[phowon]

Put another way, even if you take the strict definition of a rational (optimal) agent, in probably any non-trivial or non-contrived corner case, for any arbitrary set of actions, there exists some utility function and information constraint that can justify that set of arbitrary functions.

Have a set of "irrational actions"? Add in information and computational constraints and the necessity for heuristics, the exact right kind of risk-aversion / novelty-seeking behavior, (or play with the second or higher derivatives of your utility function), add in social dynamics like signaling, repeated games (or incorporate mental burdens that negate the impact of repeated games) etc. There exists some formalized universe where your agent is indeed rational.