An agent that retrieves on every turn pays for the retrieval on every turn. The document does not replace the last one, it joins it: each turn adds its own chunks to the window alongside the question and the answer, so a turn costs far more than the words anyone typed. At two tokens a word, even a short document doubles what a turn costs, and the conversation around it is the smaller part. That is why an agent with retrieval runs out of window in a handful of turns while a plain chat runs for dozens.
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