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.
>>cmrdpo+(OP)
Yeah, I've had Claude work on my buggy, incomplete Ruby compiler written (mostly) in Ruby, which uses an s-expression like syntax with a custom "mini language" to implement low-level features that can't be done (or is impractical to do) in pure Ruby, and it only had minor problems with the s-expression language that was mostly fixed with a handful of lines in CLAUDE.md (and were, frankly, mostly my fault for making the language itself somewhat inconsistent) and e.g. when it write a bigint implementation, I had to "tell it off" for too readily resorting to the s-expression syntax since it seemed to "prefer it" over writing high-level code in Ruby.
>>vidarh+hR
Even 3 years ago, GH Copilot, hardly the most intelligent of LLMs was suggesting/writing bytecode in my custom VM, writing full programs in bytecode for a custom VM just by looking at a couple examples.
That's when I smelled that things were getting a little crazy.