Not my experience, honestly. With a good code base for it to explore and good tooling, and a really good prompt I've had excellent results with frankly quite obscure things, including homegrown languages.

As others said, the key is feedback and prompting. In a model with long context, it'll figure it out.

But isn't this inefficient since the agent has to "bootstrap" its knowledge of the new language every time it's context window is reset?