HTTP SSE

If you mean HTTP SSE, it stands for HTTP Server-Sent Events (SSE).

It is a way for a server to continuously send updates to a client over a normal HTTP connection.

Normal HTTP

Client ── request ──> Server
Client <─ response ── Server
       connection ends

SSE

Client ── request ──> Server
Client <──── event 1 ── Server
Client <──── event 2 ── Server
Client <──── event 3 ── Server
Client <──── event 4 ── Server
       connection stays open

For example, a live notification system:

Server
  │
  ├── "New message"
  ↓
Browser
 
  │
  ├── "Someone liked your post"
  ↓
Browser
 
  │
  ├── "New follower"
  ↓
Browser

SSE vs WebSocket

The important difference is direction:

SSE:
 
Server ──────────> Client
       one-way
 
 
WebSocket:
 
Server <─────────> Client
       two-way

With SSE, the client doesn’t send messages back through the SSE connection. It normally uses regular HTTP requests for that.

Example SSE endpoint

Server:

app.get("/events", (req, res) => {
  res.setHeader("Content-Type", "text/event-stream");
  res.setHeader("Cache-Control", "no-cache");
 
  res.write(`data: Hello\n\n`);
});

Client:

const events = new EventSource("/events");
 
events.onmessage = (event) => {
  console.log(event.data);
};

SSE vs Streamable HTTP

They’re related, but not the same thing.

SSE
└── HTTP mechanism for server → client event streams
 
Streamable HTTP
└── HTTP transport approach that can support streaming
    and is used by MCP

So if you’re learning MCP, you may encounter SSE as an older MCP HTTP transport approach, while Streamable HTTP is the newer MCP transport approach.

Mental model:

SSE = “Keep an HTTP connection open so the server can push events to me.”
WebSocket = “Keep a connection open so both sides can communicate.”

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