Prompt-Cache Economics

Prompt caching and prompt compression are one joint optimization, not two independent levers — CAPC measures Anthropic Sonnet 4.6's cache at ρ ≈ 0.83 rather than the compression literature's assumed 1.0, finds a step change near 3,500 cached tokens, derives a provider-agnostic crossover from three pricing constants, and shows query-aware compression costing +40.1% more than sending nothing compressed on a public benchmark; the corpus's first end-to-end billed-cost audit of a production caching API ($98.96 total, reconciled to Anthropic's invoice within 1%)