Yeah, deterministic is not actually the concept I want here I think. It's fine if it's random, it just needs to be unambiguous.

So you just choose in advance what settings to run with and the stopping condition. And then it doesn't matter that if you had run it with different settings, you may have gotten a different answer.

It is less fun if the outcome is non-deterministic. It means that occasionally the win is determined randomly. That takes away a certain element of skill.

I guess if you let it run for a long time it should converge on a first move?

I don't think there's a real fix for the issue, unless someone effectively solves chess someday. Otherwise your win/loss is fundamentally based on the imperfect evaluation of a particular engine.

If it's really just the nondeterminism that bothers you (which is fair enough, preferences vary), there's engines that either are deterministic or can be made so with settings.