The protocol on the wire

When an MCP server doesn't show up in a host, the cause is rarely the tool logic. A study that drew 400 stdio servers at random from a census of 24,135 registry entries found that only 48.8% completed an initialize handshake, and 37.5% never started at all (Afsar, 2026; a single-author preprint, so treat the numbers as indicative). Most failures happen before the first tool call: at startup or in the handshake. And the wire changed in July 2026: the handshake that study measured is now the legacy path. This lesson takes MCP's messages apart, both eras of them, so you can read what a host sends, write a server that answers correctly, and debug one that doesn't.