The unreadable-frame text is 'message_invalid: the message could not be read' (the servers' wording; the Rust wire never carried decoder text, only its local rejection display). A put that names nothing (blank schemaId/recordKey/storedAt, empty payload, blank present source ids or tags) is refused as unreadable on every connection; the vectors' blankFields section lists the cases and the Rust server tests replay them.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>