Server-sent events (SSE)
Server-sent events (SSE) is a web standard, part of the WHATWG HTML specification, that lets a server push a one-way stream of text events to a client over a single long-lived HTTP connection, without the client needing to poll repeatedly. The Model Context Protocol used an HTTP-plus-SSE transport in its original 2024-11-05 specification for networked servers, pairing regular HTTP requests with an SSE stream for server-to-client messages, before replacing it with the Streamable HTTP transport in the 2025-03-26 specification, which folds both directions into ordinary HTTP request and response bodies instead. A server or client built against the older transport isn't automatically compatible with a Streamable-HTTP-only implementation, which matters when connecting an agent to an MCP server that hasn't been updated to the current specification.
By AdminKit team, Netbase JSC · Updated