Your agent starts a run with cache_read_input_tokens at 40K and climbing. Twelve tool calls later, reads drop to zero and cache_creation_input_tokens jumps to the full conversation length — on every single turn. Nothing in your prompt changed. No timestamp, no reordered tool, no model switch. The prefix is byte-identical.
You just hit the 20-block lookback window, and it is the single most expensive thing about Claude prompt caching that nobody puts in their retro.
TL;DR
A cache_control breakpoint searches backward through at most 20 content blocks to find an existing cache entry. One agentic turn with 11 parallel tool calls emits 22+ blocks and blows past that — the next request finds nothing and rewrites the whole prefix at 1.25x.
Fix it by placing rolling breakpoints every ~15 blocks, not one marker on the last block. You get 4 breakpoints per request total; spend 1 on tools+system and rotate the other 3 through the message list.
