The bug I want to talk about doesn't throw. Nothing turns red in the logs and no test fails. A reasoning model answers the first question well, and on the second turn it acts as if the first turn never happened. You blame the prompt, then the temperature, then the model. The real cause is usually one line in your own chat loop. Here's what's going on. When you call OpenAI's Responses API through Microsoft.Extensions.AI with a reasoning model, nothing stored on the server ( store=false , or ...