This variable captures the last X messages in the conversation, where X is also configurable if you want it to store more messages :{{conversation.SummaryAgent.transcript}}. But it looks like you are relying on the default knowledge agent behavior which gives you less control over how it behaves. You may have to use the specific card provided to 'Query Knowedge Bases' so you have more control over the query to the KB. If you go that route you will want to configure the KB agent to disable auto-answering so you don't get duplicate responses.