> strong guarantees on the output (i.e. 100% reliable, without requiring any retries).

Has anyone seen a good JSON library that can handle slightly broken JSON? e.g. trailing commas, unescaped newlines, etc.? I have not found a good one.

Entirely broken JSON, no — I would be surprised if one existed. If you want slightly laxer semantics like trailing commas, JSON5 [1] is a pretty good spec and is JSON-compatible. I used to use it for LLMs (while telling them to emit JSON — no need to confuse them by explaining JSON5), in order to handle things like trailing commas, but in my experience LLMs have gotten good enough over the last year I mostly don't even bother anymore.

1: https://json5.org/

Should be easy to build on top of a lexer as a pre-parsing pass.

Dirtyjson

You can force LLMs to generate valid json by using a context free grammar FWIW

Please elaborate

Matt Rickard has a good entry level blog post about it, from the angle of regex constraining [0]. Context free grammars follow the same principle, except using a finite state machine to restrict the action space.

[0]: https://matt-rickard.com/rellm

Look at llama.cpp grammars, lmql, or guidance-ai

I've been doing a lot of indie work with structured generation and llama.cpp, you can get extremely fast responses with caching and deterministic token skipping.

When generating json, calling the llm when you already know that after

    { "brand": "Toyota"

Comes

     , "year": 

Is a massive waste. If the data itself is constrained too you can skip most of that too! You'll go down from needing 20 calls to the llm to just three for a simple piece of data like

    { "brand": "Toyota", "year": 1995 }

If they combine these techniques with a model that's specifically trained for structured output, along with a novel inference-time pruning technique that they were talking about in the post I can definitely see them getting these kinds of inference speeds.

I'm experimenting with a self hosted api that is fast enough to not even need a gpu for single user use cases (because the latency is good, but not batching). Once I'm done with the finishing touches I'll rent a GPU server for actual hosting.

Have you seen any blog posts that have some details on this? I can imagine, roughly, the concept but it sounds interesting and I'd like to get a better understanding.

Yeah, dottxt has a post on this, it's a technique starting with a c I believe, something like continuum or something?

The op also has something about compressing finite state machines, it's basically the same thing with slightly different details in the implementation.

Check out outlines by .txt : https://github.com/outlines-dev/outlines

sglang: https://arxiv.org/abs/2312.07104

Thanks!

I thought they pretty clearly explained where the additional performance came from. If there is only 1 valid schema confirming option you can skip the LLM entirely. If there are only a limited number of possible tokens (eg. only } or ,) then you run it on smaller subset of the model. Between these two you capture a large amount of the actual token count of most json.