Cache the Stable Spine of Your Prompt
Identify which prompt elements belong in a cacheable prefix and which should stay dynamic.
Prompt caching is an economic and architectural hint: the platform can reuse previously processed prompt prefixes when the repeated portion stays identical. Both OpenAI and Anthropic document the same core rule. Put static content first, keep dynamic content later, and avoid touching cached definitions unless you really changed the task. That means context engineering is partly prompt layout. Tool definitions, policies, formatting rubrics, long examples, and immutable background belong in the stable spine. User-specific facts, fresh retrieval results, timestamps, and intermediate tool outputs belong after the cache boundary. If you mix them together, you do not just lose a discount.…
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