Topic 4: Transports

A transport carries the JSON-RPC messages between client and server. The messages are the same on every transport; the transport decides framing, where extra metadata travels, and how cancellation is signaled. The 2026-07-28 revision defines two standard transports, stdio and Streamable HTTP.

stdio: the subprocess model

With stdio, the client launches the server as a child process and they talk over its standard streams:

• The client writes messages to the server's stdin, one JSON object per line. Messages must not contain raw newlines.
• The server writes messages to its stdout, one per line. Nothing else may ever appear there.
• The server may write anything to stderr: logs, warnings, tracebacks. Clients may capture, forward, or ignore it, and should not treat it as a sign of failure.
• Closing stdin asks the server to shut down, and it should exit promptly. If it does not, the client escalates to SIGTERM and then SIGKILL.

The raw run in Part A showed all of this: messages as single lines, logs on stderr, and exit code 0 after stdin closed.