UCPOP's soundness depends on the perfect knowledge assumptions discussed in Section 1. In particular, UCPOP's plans are sound if the initial conditions are fully specified, and if all possible effects of actions are specified in the operators that represent them. If no uncertainties are involved in the plan, Cassandra is equivalent to UCPOP and therefore constructs sound plans.
If uncertainties are involved in the plan, it can no longer be assumed that the initial conditions and effects of actions are fully specified. Indeed, the uncertainties arise because these assumptions are violated. However, the assumptions can be adapted to account for the presence of uncertainty: it would be possible, for example, to insist that all possible initial conditions and action effects are specified. In Cassandra's representation, this means that every source of uncertainty must be specified through the use of unknown secondary preconditions, and every possible outcome of each source of uncertainty must be specified.
We conjecture that Cassandra is sound under these conditions. The proof would follow because the procedure for adding in new goals whenever a new source of uncertainty is encountered ensures that every goal is achieved in every possible outcome of the uncertainty.