Today I found the weirdest bug in my life. I was making a chatbot for Signal using Ollama in Rust. I finished a basic demo and tried it. For any message I would get { }, {}, {} or { } .

Do you know how hard is to debug something like this???

What was the problem? Not my program. Ollama bug combined with ollama-rs bug (rust library for ollama). And both bugs are not even bugs if you don’t combine them.

Ollama released a new feature yesterday called “Structured outputs”. Basically you can specify a format of the output using format field in json request. Format field already existes for something but I don’t know for what. In ollama-rs you can specify the format to json or leave it empty. By default its empty. Where is the bug?

There is a difference betwen "format": null and not specifying the format at all. Ollama-rs will set format to null if you dont specify it. Ollama will interpret null as a valid format. What happens? LLM WILL ACTUALLY GIVE YOU FORMAT OF OUTPUT AS NULL - { }!