Anthropic Launches Fable 5.1 With 75% Cheaper Cache Reads and EU Watermarks

Anthropic shipped Claude Fable 5.1 and its restricted sibling Mythos 5.1 on September 1, and while the base price holds at $10/$50 per million input/output tokens, the change most agentic-workload teams will feel is in the cache: cache reads drop 75%, from $1.00 to $0.25 per million tokens. Per MacRumors and community math, that nets out to roughly 25% total savings for token-billed agent runs that lean on long cached contexts, which is most of them.

The Pricing Detail That Rewards Agent Architecture

The cache math is worth reading closely because it changes optimization priorities. Cache writes now cost $12.50 (five-minute TTL) or $20 (one-hour) per million tokens against the $10 base input price, so writing cache is a premium; reading it back is an 87.5% discount. Workloads that repeatedly reference a large, stable context, the exact shape of long-running agents, codebase analysis, and document pipelines, get rewarded twice: once by the discount and once by the 512-token minimum staying unchanged. Teams that re-send fresh contexts every turn save nothing. The pricing is effectively a nudge toward architectures that persist and reuse context, which happens to be the architecture agents need anyway.

Mythos 5.1 Stays Gated, and Every Output Is Watermarked

The two releases are the same underlying model with different safeguard configurations: Fable 5.1 is generally available through the API, while Mythos 5.1 remains restricted to trusted partners in cybersecurity and life sciences, consistent with how the Mythos line has been handled since its UK cyber-capability warning. The other quiet rollout: every Fable 5.1 and Mythos 5.1 output carries Anthropic’s invisible statistical text watermark on all platforms, per the official documentation, with an EU AI Act watermark-detection API entering private preview. That makes Anthropic the first major lab to ship statutory watermarking at scale, days after the EU designated ChatGPT a VLOSE under the DSA, and it sets the compliance template every lab serving Europe now has to match.