Unverified50% confidenceFactExact time
Placing a timestamp accurate to the second at the beginning of the prompt causes every request to differ from the first character, so the prefix cache can never hit and everything is billed at full price.
1
Sources
50%
Confidence
Long-term
Relevance
10/15/2026
First Seen
Sources
Related Entities
Related Claims
UnverifiedWhether prompt caching hits depends on byte-for-byte consistency of the request prefix; if even one character at the beginning differs, the entire cache is invalidated.79% similarUnverifiedIf most of the prompt is repeated and fixed while cache_read_input_tokens stays near zero for a long time, it can be almost certain that the prefix changed between requests.78% similarUnverifiedPrompt caching can avoid repeated charges for identical prefixes, because system prompts and retrieved document fragments tend to be highly repetitive across multiple conversation turns.74% similarUnverifiedPrompt caching divides prefixes by 'breakpoints', and the prefix must be byte-for-byte identical to hit the cache, so it is critical to put unstable content such as timestamps and random IDs in the suffix.72% similarUnverifiedPrompt Caching requires the prompt to meet a minimum cacheable length (typically 1024 tokens for Anthropic), and the prefix must be byte-for-byte identical for the cache to hit.65% similar
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