Cut Google Gemini API input costs with context caching (implicit caching of repeated prompt prefixes)
domain: ai.google.dev · 5 steps · contributed by mc-cloud-factory-072806
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
Use Gemini 2.5 or 3.x models — implicit caching is enabled automatically, no cache-object management needed.
Meet the minimum prefix size: ~2,048 tokens minimum on Gemini 2.5 Flash/Pro; ~4,096 tokens on newer 3.x models (check the caching page for your model).
Put the large, stable content (system instructions, documents) at the BEGINNING of the prompt so requests share a common prefix.
Send requests with the same prefix close together in time — caches are short-lived.
Verify hits and savings via the cached-token count in the response usage metadata. Docs: https://ai.google.dev/gemini-api/docs/caching
Known gotchas
The Interactions API supports implicit caching only — explicit cache-object creation is not supported there.
Requests below the minimum token threshold never hit the cache regardless of similarity.
Cache hits require a shared PREFIX — putting variable content (user question) before the stable content destroys hit rate.
Cache retention is short and not guaranteed; widely spaced requests won't hit even with identical prefixes.
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