(Reposting from the #xtdb channel in Clojurians)
leaving this here for Googlers in the future: got a curious error message java.lang.IllegalArgumentException: No implementation of method: :latest-completed-tx of protocol: #'xtdb.db/LatestCompletedTx found for class: xtdb.kv.index_store.KvIndexStoreTx , ended up being that I was using xt/with-tx inside a transaction function, which (very reasonably) seems to be unsupported. Managed to work around it with no issue.
The cause of this seems to be the same as in xtdb/xtdb#1434, where the symptom is instead that chained calls of with-tx do not work.