/v1/responses with background:true does not append I/O to conversation (works when background:false)

API Bugs
1

When calling the Responses API with a valid conversation (string or { id }) and background:true, the request completes but the “user input” and “assistant output” are not appended to that conversation.

If I change only background to false, the items are appended as expected.

Environment (my setup)

  • Endpoint: POST /v1/responses with conversation (string or object)

  • Option causing issue: background:true

  • Model: gpt-5

  • Storage: store:true

  • Context: I am not using previous_response_id in the same request

  • Same API key, same conversation ID across the A/B tests