Question 1

Why does a long chat cost more than the number of messages suggests?

Accepted Answer

Because an LLM has no memory between calls, every turn must re-send the entire prior conversation as input so the model can see the context. On turn ten the model re-reads all nine previous exchanges plus the system prompt, on turn twenty it re-reads nineteen, and so on. Input tokens therefore grow with each turn, and the cumulative cost of the whole conversation grows with roughly the square of the turn count rather than linearly. A hundred-turn conversation can cost far more than a hundred single-turn calls of the same message size — the history is what you pay for, over and over.

Question 2

How can I stop conversation cost from exploding?

Accepted Answer

Three levers. A sliding window keeps only the last few turns of history, capping the input at a fixed size so cost grows linearly instead of quadratically — at the price of the model forgetting older context. Prompt caching lets the provider store the repeated prefix and bill it at a large discount on every reuse, which is ideal when the early history and system prompt stay identical. And summarisation compresses old turns into a short recap so you keep the meaning without keeping the tokens. This calculator shows the naive full-history cost alongside a windowed and a cached version so you can see which lever pays off for your conversation length.

Question 3

Does a bigger system prompt matter in a long conversation?

Accepted Answer

Yes, more than people expect, because the system prompt is re-sent on every single turn. A 2,000-token system prompt in a fifty-turn conversation is billed fifty times — a hundred thousand input tokens before any user message is counted. In long or high-volume chats the fixed system prompt is often a larger share of the bill than the actual conversation, which is exactly the kind of cost that prompt caching is designed to remove. The calculator separates the system-prompt contribution so you can see whether trimming or caching it is worthwhile.

マルチターン会話コスト計算ツール

ターン数に応じてコストが加速する

毎ターン、歴史の対価を支払うことになる

この計算機の仕組み

よくある質問

長時間のチャットの料金がメッセージ数よりも高くなるのはなぜですか?

会話コストの爆発を防ぐにはどうすればよいですか?

長い会話では、より大きなシステムプロンプトが重要ですか?