We've merged a draft of Proposal #349 to torspec: https://gitlab.torproject.org/tpo/core/torspec/-/blob/main/proposals/349-com...
Also available on the tor spec site at: https://spec.torproject.org/proposals/349-command-state-validation.html
This proposal is meant to deal with the third class of highly severe protocol side channels in tor: Dropped Cells. See Prop #344 for background: https://spec.torproject.org/proposals/344-protocol-info-leaks.html
Note that there still are some details that need to be ironed out wrt how and when to perform checks that depend upon full parsing and protocol context, as opposed to just relay message command.
This work is part of Sponsor 112; arti-client support is due by EOY 2024.
C-Tor will not implement this proposal.