{"i":20,"slug":"prompt-cache-break-even","chart":{"id":"cache-break-even-reads","title":"Reuses before a cached prefix costs less, by model and write type (calculation)","subtitle":"The break-even point: reuses at which the cached and the uncached cost are equal, at list prices","kind":"grouped-bar","unit":"score","yLabel":"Reuses at break-even","series":{"$k":["name","points"],"$r":[["1-hour write (2× input), whole prefix new",{"$k":["label","value"],"$r":[["Claude Haiku 4.5",1.11],["Claude Sonnet 5.5",1.11],["Claude Opus 5.5 (cache read $0.2 per M)",1.05],["Claude Opus 5.5 (cache read $0.4 per M)",1.11],["Claude Fable 5.1",1.03]]}],["1-hour write, pooled n = 6 session share, 19% already cached (as recorded)",{"$k":["label","value"],"$r":[["Claude Haiku 4.5",0.72],["Claude Sonnet 5.5",0.72],["Claude Opus 5.5 (cache read $0.2 per M)",0.67],["Claude Opus 5.5 (cache read $0.4 per M)",0.72],["Claude Fable 5.1",0.65]]}],["5-minute write (1.25× input, an assumption)",{"$k":["label","value"],"$r":[["Claude Haiku 4.5",0.28],["Claude Sonnet 5.5",0.28],["Claude Opus 5.5 (cache read $0.2 per M)",0.26],["Claude Opus 5.5 (cache read $0.4 per M)",0.28],["Claude Fable 5.1",0.26]]}]]},"note":"Calculation, not a run: break-even reuses = (write price − input price) ÷ (input price − cache-read price). A cached prefix costs less once the reuses pass that point. On a new prefix, a 1-hour write needs 2 reuses (the 3rd request). With 19% already cached it needs 1 reuse, and a 5-minute write needs 1 reuse. The 1.25 of the 5-minute write is an assumption. The caching study protocol states it as Anthropic’s published figure. No 5-minute write occurred in the recorded sessions. We recorded the 19% on Sonnet 5.5 and Opus 5.5 sessions. For Haiku 4.5 and Fable 5.1 it is a what-if. GPT-6.1 Sol and GPT-6 Luna list no write surcharge, so their break-even is 0 reuses and the chart leaves them out. The price list gives Opus 5.5 a cache read of $0.2 per million. Another table of the product lists $0.4. We did not check the vendor price, so the chart shows both.","sourceIds":["calc-cache-pricing","price-anthropic","price-openai"]}}