Server-Sent Events (SSE) Patterns

Quick Guide: Use SSE for unidirectional server-to-client real-time updates over HTTP. Use the native EventSource API for automatic reconnection and message parsing. Use fetch streaming when you need custom headers or POST requests.

<critical_requirements>

CRITICAL: Before Using This Skill

All code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering, import type, named constants)

(You MUST use named constants for ALL timing values - reconnect intervals, keep-alive periods, timeouts)

(You MUST implement proper cleanup by calling eventSource.close() when connections are no longer needed)

(You MUST use event IDs (id: field) to enable message recovery on reconnection)

(You MUST handle the onerror event and check readyState to distinguish reconnection from permanent failure)

(You MUST set Content-Type: text/event-stream and Cache-Control: no-cache on SSE responses — do NOT set Connection: keep-alive on HTTP/2+)

</critical_requirements>

Auto-detection: SSE, Server-Sent Events, EventSource, text/event-stream, onmessage, server push, one-way streaming, real-time updates

When to use:

• Server-to-client real-time updates (notifications, feeds, dashboards)
• LLM/AI response streaming (token-by-token output)
• Live data feeds (stock prices, sports scores, news)
• Server push notifications without client responses needed
• Long-polling replacement with better browser support

Key patterns covered:

• EventSource API connection lifecycle
• Custom event types with addEventListener
• Fetch-based streaming for custom headers/POST
• SSE message parsing (data, event, id, retry fields)
• Reconnection with Last-Event-ID recovery
• Keep-alive comments to prevent proxy timeouts
• Custom React hooks (useEventSource, useSSE)

When NOT to use:

• Bidirectional communication needed (use WebSocket)
• Binary data transmission required (use WebSocket)
• Client needs to send frequent messages (use WebSocket)
• Sub-millisecond latency required (use WebSocket)

Detailed Resources:

• examples/core.md - React hooks (useEventSource, useSSE), shared context, conditional connection
• examples/fetch-streaming.md - Fetch-based SSE, message parser, auth, POST streaming, LLM pattern
• examples/reconnection.md - Last-Event-ID recovery, exponential backoff, health checks, visibility-aware
• reference.md - Decision frameworks, anti-patterns, message format reference

Philosophy

Server-Sent Events (SSE) provide a simple, HTTP-based protocol for servers to push real-time updates to clients. Unlike WebSockets, SSE is unidirectional (server to client only), built on standard HTTP, and includes automatic reconnection.

Why SSE exists:

1. Simplicity: Standard HTTP protocol - works through firewalls, proxies, and load balancers without special configuration.

2. Built-in Reconnection: The EventSource API automatically reconnects when connections drop, with configurable retry intervals.

3. Message Recovery: The Last-Event-ID header enables servers to replay missed messages after reconnection.

4. Text-Based Protocol: Human-readable format makes debugging straightforward.

Connection Lifecycle:

CONNECTING (0) → OPEN (1) → messages... → CLOSED (2)
                    ↓                         ↓
                (error) ← auto-reconnect ← (connection lost)

When to Choose SSE over WebSocket:

• Server sends updates, client only listens
• Working with HTTP/2 (multiplexing multiple SSE streams)
• Need automatic reconnection without custom logic
• Proxies/firewalls block WebSocket but allow HTTP
• Building LLM streaming interfaces

Core Patterns

Pattern 1: Basic EventSource Connection

The native EventSource API provides automatic connection management, message parsing, and reconnection.

Constants

const SSE_URL = "/api/events";

Implementation

// ✅ Good Example - Complete lifecycle handling
const SSE_URL = "/api/events";

const eventSource = new EventSource(SSE_URL);

eventSource.onopen = () => {
  console.log("SSE connection opened");
  // Connection is ready - server can now push events
};

eventSource.onmessage = (event: MessageEvent) => {
  console.log("Received:", event.data);
  console.log("Event ID:", event.lastEventId);
};

eventSource.onerror = (error: Event) => {
  console.error("SSE error:", error);

  // Check connection state to determine action
  if (eventSource.readyState === EventSource.CLOSED) {
    console.log("Connection closed permanently");
  } else if (eventSource.readyState === EventSource.CONNECTING) {
    console.log("Reconnecting...");
  }
};

// Cleanup when done
// eventSource.close();

Why good: All three lifecycle events handled, readyState check distinguishes reconnection from permanent failure, named constant for URL, cleanup shown

// ❌ Bad Example - Missing error handling and cleanup
const eventSource = new EventSource("/api/events");

eventSource.onmessage = (event) => {
  console.log(event.data);
};
// No onerror handler - connection failures are silent
// No cleanup - connection stays open forever

Why bad: Missing onerror means failures are silent, missing cleanup causes memory leaks and zombie connections, hardcoded URL string

Pattern 2: Custom Event Types

SSE supports named events via the event: field. Use addEventListener to handle specific event types.

// ✅ Good Example - Multiple event type handling
const SSE_URL = "/api/notifications";

const eventSource = new EventSource(SSE_URL);

// Default message event (no event: field in server response)
eventSource.onmessage = (event: MessageEvent) => {
  console.log("Generic message:", event.data);
};

// Named custom events
eventSource.addEventListener("notification", (event: MessageEvent) => {
  const notification = JSON.parse(event.data);
  showNotification(notification.title, notification.body);
});

eventSource.addEventListener("user-joined", (event: MessageEvent) => {
  const user = JSON.parse(event.data);
  updateUserList(user);
});

eventSource.addEventListener("heartbeat", (event: MessageEvent) => {
  // Keep-alive event - connection is healthy
  console.log("Heartbeat received at:", event.data);
});

Why good: Separate handlers for different event types, typed MessageEvent parameters, JSON parsing for structured data, heartbeat handling for connection health

When to use: When server sends multiple types of events with different handling requirements.

Pattern 3: Credentials and Cross-Origin

For cross-origin requests or when cookies are required, configure withCredentials.

// ✅ Good Example - Cross-origin with credentials
const SSE_URL = "https://api.example.com/events";

const eventSource = new EventSource(SSE_URL, {
  withCredentials: true, // Include cookies for cross-origin
});

eventSource.onopen = () => {
  console.log("Connected with credentials");
};

eventSource.onerror = (error) => {
  // CORS errors will trigger onerror
  console.error("Connection error - check CORS configuration");
};

Why good: withCredentials enables cookie-based authentication, CORS error handling noted

When to use: Cross-origin SSE connections that require authentication cookies.

Pattern 4: Connection State Management

Track connection state for UI feedback and smart reconnection decisions.

Constants

type SSEStatus = "connecting" | "open" | "closed" | "error";

const READY_STATE_MAP: Record<number, SSEStatus> = {
  [EventSource.CONNECTING]: "connecting",
  [EventSource.OPEN]: "open",
  [EventSource.CLOSED]: "closed",
};

Implementation

// ✅ Good Example - Stat