@modelcontextprotocol/sdk
Version:
Model Context Protocol implementation for TypeScript
166 lines • 6.34 kB
JavaScript
import { Client } from '../../client/index.js';
import { StreamableHTTPClientTransport } from '../../client/streamableHttp.js';
import { SSEClientTransport } from '../../client/sse.js';
import { ListToolsResultSchema, CallToolResultSchema, LoggingMessageNotificationSchema, } from '../../types.js';
/**
* Simplified Backwards Compatible MCP Client
*
* This client demonstrates backward compatibility with both:
* 1. Modern servers using Streamable HTTP transport (protocol version 2025-03-26)
* 2. Older servers using HTTP+SSE transport (protocol version 2024-11-05)
*
* Following the MCP specification for backwards compatibility:
* - Attempts to POST an initialize request to the server URL first (modern transport)
* - If that fails with 4xx status, falls back to GET request for SSE stream (older transport)
*/
// Command line args processing
const args = process.argv.slice(2);
const serverUrl = args[0] || 'http://localhost:3000/mcp';
async function main() {
console.log('MCP Backwards Compatible Client');
console.log('===============================');
console.log(`Connecting to server at: ${serverUrl}`);
let client;
let transport;
try {
// Try connecting with automatic transport detection
const connection = await connectWithBackwardsCompatibility(serverUrl);
client = connection.client;
transport = connection.transport;
// Set up notification handler
client.setNotificationHandler(LoggingMessageNotificationSchema, (notification) => {
console.log(`Notification: ${notification.params.level} - ${notification.params.data}`);
});
// DEMO WORKFLOW:
// 1. List available tools
console.log('\n=== Listing Available Tools ===');
await listTools(client);
// 2. Call the notification tool
console.log('\n=== Starting Notification Stream ===');
await startNotificationTool(client);
// 3. Wait for all notifications (5 seconds)
console.log('\n=== Waiting for all notifications ===');
await new Promise(resolve => setTimeout(resolve, 5000));
// 4. Disconnect
console.log('\n=== Disconnecting ===');
await transport.close();
console.log('Disconnected from MCP server');
}
catch (error) {
console.error('Error running client:', error);
process.exit(1);
}
}
/**
* Connect to an MCP server with backwards compatibility
* Following the spec for client backward compatibility
*/
async function connectWithBackwardsCompatibility(url) {
console.log('1. Trying Streamable HTTP transport first...');
// Step 1: Try Streamable HTTP transport first
const client = new Client({
name: 'backwards-compatible-client',
version: '1.0.0'
});
client.onerror = (error) => {
console.error('Client error:', error);
};
const baseUrl = new URL(url);
try {
// Create modern transport
const streamableTransport = new StreamableHTTPClientTransport(baseUrl);
await client.connect(streamableTransport);
console.log('Successfully connected using modern Streamable HTTP transport.');
return {
client,
transport: streamableTransport,
transportType: 'streamable-http'
};
}
catch (error) {
// Step 2: If transport fails, try the older SSE transport
console.log(`StreamableHttp transport connection failed: ${error}`);
console.log('2. Falling back to deprecated HTTP+SSE transport...');
try {
// Create SSE transport pointing to /sse endpoint
const sseTransport = new SSEClientTransport(baseUrl);
const sseClient = new Client({
name: 'backwards-compatible-client',
version: '1.0.0'
});
await sseClient.connect(sseTransport);
console.log('Successfully connected using deprecated HTTP+SSE transport.');
return {
client: sseClient,
transport: sseTransport,
transportType: 'sse'
};
}
catch (sseError) {
console.error(`Failed to connect with either transport method:\n1. Streamable HTTP error: ${error}\n2. SSE error: ${sseError}`);
throw new Error('Could not connect to server with any available transport');
}
}
}
/**
* List available tools on the server
*/
async function listTools(client) {
try {
const toolsRequest = {
method: 'tools/list',
params: {}
};
const toolsResult = await client.request(toolsRequest, ListToolsResultSchema);
console.log('Available tools:');
if (toolsResult.tools.length === 0) {
console.log(' No tools available');
}
else {
for (const tool of toolsResult.tools) {
console.log(` - ${tool.name}: ${tool.description}`);
}
}
}
catch (error) {
console.log(`Tools not supported by this server: ${error}`);
}
}
/**
* Start a notification stream by calling the notification tool
*/
async function startNotificationTool(client) {
try {
// Call the notification tool using reasonable defaults
const request = {
method: 'tools/call',
params: {
name: 'start-notification-stream',
arguments: {
interval: 1000, // 1 second between notifications
count: 5 // Send 5 notifications
}
}
};
console.log('Calling notification tool...');
const result = await client.request(request, CallToolResultSchema);
console.log('Tool result:');
result.content.forEach(item => {
if (item.type === 'text') {
console.log(` ${item.text}`);
}
else {
console.log(` ${item.type} content:`, item);
}
});
}
catch (error) {
console.log(`Error calling notification tool: ${error}`);
}
}
// Start the client
main().catch((error) => {
console.error('Error running MCP client:', error);
process.exit(1);
});
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