Anthropic Ships Claude Fable 5.1 and Mythos 5.1, Cuts Cache Reads 75%
On September 1, 2026, Anthropic launched Claude Fable 5.1 generally and Mythos 5.1 for trusted-access programs, cut cache reads to $0.25 per million tokens, and opened Enterprise Frontier Safeguards so monitoring data stays in the customer’s cloud.
TLDR
Anthropic on Tuesday, September 1, 2026 launched Claude Fable 5.1 (generally available) and Claude Mythos 5.1—same weights, looser cyber/bio safeguards, gated to trusted-access. Input/output stays $10 / $50 per million tokens; cache reads drop 75% to $0.25 / MTok. Anthropic: typical workloads ~25% cheaper, highly agentic up to ~45%. Same day: Enterprise Frontier Safeguards (EFS) stores monitoring data in the customer’s AWS / Azure / GCP account; Anthropic does not charge for EFS; interim ZDR on Fable 5/5.1 until fall rollout. Fable may now discover vulns, not write exploits. New API accounts cannot edit prior thinking while keeping the transcript (anti-distillation). Vendor: Terminal-Bench-Science 0.1 52.6% vs Fable 5 24.7% / Opus 5 29.0% / GPT‑5.6 Sol 22.4%. Mythos 5.1 protein binders: ~50% hit rate vs typical 10–15%. Jane Street: “solves more of our coding problems than Fable 5 or Opus 5.” Cognition: moving Opus 5 Devin traffic to Fable 5.1 on launch day. Mythos currently US orgs, expansion coordinated with USG.
Ranking context
Lab benches (Terminal-Bench-Science, protein-binder hit rate) are vendor. Check LMArena / Artificial Analysis for 5.1 when listed; do not invent Elo. Treat 52.6% as Anthropic-reported.
Product-line de-dupe: not Jun 9 Fable 5 / Mythos 5, not Aug 7 biology safeguards, not Aug 20 Platform computer-use GA. Point release + EFS/ZDR GTM.
Why this story matters
Anthropic is selling cheaper cache and customer-held monitors as the enterprise answer to OpenAI ZDR—the same week OpenAI says Astra is Critical cyber. Watch: Mythos trusted-access scale, and whether Cognition’s Devin flip shows up in usage.