WebSocket vs SSE for AI Streaming: When to Use Which in 2026

If your AI app is "user types prompt, server streams tokens," use SSE. If it is "user and server have an ongoing conversation with audio, tools, and interruption," use WebSocket. The middle ground is smaller than it looks.

Why has SSE become the default for AI streaming?

flowchart LR
  Twilio["Twilio Media Streams"] -- "WS · μlaw 8kHz" --> Bridge["FastAPI Bridge :8084"]
  Bridge -- "PCM16 24kHz" --> OAI["OpenAI Realtime"]
  OAI --> Bridge
  Bridge --> Twilio
  Bridge --> Logs[(structured logs · OTel)]

Because LLM token streaming follows a request-then-stream pattern, which is exactly what SSE was designed for. The user POSTs a prompt; the server responds with text/event-stream and pushes tokens as they generate. The connection closes when generation finishes. There is no ongoing bidirectional state.

Under the hood, OpenAI's SDK, Anthropic's SDK, and the Vercel AI SDK all use SSE. It works through every corporate proxy because it is plain HTTP. It survives firewalls. It auto-reconnects. It does not require any special infrastructure beyond an HTTP server. For 95% of LLM-powered features, SSE is correct.

How do you decide between them?

The decision is mostly about whether the client also needs to send things mid-response:

The cost of WebSocket is operational. The cost of SSE is "I cannot send back to the server on the same connection." For a chatbot that streams answers, SSE wins. For a voice agent with real-time interruption, WebSocket is the only realistic answer.

CallSphere's implementation

CallSphere uses both, deliberately:

• Chat agents (Healthcare chat, Salon booking chat) — SSE from /api/chat/stream. The client POSTs a message, the server streams the AI response. Reconnection is handled by EventSource. Simple, debuggable.
• Voice agents (Healthcare, Sales Calling, After-hours, Real Estate) — WebSocket. Audio is bidirectional, interruption is native, and the OpenAI Realtime API requires WebSocket on this path.
• Live dashboard for the 37-agent Sales Calling team — WebSocket (Socket.IO). Multiple subscribers, presence, and state mutation.

This pattern keeps the chat surface dirt simple while reserving WebSocket complexity only for paths that genuinely need it.

Code: SSE token stream from a Next.js route

export async function POST(req: Request) {
  const { messages } = await req.json();
  const stream = new ReadableStream({
    async start(controller) {
      const enc = new TextEncoder();
      const completion = await openai.chat.completions.create({
        model: "gpt-4o-mini",
        messages,
        stream: true,
      });
      for await (const chunk of completion) {
        const tok = chunk.choices[0]?.delta?.content ?? "";
        controller.enqueue(enc.encode(`data: ${JSON.stringify({ tok })}\n\n`));
      }
      controller.enqueue(enc.encode("event: done\ndata: {}\n\n"));
      controller.close();
    },
  });
  return new Response(stream, {
    headers: { "Content-Type": "text/event-stream", "Cache-Control": "no-cache" },
  });
}

Build steps

1. Default to SSE for any AI feature that is "send prompt, stream response."
2. Use WebSocket when bidirectional state is genuine — voice, collaborative editing, multi-user agents.
3. For SSE, set Cache-Control: no-cache and X-Accel-Buffering: no (for nginx) so intermediaries do not buffer the stream.
4. For WebSocket, follow the rest of this batch — auth, backpressure, reconnection.
5. Do not "upgrade" SSE to WebSocket later if you do not need bidirectional. The migration cost is real.
6. Test through every proxy in your stack. Some corporate firewalls strip SSE chunked encoding.

FAQ

Does SSE work in mobile WebViews? Yes — every modern WebView ships EventSource. Older Android WebViews need a polyfill.

Can SSE handle voice? No, not practically. SSE is text-only and one-direction. Voice means WebSocket or WebRTC.

Does HTTP/3 change anything? SSE benefits significantly — multiplexed streams over QUIC are faster and more robust. Use HTTP/3 if your edge supports it.

Does the Vercel AI SDK use SSE? Yes. useChat() hits an SSE endpoint by default.

What about Server Components streaming in React? That uses a custom RSC protocol, but it is layered on the same SSE-like chunked transport.

CallSphere serves six verticals with the right protocol for each surface. Start the 14-day trial, or join the affiliate program — pricing is $149/$499/$1499.

Sources

• WebSocket vs SSE: Which One Should You Use?
• Server-Sent Events vs WebSockets: Key Differences and Use Cases
• Streaming in 2026: SSE vs WebSockets vs RSC
• Server-Sent Events Beat WebSockets for 95% of Real-Time Apps