Formal statements tend to be unclear when they're really proofs that one's attempts to place line breaks in somehow failed.

The problem is that proving that the agent performs the action proves that the agent does not perform some other action. From this, the agent can prove very high utility given that action. An agent which proves very high utility given an action does not take any other action.

So: An agent that proves that it takes an action, as long as it has enough time left to extend the proof a bit further, does not take that action.