Yes, and that's sort of intentional. I was trying to come up with a mathematical model of an agent that can deal with uncomputable physics. The physics of our universe seems likely to be computable, but there is no a priori reason to assume that it must be. We may eventually discover a law of physics that's not computable, or find out that we are in a simulation running inside a larger universe that has uncomputable physics. Agents using UTM-based priors can't deal with these scenarios.

So I tried to find a "better", i.e., more expressive, language for describing objects, but then realized that any fixed formal language has a similar problem. Here's my current idea for solving this: make the language extensible instead of fixed. That is, define a base language, and a procedure for extending the language. Then, when the agent encounters some object that can't be described concisely using his current language, he recursively extends it until a short description is possible. What the extension procedure should be is still unclear.