Very confused. When you enable structured output the response should adhere to the JSON schema EXACTLY, not best effort, by constraining the output via guided decoding. This is even documented in OpenRouter's structured output doc
> The model will respond with a JSON object that strictly follows your schema
Gemini is listed as a model supporting structured output, and yet its fail rate is 0.39% (Gemini 2.0 Flash)!! I get that structured output has a high performance cost but advertising it as supported when in reality it's not is a massive red flag.
Worst yet response healing only fixes JSON syntax error, not schema adherence. This is only mentioned at the end of the article which people are clearly not going to read.
WTF
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
Cool stuff! I don't get how all the open source inference framework have this down but the big labs doesn't...
Gemini [0] is falsely advertising this:
> This capability guarantees predictable and parsable results, ensures format and type-safety, enables the programmatic detection of refusals, and simplifies prompting.
[0]: https://ai.google.dev/gemini-api/docs/structured-output?exam...