Whether you use 3.15 or not, if your project already passes a modern type checker, one thing that you can do easily using AI is to significantly tighten (narrow) the type annotations of your functions. Run this two or three times until the annotations are sufficiently but not excessively narrowed. This prevents a whole lot of bugs, and increases clarity of the code for AI.

This matters more for newer versions of Python which actually offer the constructs needed for it. Python 3.15 extends this with sentinel and enhancements to TypedDict.