Is Implicit Caching Prompt Retention?

Implicit caching in large language models, such as those used by OpenRouter, has sparked debate over whether it constitutes data retention, particularly in light of Google's stance that it does. Implicit caching involves automatically maintaining a cache of key and value tensors that represent a model's intermediate activations, enhancing performance by reducing computational redundancy without explicit user control. This process typically uses NVMe/SSD storage for efficiency, but only ephemeral KV tensors are stored, which are difficult to reverse-engineer into the original prompt data. Despite Google's conservative interpretation of implicit caching as disqualifying from Zero Data Retention (ZDR) compliance, OpenRouter argues that this form of caching does not equate to data retention since the data is transient and not operationally extractable. OpenRouter maintains transparency about which endpoints use implicit caching and continues to work with providers to safeguard sensitive data while ensuring that customers can make informed decisions about their data protection preferences.

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