MCP transport (stdio and Streamable HTTP)

MCP defines what clients and servers say to each other; a transport defines how those messages travel. The specification defines two standard transports.

With stdio, the client launches the server as a local process and they exchange JSON-RPC messages over standard input and output. This is the simplest option and is common for servers that run on your own machine, such as one that reads local files.

With Streamable HTTP, the server runs as an independent web service at a URL, and the client sends requests over HTTP, with the option of streaming responses. This is how remote and hosted MCP servers work, and it is where authentication becomes important. It replaced an older HTTP plus server-sent events transport, which you may still see in older servers and documentation.