[benatkin]

Indeed, it does deal with the message loss. I was momentarily confused because in my many thousands of bullet chess games on Lichess I haven't had much of any message loss that can be attributed to Lichess's servers (but plenty when my Internet connection is down or unstable).

I will have to take a look, because whatever it's doing, it works very well!

[crabmusket]

The at-most-once delivery could be an issue if lichess's backend services (lila or lila-ws) crash. Presumably this a rare enough occurrence that message loss is more of a theoretical concern.