I think the delta may be an overloaded use of "turn"? The Responses API does preserve reasoning across multiple "agent turns", but doesn't appear to across multiple "user turns" (as of November, at least).

In either case, the lack of clarity on the Responses API inner-workings isn't great. As a developer, I send all the encrypted reasoning items with the Responses API, and expect them to still matter, not get silently discarded[0]:

> you can choose to include reasoning 1 + 2 + 3 in this request for ease, but we will ignore them and these tokens will not be sent to the model.

[0]https://raw.githubusercontent.com/openai/openai-cookbook/mai...