Good catch! It really is a combination of guidance guaranteeing JSON output and OpenAI getting it right a good majority of the time[0]. But yeah, I can see how it can be frustrating that the JSON output is not guaranteed by the docs.

[0] >>99% in my experience

That said, I am definitely going to look into this library and compare its results to guidance, since they claim it blows it out of the water (which is very enticing!)

Figure 2 in our paper (https://arxiv.org/abs/2307.09702) shows the difference for a single regex.