How to use WebSockets with the Vercel adapter?

[alexbjorlig]

I would like to experiment with WebSockets. Our application is delpoyed using the official Vercel adapter.
Now that Vercel announced support for websockets, this made me think - is there an official way to use websockets in Sveltekit when using the Vercel adapter?

The end-goal would be to build something simple with socket.io, so we don't need a pusher subscription.

[brunnerh]

WebSocket support is in limbo, see #12973

@teemingc any news on when there will be websockets support? really really looking forward to it!

We're focusing on SvelteKit 3 right now. We will revisit WebSockets after

[brunnerh]

(In case you only need live data updates from the server rather than duplex communication, I would recommend using server-sent events for now.)

[MILLERMARRU]

Worth separating what's actually in limbo from what already exists. adapter-vercel itself doesn't wire up WebSocket upgrade handling today, that's exactly the gap tracked in #12973, and teemingc's comment confirms it's deprioritized behind SvelteKit 3 work.

The Vercel platform side is real though, and it isn't Next.js-specific. @vercel/functions ships experimental_upgradeWebSocket, and any Vercel Function that returns a Response from a Fetch-API-shaped handler can use it:

import { experimental_upgradeWebSocket, type WebSocketData } from '@vercel/functions'

export async function GET() {
  return experimental_upgradeWebSocket((ws) => {
    ws.on('message', (data: WebSocketData) => {
      ws.send(data)
    })
  })
}

A SvelteKit +server.js GET handler is exactly that shape, a function returning a Response, so in principle there's nothing framework-specific stopping you from calling this directly inside one, since adapter-vercel compiles each server route down to a Vercel Function already. I haven't seen anyone confirm adapter-vercel's build step passes that response through untouched rather than wrapping or re-serializing it, and that's the actual unknown behind the open issue, not whether Vercel supports WebSockets at all.

For getting unblocked now rather than experimenting: deploy the socket.io server as its own separate Vercel Function (outside the SvelteKit build, or even a separate minimal Vercel project) using experimental_upgradeWebSocket directly, and point socket.io-client at that endpoint from your SvelteKit frontend. That sidesteps the adapter question entirely and is the path most likely to just work today.

[latent-9]

Short version: you cannot run a WebSocket server on adapter-vercel. Vercel's serverless and edge functions are request-scoped and get torn down after each response, so they cannot hold the long-lived connection a WebSocket needs. SvelteKit's experimental WebSocket support only works on adapters that keep a persistent Node server alive (adapter-node or a custom server).

Practical options:

• Offload realtime to a managed service (Ably, Pusher, Supabase Realtime, or PartyKit) and keep the rest of the app on Vercel.
• Deploy just the WebSocket piece on a host that stays alive (Fly, Render, Railway) and point the client at it.
• If you only need one-way server to client updates, Server-Sent Events over a streamed endpoint do work on Vercel and are much lighter.

So the app itself can stay on Vercel. The socket server just has to live somewhere persistent.

[teemingc]

I don't think that's true. https://vercel.com/docs/functions/websockets seems to imply it's possible now

[brunnerh]

...and most server-sent events connections are also long-lived.