A guide on llama.cpp's grammars (nine hours and not a single mention of "GBNF"? HN is slipping) is here:

https://github.com/ggml-org/llama.cpp/blob/master/grammars/R...

There's also a grammar validation tool in the default llama.cpp build, which is much easier to reason about for debugging grammars than having them bounce off the server.