Author here. Short version is that YAML 1.1 behavior has a lot of bad, but the 1.2 spec addressed the major problems (way back in 2009!). I don't really know why PyYAML never updated to 1.2. So for the intervening years Python users kept hitting 1.1 bugs and blaming the format.

I wonder if YAML is better than JSON for LLMs, because the indentation keeps the nesting level information local to the content.

So now there are two mutually incompatible versions of YAML that silently apply different semantics: both do implicit coercion but _different kinds_ of implicit coercion. YAML 1.2 still coerces an arbitrary set of ways to spell "true" and that's exactly the insanity that created this disaster in the first place. The YAML spec writers just couldn't let go of their bad habits.

Honestly, this seems like an even worse situation than before. If you're making a new tool and choosing a configuration format, stay away from YAML.