@fastify/sse

Server-Sent Events plugin for Fastify. Provides first-class SSE support with clean API integration, session management, and streaming capabilities.

Features

• 🚀 Clean API: Route-level SSE support with { sse: true }
• 📡 Streaming: Native support for Node.js streams and async iterators
• 🔄 Reconnection: Built-in message replay with Last-Event-ID
• 💓 Heartbeat: Configurable keep-alive mechanism
• 🎯 Lifecycle: Full integration with Fastify hooks and error handling
• 📝 TypeScript: Complete type definitions included
• ⚡ Performance: Efficient backpressure handling

Install

npm i @fastify/sse

Quick Start

const fastify = require('fastify')({ logger: true })

// Register the plugin
await fastify.register(require('@fastify/sse'))

// Create an SSE endpoint
fastify.get('/events', { sse: true }, async (request, reply) => {
  // Send a message
  await reply.sse.send({ data: 'Hello SSE!' })

  // Send with full options
  await reply.sse.send({
    id: '123',
    event: 'update',
    data: { message: 'Hello World' },
    retry: 1000
  })
})

await fastify.listen({ port: 3000 })

API

Plugin Registration

await fastify.register(require('@fastify/sse'), {
  // Optional: heartbeat interval in milliseconds (default: 30000)
  heartbeatInterval: 30000,

  // Optional: default serializer (default: JSON.stringify)
  serializer: (data) => JSON.stringify(data)
})

Route Configuration

Routes opt into SSE handling by setting the sse field. There are three forms, each describing what the route serves:

// SSE-only route: handler always streams. Clients that explicitly refuse
// SSE (e.g. `Accept: application/json`) receive 406 Not Acceptable.
// Ambiguous Accept headers (*/*, text/*, missing) admit SSE per RFC 9110.
fastify.get('/events', { sse: 'only' }, handler)

// Dual-mode route: the same handler serves both SSE and non-SSE clients.
// Only an explicit `text/event-stream` token routes to SSE; everything
// else falls through to the handler with `reply.sse` undefined, so the
// handler is expected to produce a non-SSE response in that branch.
fastify.get('/data', { sse: 'dual' }, handler)

// Manual route: no Accept negotiation. `reply.sse` is always attached and
// the handler decides at runtime whether to stream (by calling reply.sse.*)
// or to return a normal response. Useful for streaming APIs that signal
// streaming via the request body (e.g. OpenAI-style `{ stream: true }`)
// rather than the Accept header.
fastify.post('/v1/chat', { sse: 'manual' }, handler)

// Back-compat: `sse: true` behaves like `'dual'` for routing. If the
// handler is actually SSE-only and trips a TypeError on `reply.sse`
// because of a wildcard Accept, the plugin rethrows with a message
// that names `sse: 'only'` as the fix.
fastify.get('/legacy', { sse: true }, handler)

// Object form (supports per-route options):
fastify.get('/events', {
  sse: {
    kind: 'only',                // 'only' | 'dual' | 'manual' — omit for back-compat
    heartbeat: false,            // Disable heartbeat for this route
    serializer: customSerializer // Custom serializer for this route
  }
}, handler)

reply.sse.send(source)

Send SSE messages. Accepts various source types:

Single Message

// Simple data
await reply.sse.send({ data: 'hello' })

// Full SSE message
await reply.sse.send({
  id: '123',
  event: 'update',
  data: { message: 'Hello' },
  retry: 1000
})

// Plain string
await reply.sse.send('plain text message')

Streaming Sources

// Async generator
async function* generateEvents() {
  for (let i = 0; i < 10; i++) {
    yield { id: i, data: `Event ${i}` }
    await sleep(1000)
  }
}
await reply.sse.send(generateEvents())

// Node.js Readable stream
const stream = fs.createReadStream('data.jsonl')
await reply.sse.send(stream)

// Transform existing stream
const transformStream = new Transform({
  transform(chunk, encoding, callback) {
    callback(null, { data: chunk.toString() })
  }
})
someSource.pipe(transformStream)
await reply.sse.send(transformStream)

reply.sse.stream()

Create a transform stream for use in pipeline operations:

// Use with pipeline for efficient streaming
const { pipeline } = require('stream/promises')
const fs = require('fs')

fastify.get('/file-stream', { sse: true }, async (request, reply) => {
  const fileStream = fs.createReadStream('data.jsonl')

