WebSockets and Server-Sent Events (SSE) solve the same immediate problem: getting data from a server to a browser without the client asking for it first. Most comparisons stop at how their connection types and browser support compare, to help you decide which one to use.
This one covers that ground too, but goes further, into what neither protocol solves for you and whether to build that yourself or hand it to a managed platform.
What are WebSockets?
WebSockets are a thin transport layer on top of TCP, giving you a single, full-duplex connection between the browser and the server after one HTTP handshake. Once that handshake completes, both sides can send messages at any time, with no further requests needed. Because the connection stays open, WebSockets also avoid the repeated handshakes and headers of a traditional HTTP request/response cycle, which keeps latency low even at high message volumes.
Pros: full-duplex and low-latency, since the connection stays open with no repeated handshakes.
Tradeoff: no built-in reconnection, so a dropped connection is yours to handle.
What are Server-Sent Events?
Server-Sent Events use the EventSource API to let a browser subscribe to a one-way stream of updates over plain HTTP. There's no separate protocol to implement, no handshake beyond the initial request, and browsers reconnect automatically if the connection drops.
Pros: reconnects automatically and runs over plain HTTP, with no separate protocol to implement.
Tradeoff: one-way, text-only, and limited to six connections per origin under HTTP/1.1.
What are the differences that matter between WebSockets and Server-Sent Events?
The headline differences are easy to list. What matters more, especially once this is running in production, is why each one matters and how it should impact your decision:
Should I use WebSockets or Server-Sent Events?
This isn't only a decision about the feature you're building right now. It's about whether SSE's one-way limit, or WebSockets' lack of built-in reconnection, will still hold up for the product that the feature lives inside, over time.
Selecting the right protocol for your product means walking through four questions, roughly in this order:
- Does the client ever need to send something back once the stream is open, not only when the request starts, now or on any near-term roadmap? If yes, that alone tips you toward WebSockets.
- Is the data binary, or will volume push past SSE's six-connections-per-origin ceiling under HTTP/1.1? Either constraint rules SSE out on its own.
- Will a meaningful share of your users sit behind corporate networks or proxies you don't control? SSE has a real edge here if you don't need bidirectional communication: it runs over plain HTTP and doesn't trip firewall inspection the way WebSockets can. If you do need something that can talk back regardless, budget for a fallback path (HTTP streaming, long polling) or a managed provider that ships one.
- Are you able and willing to staff the complexity that comes with WebSockets, such as reconnection, retries, and connection state, or would you rather hand that to a managed platform? It's tempting to defer this question when you're early in the build. But that complexity is often the real reason teams rule out WebSockets before weighing the tradeoffs properly.
And if you're still undecided after answering these four questions, here's a breakdown of the scenarios where WebSockets or SSE is the better fit.
When to use WebSockets
WebSockets are the right default when the interaction is genuinely two-way. Chat, live streaming, an AI agent that a user needs to interrupt or redirect mid-response: anything where the client needs to push data back as often as the server does.
If you're building or self-hosting this, you take on more upfront work (reconnection logic, testing behavior behind corporate proxies) in exchange for a channel that can carry binary data and handle high message volumes on a single connection. A managed platform absorbs most of that work instead, which is why question 4 above is worth revisiting once you know WebSockets is the direction you're heading.
That reconnection logic isn't optional, either. A dropped WebSocket connection doesn't try to come back on its own, so if you don't build or buy that layer, your users find out about a dropped connection before you do.
Why use Server-Sent Events over WebSockets?
SSE earns its place when the traffic really is one-way and built-in reconnection matters more to you than bidirectional support. It comes with a side benefit too: because it runs over plain HTTP, it doesn't trip the firewall and packet-inspection issues WebSockets can hit. A live sports score, a build status page, a notification feed: none of these need the client to talk back, so the extra complexity of a two-way channel buys you nothing here.
The one-way limit itself is the risk. If a single feature grows even a small two-way requirement (e.g. a stop button, a read receipt, or a way to approve an agent's next action), SSE can't absorb it on its own. You either bolt on a second channel, typically a plain HTTP POST alongside the SSE stream, or migrate that surface to WebSockets. Either way, the decision stops being about one feature: once part of your product needs bidirectional support, you're maintaining two transport patterns side by side, or moving everything over. Decide upfront whether SSE's simplicity is worth that risk for the product as a whole, not only for the feature in front of you today.
Is WebSockets or SSE better for AI streaming?
