/examples/server/simpleSseServer.ts‎

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const transports: Record<string, SSEServerTransport> = {};
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// SSE endpoint for establishing the stream
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app.get('/sse', async (req: Request, res: Response) => {
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app.get('/mcp', async (req: Request, res: Response) => {
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console.log('Received GET request to /sse (establishing SSE stream)');
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try {

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