X_MCP_PROTOCOL_UNSUPPORTED
Means
the request names an MCP protocol version this server does not serve
Typical cause
a request whose params._meta["io.modelcontextprotocol/protocolVersion"] (or MCP-Protocol-Version header) names a revision outside MCP_MODERN_VERSIONS and MCP_LEGACY_VERSIONS. Answered JSON-RPC -32022 (UnsupportedProtocolVersionError) with data: { supported, requested }, HTTP 400
Fix
resend the request with params._meta["io.modelcontextprotocol/protocolVersion"] set to a version from error.data.supported — MCP_MODERN_VERSIONS from @ultimat3/mcp lists the modern ones