Every turn resends the chat — see why a long conversation costs far more than its words.
Resending history every turn means long chats cost far more than the words in them. Cut the repeated prefix with caching — see the break-even on the prompt cache calculator — and check the running total fits the window on the context window calculator.
Free cost-per-conversation calculator — model how resending context each turn makes a multi-turn chat cost far more than the tokens it adds.
Because the model is stateless: every turn you resend the entire prior conversation as input. By turn eight you are paying for the first turn eight times over. Total billed tokens grow roughly with the square of the turn count, so length, not just message size, drives the cost.
Trim or summarise old turns instead of resending everything, use prompt caching so the repeated prefix is billed at a discount, cap conversation length, and route simple turns to a cheaper model. These attack the re-sent context that makes long chats expensive.