Has anyone actually written a verifier for a business / project?

I used it (well, a skill based on the same idea) to optimise a prompt that does data extraction from UGC.

However there isn't really a "correct" answer that's easy to define in code (I could manually label a training set, but wanted to avoid that) so I had the LLM just analyse the results itself and decide if they are better or not. It wrote deterministic rules for a few things, but overall it just reviewed the results of each round and decided if the are better or not.

Reviewing the before and after results, I would say yes, it's a big improvement in quality. It also optimised the prompt size to reduce input tokens by 25% and switched to a smaller/cheaper model.

I'd say "a verifier" here is a loose term. A great testsuite is a verifier. I've done reverse-engineering projects that involved generating trace logs from the object under test, having a reimplementation emit the same logs, and running strict comparisons.

OP's post is basically pointing out what certainly many others have independently discovered: Your agent-based dev operation is as good as the test rituals and guard rails you give the agents.

Can you explain your question a little more? The recursive agents will find the minimum to satisfy the deterministic termination condition including cheating. In other words, it will be literally correct yet wrong. I would go so far to say malicious compliance.

I have recursive agent that finds trading strategies after recreating academic research and probing the model using its training on everything. It works really well but I have to force it to write out every line and write a proof that data in the future from the time of the wall clock didn't enter the system. Even then some stupid thing like not converting the timezone with daylight savings will allow it to peek into the future 1 hour. These types of bugs are almost impossible to find. Now there needs to be another agent whose only purpose to write out every line explaining that the timezone for that line of code was correct.

Its tangential, but: I’m currently doing a rewrite of the backend of a project, and the verifier is basically the instruction of “maintain v1 functionality if observed from the api side externally”. This allows making a lot of tests based on existing data in the system and how the frontend expects data.