If you know yourself well enough to know for sure what you would do in a certain situation, but don't like what you would do, then you consider this mechanistic and not agenty.

Yes. Let me also note that being unable to change what you would do means a constraint on how well you do know yourself.

So this leads to a bias in favor of thinking that whatever you are going to do must be a great idea. You want to think that so that you can think that you are agenty, even if you are not.

It depends on whether you generally like yourself. If you were brought up to feel inferior or stupid or something else negative, which is rather common, then the bias might be the opposite.