As a Roblox developer, it is currently impossible to get the tri count for a meshpart without hardcoding the value ahead of time.
When working on optimizations for an experience, figuring out how expensive meshes are to render is a very important factor in determining what should and shouldn’t be rendered. This information can be very beneficial to grab dynamically for both static optimizations (while in the editor) and runtime optimizations such as custom culling based on how expensive meshes are.
Currently the process of grabbing this information from a 3rd party system and hardcoding this information somewhere is both tedious and time-consuming and could easily be avoided with a simple API call.