  // Parse each line as JSON and convert to SSE format
  const parseTransform = new Transform({
    transform(chunk, encoding, callback) {
      const lines = chunk.toString().split('\n').filter(Boolean)
      for (const line of lines) {
        try {
          const data = JSON.parse(line)
          this.push({ id: data.id, data })
        } catch (err) {
          // Skip invalid JSON lines
        }
      }
      callback()
    }
  })

  // Stream file data through SSE
  await pipeline(
    fileStream,
    parseTransform,
    reply.sse.stream(),
    reply.raw,
    { end: false }
  )
})

Connection Management

fastify.get('/live', { sse: true }, async (request, reply) => {
  // Keep connection alive (prevents automatic close)
  reply.sse.keepAlive()

  // Send initial message
  await reply.sse.send({ data: 'Connected' })

  // Check if keepAlive was called
  console.log('Keep alive status:', reply.sse.shouldKeepAlive) // true

  // Set up periodic updates
  const interval = setInterval(async () => {
    if (reply.sse.isConnected) {
      await reply.sse.send({ data: 'ping' })
    } else {
      clearInterval(interval)
    }
  }, 1000)

  // Clean up when connection closes
  reply.sse.onClose(() => {
    clearInterval(interval)
    console.log('Connection closed')
  })
})

Message Replay

Handle client reconnections with Last-Event-ID:

const messageHistory = []

fastify.get('/events', { sse: true }, async (request, reply) => {
  // Handle replay on reconnection
  await reply.sse.replay(async (lastEventId) => {
    // Find messages after the last received ID
    const startIndex = messageHistory.findIndex(msg => msg.id === lastEventId)
    const messagesToReplay = startIndex !== -1
      ? messageHistory.slice(startIndex + 1)
      : messageHistory

    // Send missed messages
    for (const message of messagesToReplay) {
      await reply.sse.send(message)
    }
  })

  // Send new message
  const newMessage = { id: Date.now(), data: 'New event' }
  messageHistory.push(newMessage)
  await reply.sse.send(newMessage)
})

Properties and Methods

Properties

• reply.sse.lastEventId: Client's last received event ID (string | null)
• reply.sse.isConnected: Connection status (boolean)
• reply.sse.shouldKeepAlive: Whether connection should be kept alive after handler completion (boolean)

Methods

• reply.sse.send(source): Send SSE messages from various source types
• reply.sse.stream(): Create a transform stream for pipeline operations
• reply.sse.keepAlive(): Prevent connection from auto-closing after handler returns
• reply.sse.close(): Manually close the connection
• reply.sse.replay(callback): Handle message replay using Last-Event-ID
• reply.sse.onClose(callback): Register callback for when connection closes
• reply.sse.sendHeaders(statusCode?): Manually send headers (called automatically on first write)

Custom Response Status Code

The SSE response defaults to 200 OK. To use a different status, either call reply.code() before the first SSE write, or pass the status directly to a manual reply.sse.sendHeaders() call:

fastify.get('/events', { sse: 'only' }, async (request, reply) => {
  reply.code(201)
  await reply.sse.send({ data: 'created' })
})

fastify.get('/events-explicit', { sse: 'only' }, async (request, reply) => {
  reply.sse.sendHeaders(201)
  await reply.sse.send({ data: 'created' })
})

An explicit sendHeaders() argument takes precedence over reply.code(). Once the first SSE write commits the response headers, later reply.code() or sendHeaders() calls have no effect.

Advanced Usage

Accept-Header Negotiation

The plugin parses the Accept header per RFC 9110 §12.5.1 to decide whether to serve a request as SSE. The decision depends on the route's declared kind:

In short: explicit text/event-stream always routes to SSE; ambiguous wildcards route to SSE only when you've declared the route SSE-only. This keeps curl, fetch(), and Postman defaults working with SSE-only endpoints while preserving the dual-mode pattern's expectation that a wildcard prefers the richer non-SSE representation.

Fallback to Regular Responses (sse: 'dual')

Dual-mode routes serve both SSE and non-SSE clients from a single handler. The plugin attaches reply.sse only when the client explicitly requested text/event-stream; otherwise the handler is invoked without reply.sse and is expected to produce a normal response. Branch on its presence:

fastify.get('/data', { sse: 'dual' }, async (request, reply) => {
  const data = await getData()

  if (reply.sse) {
    // SSE client — stream the data
    await reply.sse.send({ data })
  } else {
    // Regular client — return JSON
    return { data }
  }
})

The SSE-specific response headers (Content-Type: text/event-stream etc.) are not committed until the first reply.sse.send() / reply.sse.stream() call, so a handler that decorates reply.sse but then returns a plain value falls through to Fastify's normal serialization path without corrupting the response.