SSE only sends messages in one direction: the server streams tokens to the client, but the client can't send anything back while the stream is active. That's fine for a single prompt and a single response. It breaks down the moment you need cancellation, redirection, or tool-call approval mid-stream, because each of those needs a signal to travel from the client to the agent, not only tokens flowing the other way.
The Vercel AI SDK uses SSE as its default transport, and for a single prompt and a single response, it works well. That's the same one-way tradeoff covered above, applied to token streaming instead of a stock ticker or a notification feed.
Vercel's own native WebSocket support, in public beta since June 2026, narrows this bidirectional limitation without closing it. Connections inherit the function's duration limit, and they're still pinned to a single function instance with no fan-out to other devices. That's why Vercel's own Knowledge Base points developers towards using a third-party WebSocket provider, plugged in through the AI SDK's ChatTransport interface, for anything beyond a single, session-scoped connection.
For more on this, see Why WebSockets beat SSE for AI streaming at scale, which walks through the failure modes, the reconnection and fan-out work WebSockets still leave you to manage, and what changes once that moves to a managed platform.
What both WebSockets and SSE leave you to solve
Picking a protocol answers how data moves between the server and the client. It doesn't answer what happens once that connection is live, because neither protocol comes with guarantees about delivery, continuity, or visibility into problems. Let’s look at each area in turn.
- Delivery: Neither protocol guarantees a message arrives, or tells you when it doesn't. A WebSocket message sent during a network drop can be lost with no signal that it happened, and an SSE stream that drops mid-update leaves a gap, not a pause, with no way to tell the client what it missed.
- Continuity: Neither protocol guarantees a session survives a second device or a refresh. Opening the same session elsewhere, or refreshing the page, starts both protocols from zero: there's no shared state to resume from, only a new connection.
- Observability: Neither protocol guarantees you'll see a problem before your customer does. Debugging a production issue usually means digging through logs after the fact, rather than watching the connection's health in realtime.
A managed platform exists to solve exactly these problems, on top of whichever protocol you choose, which is why the decision above isn't the last one you need to make.
What a managed platform gives you instead
A managed, WebSocket-based platform removes the need to choose between WebSockets and SSE in most cases. Ably, for instance, defaults to a WebSocket connection and falls back automatically to HTTP streaming, then long polling, if a proxy or firewall blocks it. You get a two-way channel where the network allows it, and something that still works where it doesn't, without building that fallback logic yourself.
It also provides the delivery, continuity, and observability guarantees described above. And that matters even if you never need two-way communication. A one-way feed still has to arrive reliably, survive a user switching devices, and surface problems before your customers do.
For Ably, these guarantees sit under what is known as our Pillars of Dependability.
- Delivery: Ably ensures that messages are delivered in order, with automatic recovery if a connection drops briefly, so a disconnection doesn't mean lost data. That guarantee holds across 20+ SDKs spanning every major language and platform. Fin, for example, sees 99.9999% delivery reliability across workspaces on Ably.
- Continuity: sessions can follow a user across devices instead of starting from zero on every new connection. Ably keeps that session state server-side, so resuming on a new device or after a dropped connection picks up where it left off, rather than replaying everything.
- Observability: because the platform runs the infrastructure, you get visibility into connection health as it happens, rather than logs to dig through after the fact. That visibility comes via a live dashboard and the Control API.
What you're trading for these guarantees is control. You're relying on someone else's infrastructure, uptime, and pricing instead of your own, which is worth weighing carefully if you have strict data residency requirements or you're already deep into a self-hosted stack. For most teams outside those cases though, the trade holds up: the engineering time you get back from not maintaining realtime infrastructure yourself outweighs what you give up in control.
Colin Kennedy, Principal Product Engineer at Fin (formerly Intercom), put it this way after moving Fin's realtime layer onto Ably:
"The biggest win isn't just that things work. It's that we trust the system. We're not designing around gaps anymore. We're building what we actually want to build."
Choosing the right managed platform
Not every managed provider backs the same delivery, continuity, and observability guarantees. Before picking one, it's worth checking what it actually promises, rather than taking a marketing claim like "reliable" at face value:
Ably meets each of these criteria, but it's worth checking that yourself against Ably's status page and security and compliance documentation, rather than taking our word for it.
Ready to get started?
Sign up for a free Ably account and work through our quickstart guide to see how quickly you can get a production-ready connection running, without building and maintaining that infrastructure yourself.
