Claude Fable 5.1 cuts cache-read costs 75%, but savings depend on usage patterns

Anthropic released Claude Fable 5.1 on September 1, keeping standard input and output prices unchanged at $10 and $50 per million tokens respectively, while slashing cache-read costs from $1 to $0.25 per million tokens. Despite remaining twice as expensive as Opus 5 on fresh input and output, Fable 5.1 becomes cheaper than Opus 5 once cached tokens exceed a calculated threshold of roughly 20 times fresh input plus 100 times output tokens per request. Analysis of two sample workloads shows the model cuts per-request costs by around 44% for heavy agent use with large cached contexts, while offering minimal savings for typical chat interactions where output costs dominate. However, a one-time cache-write premium means users need more than 80 requests reusing the same prefix before Fable 5.1 breaks even against Opus 5. Developers are advised to measure their actual cache reuse rates before switching, as those with frequently rebuilt or short-lived contexts may see no net savings.