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Instrument each request to record five quantities: new input tokens, cache-write tokens, cache-read tokens, visible output tokens, and reasoning tokens. The cost of a single request is then $c = p_{\mathrm{in}} n + p_{\mathrm{cw}} w + p_{\mathrm{cr}} r + p_{\mathrm{out}} \left( y + z \right)$. where n , w , r , y and z are the five counts in that order and each p is the corresponding published rate. The important feature is the last term: visible output and reasoning share one price, and the reasoning count is the only one of the five you cannot recover by inspecting what came back.

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