How to solve "BadRequestError: 400 Item 'rs_ ... ' of type 'reasoning' was provided without its required following item" error in Responses API

[matt.ambrogi]

This is what worked for me

One potential cause of “Item ‘rs_xx’ of type ‘reasoning’ was provided without its required following item.” error. Stateless, using Agents SDK

Overview I work on an agent which uses the OpenAI Agents SDK and GPT-5. We manage state ourselves and I recently did some work to make sure we included reasoning items in the chain of previous messages we send to the API. I could not get this to work for quite some time, running into this error and wanted to share what fixed it for me in case anyone else runs into it. Lots of other threads on this but the root cause seems to vary on each. “Item ‘rs_xx’ of type ‘reasoning’ was provided without…