Why was the HTTP+SSE transport deprecated as an MCP transport in 2025?

The original remote transport for MCP used two separate channels. The server pushed messages back to the client over a long-lived **Server-Sent Events** stream, and the client sent its own messages with separate HTTP POST requests. Two directions, two mechanisms. That split caused real headaches. The SSE stream had to stay open the whole session. If it dropped, the client lost the link and had to reconnect and rebuild state. Managing the lifecycle of that long connection was fiddly and error-prone. It also clashed with modern hosting. Serverless platforms like Cloudflare Workers or AWS Lambda spin up a fresh worker per request and do not hold a socket open between calls. A transport that demands a persistent open stream simply does not fit that model. Streamable HTTP fixed this by collapsing both directions onto a single HTTP endpoint. The client POSTs a request; the server replies on that same connection, optionally streaming chunks back when it needs to. One endpoint, no mandatory long-lived channel, and it works cleanly on stateless infrastructure.

HTTP+SSE was asymmetric. Server-to-client traffic flowed over a long-lived **Server-Sent Events** stream, while client to server traffic used separate HTTP POSTs. Two directions ran on two different connection types. That design created connection lifecycle pain. The SSE stream had to stay open for the whole session. A dropped stream meant reconnecting and re-establishing state. It also assumed a stateful server holding an open socket, which clashes with serverless and load-balanced deployments. A platform like Cloudflare Workers cannot hold one stream open across many short-lived workers. Streamable HTTP, introduced in the 2025-03-26 spec revision, unifies both directions on a single endpoint. The client sends an HTTP POST. The server can reply with a plain JSON response, or upgrade that same response to an SSE stream when it wants to push multiple messages. Streaming becomes optional rather than mandatory. The payoff is statelessness. A server can answer one request and forget everything, which maps cleanly onto serverless functions. Sessions, when needed, ride an explicit session id header rather than an implied open socket. Same protocol semantics, far friendlier transport.