A team enabled prompt caching with a cache_control breakpoint after the system prompt. Usage data shows cache write tokens on nearly every request and almost no cache read tokens. The prompt order is: tool definitions, then a system prompt whose first line is 'Session: {session_id}, opened {timestamp}', then a long policy document, then the user message. What is the most likely cause?

Answer: A — Per-request session and time values before the breakpoint make every prefix unique, so none is reused

(a) is right: caching only hits on an identical prefix, and a session ID and timestamp near the top change it on every request, so each call writes a new cache entry that is never read. Move volatile values after the breakpoint. (b) would show no cache writes at all, not writes on every request. (c) is false; usage data reports cache reads and writes. (d) is false; tool definitions are part of the cacheable prefix.