What is an MCP Streamable HTTP?

Streamable HTTP replaced the earlier SSE-based transport by collapsing MCP's client-to-server and server-to-client traffic onto one POST endpoint. A client sends a JSON-RPC message to that endpoint, and the server can reply immediately with a single JSON response, or keep the HTTP response open and stream multiple messages down it, such as progress updates and intermediate results, before closing. This removes the need for a separate persistent connection just to receive server-initiated messages. This design plays much better with ordinary web infrastructure — reverse proxies, CDNs, serverless functions, and load balancers — because it looks like standard HTTP traffic rather than a long-lived out-of-band stream that some infrastructure mishandles. It also supports resumability: a client can reconnect with a last-seen message ID and the server can replay what was missed, which matters for flaky networks like a mobile or sandboxed client connection. Streamable HTTP is now the recommended transport for any MCP server that isn't a same-machine local subprocess, effectively the network-facing counterpart to stdio's local-process model.