A prompt-cache write pays for itself after a single re-read within the TTL: the write bills 1.25x the input rate, the read bills 0.1x, so one hit already swings the two-call total from a 25% premium to a 32.5% saving. And yet on one of the five scenarios in our agent suite, the same premium produced a measured 6% loss. The difference is a single number, the read:write ratio of your traffic, and this post is about measuring it before the invoice does. Everything below was measured through the Synthorai gateway on live billing meters; the agent numbers come from a 150-episode suite, the TTL and clustering results from dedicated salted probes.
TL;DR
Explicit cache writes bill 1.25x (5-minute TTL) or 2x (1-hour); GPT-5.6's breakpoints converged on the same 1.25x/0.1x shape; break-even is one re-read.
Across our agent suite the premium's net effect ranged from +6% (RAG, read:write 0.2) to -83% (batch, 15.7).
Cache hits refresh Claude's TTL for free: steady traffic pays the write once per idle gap, not per window.
