Fable 5.1 Cuts Cache Costs and Plans Customer-Held Monitors; Mythos Stays Gated
Anthropic launched Fable 5.1 with $0.25 cache reads and kept Mythos 5.1 on trusted access. Enterprise Frontier Safeguards were announced for a phased fall rollout, with interim zero data retention for eligible customers.
Correction / update, October 14, 2026: EFS was announced for a phased fall rollout, not opened generally on launch day. Removed an unsupported same-week comparison and added benchmark limitations.
Anthropic paired cheaper Fable 5.1 cache reads with plans for customer-controlled monitoring storage.
Anthropic (September 1) 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%. Announced for a phased fall rollout: Enterprise Frontier Safeguards (EFS) can store 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 context while preserving the transcript of prior thinking (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.
Claims vs checks#
Lab benches (Terminal-Bench-Science, protein-binder hit rate) are vendor. Treat 52.6% as Anthropic-reported. The company gives a 3.5–4.5 percentage-point standard error and notes that its setup differs from the public leaderboard; production safeguards also affect comparisons. Watch: Mythos trusted-access scale, and whether Cognition’s Devin flip shows up in usage.