🗓️ 19092026 1003
Server-Sent Events (SSE) keeps an HTTP response open so a server can send a sequence of text events to a client.
What problem does it solve?
Polling makes a client repeatedly ask whether anything has changed. SSE lets the server push updates as they become available, without requiring a new request for every update.
It is designed for one-way communication:
client -- opens HTTP request --> server
client <-- update 1 -- server
client <-- update 2 -- server
client <-- update 3 -- server
The response uses the text/event-stream content type. An event can carry text or JSON, but SSE does not define the application's data model.
Where it fits
SSE is a natural choice for:
- notifications and live feeds;
- dashboards and log streams;
- job progress;
- incremental assistant responses.
The client can still send commands through ordinary HTTP requests. This makes SSE useful when traffic is asymmetric: occasional client actions, but frequent server updates.
Tradeoffs
SSE is simpler than a bidirectional socket because it stays within ordinary HTTP and provides a browser-native client API. The browser can reconnect when the connection drops.
The costs are equally important:
- the connection stays open and consumes server and proxy resources;
- the stream is text-only and server-to-client;
- authentication and custom request headers can be awkward with native
EventSource; - reliable replay, cancellation, buffering, and backpressure remain application concerns.
Reconnection is not the same as guaranteed delivery. If an application needs to resume missed updates, it must retain enough history and define how a client catches up.
Choose websocket_protocol when both sides need a continuous bidirectional channel or binary frames. Choose polling or long_polling when updates are infrequent or long-lived connections are difficult to operate.
SSE, AI chat, and MCP
These concepts sit at different levels:
- AI chat streaming is the user-visible behavior of displaying partial model output.
- SSE is one HTTP mechanism for sending a sequence of server events.
- MCP Streamable HTTP is a JSON-RPC transport that may use SSE for streamed responses or long-lived change notifications.
So an AI chat API may use SSE without being MCP. An MCP server may use Streamable HTTP without keeping a permanent SSE connection open. mcp_transports explains that boundary.