Dynamic Streaming Based on Request Logic (sse: 'manual')

Some APIs decide whether to stream based on the request body rather than the Accept header. OpenAI-compatible and other LLM endpoints, for example, return a text/event-stream response when the request carries { "stream": true } and a single JSON object otherwise — the client sends no Accept: text/event-stream header either way.

sse: 'manual' covers this: it skips Accept-header negotiation entirely, always attaches reply.sse, and lets the handler decide at runtime whether to stream. As with 'dual', SSE headers are committed lazily on the first reply.sse.send() / reply.sse.stream() call, so a handler that never streams falls through to Fastify's normal serialization.

fastify.post('/v1/chat/completions', { sse: 'manual' }, async (request, reply) => {
  const completion = await model.run(request.body)

  if (request.body.stream) {
    // Stream tokens as SSE — headers commit on the first send().
    await reply.sse.send(completion.tokens) // async iterable / Readable / messages
    return
  }

  // Non-streaming client — return a normal JSON response.
  return completion.toJSON()
})

The connection is closed automatically when the handler resolves or throws (call reply.sse.keepAlive() to keep it open for out-of-band sends, then reply.sse.close() when done). Because the heartbeat only starts once the response is committed as SSE, a non-streaming fallback response is never corrupted by a heartbeat write.

Error Handling

fastify.get('/stream', { sse: true }, async (request, reply) => {
  try {
    async function* riskyGenerator() {
      yield { data: 'before error' }
      throw new Error('Something went wrong')
    }

    await reply.sse.send(riskyGenerator())
  } catch (error) {
    // Handle errors gracefully
    await reply.sse.send({
      event: 'error',
      data: { message: 'Stream error occurred' }
    })
  }
})

Custom Serialization

// Plugin-level serializer
await fastify.register(require('@fastify/sse'), {
  serializer: (data) => {
    // Custom serialization logic
    return typeof data === 'string' ? data : JSON.stringify(data)
  }
})

// Route-level serializer
fastify.get('/custom', {
  sse: {
    serializer: (data) => `CUSTOM:${JSON.stringify(data)}`
  }
}, async (request, reply) => {
  await reply.sse.send({ data: 'test' }) // Outputs: "CUSTOM:\"test\""
})

Testing

Testing SSE endpoints is simplified with standard Fastify injection:

const response = await fastify.inject({
  method: 'GET',
  url: '/events',
  headers: {
    accept: 'text/event-stream'
  }
})

assert.strictEqual(response.statusCode, 200)
assert.strictEqual(response.headers['content-type'], 'text/event-stream')
assert.ok(response.body.includes('data: "Hello SSE!"'))

Client-Side Usage

<!DOCTYPE html>
<html>
<head>
  <title>SSE Client</title>
</head>
<body>
  <div id="messages"></div>

  <script>
    const eventSource = new EventSource('/events')
    const messagesDiv = document.getElementById('messages')

    eventSource.onmessage = function(event) {
      const data = JSON.parse(event.data)
      messagesDiv.innerHTML += '<div>' + JSON.stringify(data) + '</div>'
    }

    eventSource.addEventListener('update', function(event) {
      console.log('Update event:', JSON.parse(event.data))
    })

    eventSource.onerror = function(event) {
      console.error('SSE error:', event)
    }
  </script>
</body>
</html>

TypeScript

Full TypeScript support included:

import fastify from 'fastify'
import fastifySSE, { SSEMessage } from '@fastify/sse'

const app = fastify()
await app.register(fastifySSE)

app.get('/events', { sse: true }, async (request, reply) => {
  const message: SSEMessage = {
    id: '123',
    event: 'test',
    data: { hello: 'world' }
  }

  await reply.sse.send(message)

  // TypeScript knows about SSE properties
  console.log(reply.sse.isConnected) // boolean
  console.log(reply.sse.lastEventId) // string | null
  console.log(reply.sse.shouldKeepAlive) // boolean
})

Examples

See the examples directory for complete working examples:

• Basic Usage - Simple SSE endpoints
• Dynamic Streaming - sse: 'manual', OpenAI-style stream-or-JSON based on the request body
• More examples coming soon...

Comparison with fastify-sse-v2

License

MIT