Server-Sent Events vs. WebSockets: Architecting Low-Latency Streaming for Multi-Agent UIs

When designing a user interface for multi-agent workflows, the standard HTTP request-response cycle is dead.

An agentic pipeline takes 5 to 15 seconds to execute: it searches a database, calls a tool, checks an invariant, and synthesizes a response. If your UI waits for the entire process to finish before returning data, your user will assume the app has frozen and close the tab.

You must stream intermediate tokens, tool invocation badges, and progress milestones in real time.

Should you use WebSockets or Server-Sent Events (SSE)?

Here is the engineering breakdown of why Server-Sent Events (SSE) is the superior architectural choice for 95% of AI streaming interfaces.

Protocol Comparison: SSE vs. WebSockets

The Production Implementation in FastAPI and React

from fastapi import FastAPI
from fastapi.responses import StreamingResponse
import asyncio
import json

app = FastAPI()

async def event_stream_generator(agent_runner, user_prompt: str):
    # Stream thought tokens, tool calls, and final text
    async for event in agent_runner.stream_events(user_prompt):
        payload = json.dumps({
            "type": event.type, # "thought", "tool_start", "token", "done"
            "data": event.data
        })
        # Standard SSE event framing: "data: {...}

"
        yield f"data: {payload}

"
        await asyncio.sleep(0.01)

@app.get("/api/agent/stream")
async def stream_agent_endpoint(prompt: str):
    return StreamingResponse(
        event_stream_generator(agent_runner, prompt),
        media_type="text/event-stream",
        headers={"Cache-Control": "no-cache", "Connection": "keep-alive"}
    )

React Client Consumer Hook

import { useState, useEffect } from 'react';

export function useAgentStream(url: string) {
  const [tokens, setTokens] = useState<string>('');
  const [activeTool, setActiveTool] = useState<string | null>(null);

  const startStream = (prompt: string) => {
    const eventSource = new EventSource(`${url}?prompt=${encodeURIComponent(prompt)}`);

    eventSource.onmessage = (e) => {
      const event = JSON.parse(e.data);
      if (event.type === 'token') {
        setTokens((prev) => prev + event.data);
      } else if (event.type === 'tool_start') {
        setActiveTool(event.data.name);
      } else if (event.type === 'done') {
        setActiveTool(null);
        eventSource.close();
      }
    };

    eventSource.onerror = () => {
      eventSource.close();
    };
  };

  return { tokens, activeTool, startStream };
}

SSE works natively over HTTPS, reconnects automatically, and integrates cleanly with Next.js and FastAPI.