Claude Prompt Cache Diagnostics: Stats Thread Reveals 98.9% Cache Read Ratio

Two days ago, Anthropic released the prompt cache diagnostics feature in Claude Console. It's a tool for developers to understand why a request misses the cache and to reduce costs. One developer (u/samuelroy_) shared their stats in a community thread, aiming to find patterns and improve cache performance across the board.

Key Stats from the Source

• Overall cache read ratio: 98.9%
• 80% of cache misses are due to messages changed.
• Write amortization for Sonnet: 3.69x

The developer noted that their project harness is designed to only append messages in history, making the high miss rate from messages changed surprising. The likely explanation is users forking conversations, which changes the message chain.

What This Means

Prompt caching reduces cost and latency. With a 98.9% read ratio, the developer is already efficient, but the diagnostic data reveals a clear area for improvement: reducing unnecessary message changes. If you see similar patterns, auditing how conversations are forked or edited could boost cache hit rates.

For reference, write amortization (3.69x for Sonnet) indicates how many times a cache entry is written relative to reads. A lower value is better.

First-party analytics like this are a step forward for AI API cost optimization. Other providers are expected to follow.