Responses API Token Usage for Chained Responses (Threaded Conversation State)

I want to ask about Responses API using threaded conversation state.

Let’s say I chain two responses, response A and response B.

It seems like response B’s token usage doubled from response A with pretty similar query length within each of them. Is response B’s usage accumulated from response A? And if it’s accumulated, how is it billed to me? Should I just need to consider token usage from response B or still add it with response A’s usage?