No no, you never ever build ON the models. You build WITH the models.

Never let a 3rd party LLM be the core of your product or it can be changed or taken away at any moment.

What you do is use the frontier models to build a deterministic set of tools that does what you want and MAYBE put in a small core of LLM for the ambiguous stuff you can't make deterministic (yet).

And make sure you can swap that LLM core to any other provider (even local) and have a playbook ready for that.