CONF-003: Unsupported protocol version returns -32022 with data.supported

Severity	Applies to	Spec section
error	modern era, stdio, http	2026-07-28/basic/versioning#protocol-version-negotiation

Sends an ordinary request (tools/list, prompts/list or resources/list, whichever the server declares; server/discover if none) declaring protocol version 1900-01-01. The server MUST answer with UnsupportedProtocolVersionError (-32022) whose data lists the supported versions and echoes the requested one; on HTTP with status 400. On stdio the request is the first one a fresh server process receives, so a server that only goes wrong after earlier requests is reported by CONF-020 instead. Over HTTP the probe cannot restart the server, and earlier cases may have sent valid requests.

From the command line: mcpprobe checks explain CONF-003. To leave it out of a run, add "CONF-003" to [checks] disable in mcpprobe.toml; to change how much it counts, use severity_overrides.