Live AI Chat Replies with WebSockets and SSE

§1 Token streams make AI chat feel responsive

A streaming chat interface starts showing an answer before the model finishes writing it. Instead of waiting for one complete response, the browser receives small pieces and adds them to the conversation. This reduces the wait before users see something useful, even when the full answer takes several seconds.

These pieces are often called tokens, but a network message does not necessarily contain exactly one token. It might carry a text fragment, a status update, or part of a tool request. The transport delivers those pieces; it does not generate them. Server-sent events, usually shortened to SSE, and WebSockets are two ways to handle that delivery.

§2 SSE sends updates over an HTTP response

SSE lets a server keep an HTTP response open and send a sequence of text events. For a typical chat interaction, the browser submits a prompt, and the server streams the answer back. The next user message can start another request.

This fits ordinary AI chat well because most of the ongoing traffic flows in one direction: from the server to the user. SSE also uses familiar HTTP infrastructure, although proxies still need configuration that avoids buffering the response.

One browser detail matters: the built-in EventSource interface opens a GET request and cannot freely attach custom authorization headers. Chat applications often need POST requests with a message body, so they commonly use fetch and read its response stream while parsing SSE events.

§3 WebSockets keep a two-way connection open

A WebSocket provides a persistent connection through which both the browser and server can send messages at any time. After the initial connection setup, neither side needs to create a fresh HTTP request for each application message.

This is useful when your product needs frequent interaction during a response. Examples include live audio, collaborative sessions, and control messages that interrupt or redirect ongoing work. A WebSocket connection can carry text or binary messages.

That flexibility brings extra responsibilities. Your application needs rules for message types, authentication, reconnecting, and matching responses to requests. WebSockets do not automatically make a model answer faster. Their main advantage is the communication pattern they support.

§4 Choosing a transport for your first chat app

• – Choose SSE when users submit prompts and mainly receive streamed answers.
• – Choose WebSockets when both sides need to exchange frequent updates throughout a session.
• – Check your hosting platform for connection limits, idle timeouts, and streaming support before committing to either transport.
• – Keep the browser-facing transport separate from the model-facing transport when your architecture benefits from it.

For example, your backend can consume an SSE stream from an AI provider and forward updates to the browser over a WebSocket. The two connections do not have to use the same protocol.

§5 Reliable token streams need explicit handling

Do not assume each network read contains one complete event. Data can arrive split across reads or bundled together. Buffer incomplete data and parse complete messages before updating the interface.

Also distinguish a finished answer from a broken connection. Use the API's documented completion signal, handle errors visibly, and offer a retry when appropriate. Reconnecting does not automatically resume an interrupted generation; recovery requires application support.

A stop button should cancel the browser request and tell your backend to cancel upstream work where supported. Closing the visible stream alone does not guarantee generation or billing stops. Keep API keys on your server, and avoid placing sensitive conversation content in logs.

§6 Fluxio fits the upstream streaming layer

Fluxio is a streaming-first, pay-per-token AI API relay that provides access to multiple model families through one key. For a newcomer building token-by-token chat, it offers a consistent access point for trying models such as Claude Haiku 4.5 and Gemini 2.5 Flash.

It works with OpenAI-style SDK clients, so your backend can consume streamed responses and pass updates through your chosen browser transport. Fluxio simplifies model access; your application still handles parsing, cancellation, security, and rendering.

Sign up for Fluxio to build your first streaming chat.