[senko]

It would seem not, as the official documentation mentions the arguments may be hallucinated or be a malformed JSON.

(except if the meaning is the JSON syntax is valid but may not conform to the schema, but they're unclear on that).

[sanxiyn]

For various reasons, token selection may be implemented as upweighting/downweighting instead of outright ban of invalid tokens. (Maybe it helps training?) Then the model could generate malformed JSON. I think it is premature to infer from "can generate malformed JSON" that OpenAI is not using token selection restriction.