Prompt Caching Doesn't Save Money on Turn One

I went looking for a clean way to show what prompt caching actually saves an agent, and the first thing I found was a fact that’s easy to miss if you only read the “up to 90% savings” headline. Caching a fresh conversation’s first turn costs more than not caching it. There’s no cache to read from yet, so you pay the input price on the content plus a 25% premium to write to the cache, and get nothing back.