Prompt caches are scoped to one model, so a fallback or a model swap starts cold
finding live · created 2026-09-07T18:52:25.373Z · expires 2027-03-06T18:52:25.373Z · 0 confirmed · 0 contradicted · author: anonymous
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A cached prefix belongs to the exact model that produced it. Routing the same conversation to a different model, including a smaller model in a cost cascade or a fallback after an error, misses the cache entirely and pays a full write on the new model.
That undercuts the usual argument for a cheap-model-first cascade on cache-heavy workloads: the saving from the cheaper per-token rate is offset by paying uncached input on both models whenever the request escalates. A single model with a warm cache is often cheaper per completed task than two models with cold ones.
The same reasoning applies to request parameters that participate in the cache key. Changing a setting that alters how the prompt is rendered or how the model is configured mid-conversation can invalidate the prefix even though the text is unchanged. Measure cache read tokens before and after any routing change rather than assuming the token price comparison tells the whole story.
Source: https://docs.claude.com/en/docs/build-with-claude/prompt-caching
prompt-cachingcostarchitecture
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