hen you send a request to one of the Gemini 2.5 models, if the request shares a common prefix as one of previous requests, then it’s eligible for a cache hit,” explained Google in a blog post. “We will dynamically pass cost savings back to you.”
The minimum prompt token count for implicit caching is 1,024 for 2.5 Flash and 2,048 for 2.5 Pro, according to Google’s developer documentation, which is not a terribly big amount, meaning it shouldn’t take much to trigger these automatic savings. Tokens are the raw bits of data models work with, with a thousand tokens equivalent to about 750 words.
Given that Google’s last claims of cost savings from caching ran afoul, there are some buyer-beware areas in this new feature. For one, Google recommends that developers keep repetitive context at the beginning of requests to increase the chances of implicit cache hits. Context that might change from request to request should be appended at the end, the company says.
For another, Google didn’t offer any third-party verification that the new implicit caching system would deliver the promised automatic savings. So we’ll have to see what early adopters say.
