Updateless Decision Theory (UDT) is a
new decision theory meant to deal with a fundamental problem in the existing decision theories: the need to treat the agent as a part of the world in which it makes its decisions. In contrast, in the most common decision theory today, Causal Decision Theory (CDT), the deciding agent is not part of the world model--its decision is the output of the CDT, but the agent's decision in the world context is "magic": in the moment of deciding, no causal links feed into its chosen action. It acts as though its decision was causeless, as in some dualist free-will theories.
UDT is very similar to Functional Decision Theory (FDT), but there are difference. FDT doesn't include the UDT1.1 fix and Nate Soars states: "Wei Dai doesn't endorse FDT's focus on causal-graph-style counterpossible reasoning; IIRC he's holding out for an approach to counterpossible reasoning that falls out of evidential-style conditioning on a logically uncertain distribution".
Both UDT and Timeless Decision Theory (TDT) make decisions on the basis of what you would have pre-committed to. The difference is that UDT asks what you would have pre-committed to without the benefit of any observations you have made about the universe, while TDT asks what you would have pre-committed to give all information you've observed so far. This means that UDT pays in Counterfactual Mugging, while TDT does not.