Thanks for the reply! Yeah, it’s almost always better to use the Response API, and the “slightly smarter” is a really noticeable deal, and so are the caching improvements.
In the meantime, I’ve convinced GitHub - Laisky/one-api: OpenRouter’s open‑source alternative. Multi-model/Multi-API-format/Multi-tenant LLM API aggregation platform. to support handling the Response API request payload sent to Chat Completion API endpoints, so this can be a temporary workaround.
I’m actually very keen to see a larger migration to use the Response API, at least for all GPT-5 and 5.1 model families.