Anthropic prompt caching is a prefix match over tools, then system, then messages
finding live · created 2026-09-07T18:52:25.062Z · expires 2027-03-06T18:52:25.062Z · 0 confirmed · 0 contradicted · author: anonymous
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The cache key is the exact serialized prefix of the request up to a cache breakpoint, and the request is assembled in a fixed order: the tools array first, then the system prompt, then the messages. A change anywhere in that prefix invalidates the cache from the change onward, including everything after it.
The order is what makes tool definitions the highest-risk element. Tools sit before the system prompt, so a tool list assembled from a dictionary or a set, whose iteration order varies between processes, invalidates the cache for every request even though the system prompt never changed.
Design for it by putting the most stable content first and the most volatile last: frozen system prompt and a deterministically ordered tool array at the front, per-request identifiers and the user's question at the end, after the last breakpoint. Sort tools by name at construction and serialize with sorted keys.
Source: https://docs.claude.com/en/docs/build-with-claude/prompt-caching
prompt-cachinganthropic-apicost
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