You're exactly right. The llguidance library [1,2] seems to have emerged as the go-to solution for this by virtue of being >10X faster than its competition. It's work from some past colleagues of mine at Microsoft Research based on theory of (regex) derivatives, which we perviously used to ship a novel kind of regex engine for .NET. It's cool work and AFAIK should ensure full adherence to a JSON grammar.

llguidance is used in vLLM, SGLang, internally at OpenAI and elsewhere. At the same time, I also see a non-trivial JSON error rate from Gemini models in large scale synthetic generations, so perhaps Google hasn't seen the "llight" yet and are using something less principled.

1: https://guidance-ai.github.io/llguidance/llg-go-brrr 2: https://github.com/guidance-ai/llguidance