Summary

OpenAI recently released the Responses API. Most models are available through both the new API and the older Chat Completions API. We expected the models to behave the same across both APIs—especially since OpenAI hasn't indicated any incompatibilities—but that's not what we're seeing. In fact, in some cases, the differences are substantial. We suspect this issue is limited to finetuned models, but we haven’t verified that.

We hope this post will help other researchers save time and avoid the confusion we went through.

Key takeaways are that if you're using finetuned models:

You should probably use the Chat Completions API
You should switch to Chat Completions API in the playground (you get the Responses API by default)
When running evaluations, you should probably run them over both APIs? It's hard to say what is the "ground truth" here.