Server-Sent Events transport for real-time, unidirectional communication from server to MCP clients. Enables streaming responses and notifications.

Core Purpose

• Manage SSE connections for clients
• Format and send SSE-compliant messages
• Handle JSON-RPC over SSE
• Support server notifications

Key Components

SSETransport Class

export class SSETransport {
    private clients: Map<string, Response>
    private messageId: number
    
    // Main methods
    initializeSSE(): Sets up connection
    sendEvent(): Sends SSE message
    sendJsonRpcResponse(): Sends JSON-RPC
    broadcast(): Send to all clients
    disconnect(): Close connection
}

SSE Message Format

Structure

id: 123
event: message
retry: 5000
data: {"jsonrpc":"2.0","result":{}}

Headers

Header	Value	Purpose
Content-Type	text/event-stream	SSE stream
Cache-Control	no-cache	No caching
Connection	keep-alive	Persistent
X-Accel-Buffering	no	No buffering

Integration in Application

• Used By: HTTP transport for streaming
• Protocol: SSE over HTTP
• Format: JSON-RPC messages
• Events: connect, message, notification

Event Types

Standard Events

• connect: Initial connection
• message: JSON-RPC response
• notification: Server update
• endpoint: Legacy compatibility

Custom Events

• progress: Operation progress
• error: Error notification
• ping: Keep-alive

Connection Management

Lifecycle

1. Initialize with headers
2. Send connect event
3. Handle messages
4. Clean up on close

Features

• Auto-cleanup on disconnect
• Connection status tracking
• Broadcast capabilities

Message Sending

// Send event
sendEvent(clientId, {
    event: 'message',
    data: JSON.stringify(response)
});

// Broadcast to all
broadcast({
    event: 'notification',
    data: JSON.stringify(update)
});

Client Integration

JavaScript EventSource

const source = new EventSource('/sse/connect');

source.addEventListener('message', (e) => {
    const data = JSON.parse(e.data);
});

source.addEventListener('error', (e) => {
    // Auto-reconnects
});

Key Features

Error Handling

• Connection error recovery
• Client cleanup on failure
• Structured error logging

Performance

• O(1) client lookups
• Efficient broadcasting
• Optional keep-alive

Security

• Session validation
• Connection limits
• Data sanitization

Usage Example

// Initialize connection
transport.initializeSSE(res, clientId);

// Send JSON-RPC response
transport.sendJsonRpcResponse(clientId, {
    jsonrpc: '2.0',
    result: data,
    id: requestId
});

// Send notification
transport.sendNotification(clientId, 
    'resources/changed',
    { uri: 'airs://scan-results/123' }
);

// Clean up
transport.disconnect(clientId);

Related Modules

• HTTP Transport - Main transport layer
• Transport Types - Type definitions
• Logger - Debug logging