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 - State tracking class
const MAX_MANUAL_RETRIES = 5;
const RETRY_DELAY_MS = 3000;

class SSEConnection {
  private eventSource: EventSource | null = null;
  private status: SSEStatus = "closed";
  private manualRetryCount = 0;
  private onStatusChange?: (status: SSEStatus) => void;
  private onMessage?: (data: string, eventType: string) => void;

  constructor(
    private url: string,
    options?: {
      onStatusChange?: (status: SSEStatus) => void;
      onMessage?: (data: string, eventType: string) => void;
    },
  ) {
    this.onStatusChange = options?.onStatusChange;
    this.onMessage = options?.onMessage;
  }

  connect(): void {
    if (this.eventSource) {
      this.disconnect();
    }

    this.setStatus("connecting");
    this.eventSource = new EventSource(this.url);

    this.eventSource.onopen = () => {
      this.setStatus("open");
      this.manualRetryCount = 0; // Reset on successful connection
    };

    this.eventSource.onmessage = (event: MessageEvent) => {
      this.onMessage?.(event.data, "message");
    };

    this.eventSource.onerror = () => {
      if (this.eventSource?.readyState === EventSource.CLOSED) {
        this.setStatus("closed");
        // EventSource won't auto-reconnect if server sent HTTP error
        this.attemptManualReconnect();
      } else {
        this.setStatus("error");
        // EventSource is auto-reconnecting
      }
    };
  }

  private attemptManualReconnect(): void {
    if (this.manualRetryCount < MAX_MANUAL_RETRIES) {
      this.manualRetryCount++;
      console.log(`Manual reconnect attempt ${this.manualRetryCount}`);
      setTimeout(() => this.connect(), RETRY_DELAY_MS);
    }
  }

  disconnect(): void {
    if (this.eventSource) {
      this.eventSource.close();
      this.eventSource = null;
      this.setStatus("closed");
    }
  }

  private setStatus(status: SSEStatus): void {
    this.status = status;
    this.onStatusChange?.(status);
  }

  getStatus(): SSEStatus {
    return this.status;
  }
}

export { SSEConnection };

Why good: Named constants for retry values, status tracking enables UI updates, manual retry for HTTP errors (EventSource only auto-retries network errors), cleanup resets state properly

---

Pattern 5: SSE Message Format

Messages are \n-separated fields terminated by \n\n. Five field types: data: (payload), event: (named type), id: (recovery ID), retry: (reconnect ms), : (comment/keep-alive).

event: notification
data: {"title": "New message"}
id: msg-002

: keep-alive comment (ignored by client)

Key behaviors: multiple data: lines concatenate with \n; id: persists until changed; retry: is remembered for future reconnections; comments (:) keep connection alive but are not delivered.

See reference.md for the full field reference and behavior table.

---

Pattern 6: Discriminated Unions for Message Types

Use TypeScript discriminated unions for type-safe message handling.

// ✅ Good Example - Type-safe SSE message handling

// Server message types
type SSEMessage =
  | {
      type: "notification";
      title: string;
      body: string;
      priority: "low" | "high";
    }
  | { type: "user-update"; userId: string; action: "joined" | "left" }
  | { type: "data-sync"; payload: unknown; timestamp: number }
  | { type: "heartbeat"; serverTime: number };

function parseSSEMessage(data: string): SSEMessage | null {
  try {
    return JSON.parse(data) as SSEMessage;
  } catch {
    console.error("Failed to parse SSE message:", data);
    return null;
  }
}

function handleSSEMessage(message: SSEMessage): void {
  switch (message.type) {
    case "notification":
      showNotification(message.title, message.body, message.priority);
      break;
    case "user-update":
      updateUserPresence(message.userId, message.action);
      break;
    case "data-sync":
      syncData(message.payload, message.timestamp);
      break;
    case "heartbeat":
      updateServerTime(message.serverTime);
      break;
    default:
      // Exhaustiveness check
      const exhaustive: never = message;
      console.warn("Unknown message type:", exhaustive);
  }
}

// Usage with EventSource
eventSource.onmessage = (event: MessageEvent) => {
  const message = parseSSEMessage(event.data);
  if (message) {
    handleSSEMessage(message);
  }
};

Why good: Discriminated union enables type narrowing, exhaustiveness check catches missing cases at compile time, separate parse and handle functions, error handling for malformed messages

---

Integration Guide

SSE is a transport mechanism. This skill covers the EventSource API and fetch streaming patterns only.

• Components receive SSE data via callbacks/hooks and pass it to your UI layer via props or state
• Authentication integrates via cookies (withCredentials: true) or fetch streaming with custom headers
• For bidirectional communication, SSE is not the right tool — evaluate WebSocket instead

---

<red_flags>

RED FLAGS

• No cleanup on unmount - EventSource stays open, memory leaks, zombie connections
• Ignoring onerror event - Connection failures are silent, users see stale data
• Not checking readyState in onerror - Cannot distinguish reconnection from permanent failure
• Token in URL query string - Security risk: visible in server logs, browser history
• Missing keep-alive comments - Proxies may close "idle" connections after 60-120 seconds
• JSON.parse without try-catch - Malformed messages crash the entire handler
• Creating new EventSource without closing old one - Duplicate connections, duplicate messages
• Not handling buffer boundaries in fetch streaming - Messages split across chunks are missed

Gotchas & Edge Cases:

• EventSource has no timeout - dead connections may not fire onerror for minutes
• HTTP/1.1 browsers limit 6 connections per domain (SSE counts against this)
• retry: field is in milliseconds, not seconds
• Empty data:\n\n sends empty string, not undefined
• Connection: keep-alive header is prohibited in HTTP/2+ (Safari rejects it)

See reference.md for full anti-pattern examples with code.

</red_flags>

---

<critical_reminders>

CRITICAL REMINDERS

All code must follow project conventions in CLAUDE.md

(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+)

Failure to follow these rules will result in memory leaks, missed messages, and silent connection failures.

</critical_reminders>