That said it can work surprisingly well with custom frameworks and tools provided that they are predictable and consistent.
For example, I created a platform with custom Web Components. Agents do a great job at using the components by reading the docs. I find it a lot easier and more succinct than React. I think it's because AI isn't as good with high level patterns when there are too many pieces involved and too many sub-patterns to apply, it gets so caught up in the details that it misses the forest for the trees.
My SDK abstracts away a lot of low-level complexity so that agents are able to focus on higher-level architectural patterns. Also, it's very succinct so agents can fit a lot of context/functionality into its context window. It gets faster and better as the codebase grows.
Here's the link if anyone wants to try: https://saasufy.com/