Cache Guard — stop paying 700× overhead on context rewrites

The one mechanism to understand

Prompt cache is a prefix match with three tiers: tools → system → messages. Reads from cache cost 0.1× the base input price; writes cost 1.25× — a 12.5× spread. A long conversation is cheap as long as every call re-reads the cached prefix.

Any image anywhere in the request invalidates the entire messages tier. The next call rewrites the whole conversation history at the 1.25× write price. The rewrite cost equals the current context size — in an 800k-token session, one glance at a screenshot costs ~800k cache-write tokens, even though the image itself is ~1–2k tokens. Measured in production: 91% of a session's cost was image-triggered rewrites; the images themselves were 0.13% of the tokens.

Other full-rewrite triggers: cache TTL expiry (>5 min idle, next wake rewrites everything), model switch, tool-set / MCP change mid-session, editing earlier history. Details and pricing math: references/mechanism.md.