Work it out yourself from your pre-September bill: share saved = 0.75 × (cache-read spend ÷ total), and it holds only for Fable 5.1 and Mythos 5.1. On 9/1 Anthropic cut the cache-read price from $1 to $0.25 and left input and output prices untouched.
The mainstream story repeats the press-release line of a typical bill falling around 25% and up to around 45%. The diary parts ways here: those are the declines in Anthropic's measured sample, which imply cache reads were 33% and 60% of the bill (the author's back-calculation, not an official figure). A different share gives a different saving.
Anthropic released Fable 5.1 on 9/1; per its official pricing page, cache-read fell from $1 to $0.25 (0.025x the input price), with the other five items unchanged. Anthropic says typical bills fall around 25% and highly agentic ones up to around 45%, a range and not a committed value, and it did not disclose cache-read share. OpenAI and Google are at 0.1x and DeepSeek at about 0.032x (official pricing pages, fetched 2026-09-02).
For founders: take your pre-September bill, count the cache_read share only on the Fable and Mythos SKUs, and multiply it by 0.75; mixing in Opus and Sonnet overstates the saving. For investors: compare vendors on absolute prices, since a low multiple only describes the shape of the bill.