update · TowCue Editorial Team
Anthropic launches Claude Fable 5.1 and Mythos 5.1: performance gains, cache read price cut, enterprise safeguards
Anthropic released Claude Fable 5.1 and Mythos 5.1 on September 1, 2026 — same model, split safeguards. Fable 5.1 is GA with 25–45% cost reduction via 75% cheaper cache reads, Mythos 5.1 for vetted cyber/life-science work. Enterprise Frontier Safeguards (EFS) rolling out this fall for zero-data-retention privacy with automated misuse detection.
Quick answer
Anthropic released Claude Fable 5.1 and Claude Mythos 5.1 on September 1, 2026 — the same underlying model with split safeguard tiers.
Fable 5.1 (GA): Available now on all platforms (Claude API, AWS Bedrock, Google Cloud Vertex AI, Microsoft Azure). ~25% cheaper for typical workloads, up to ~45% cheaper for highly agentic work — driven by a 75% cache read price cut (now $0.25/M tokens). Base pricing unchanged: $10/M input, $50/M output.
Mythos 5.1 (Restricted): Same model, more permissive safeguards for vetted cyberdefenders and life-science researchers. Access via Cyber Verification Program (CVP) and Life Sciences Verification Program (LSVP, US-government partnership). Currently US-only; expansion planned.
Enterprise Frontier Safeguards (EFS): Rolling out this fall — zero-data-retention privacy (customer-controlled cloud storage) + automated misuse detection. Developed with 100+ enterprise customers. Supported on Claude Code, Enterprise, Platform, Bedrock, Vertex AI, Foundry, Google Agent Platform.
EU AI Act compliance: Watermarking on post-August-2 outputs; detection API in private preview for eligible orgs.
Model ID: claude-fable-5-1 on Claude API. Available on AWS Bedrock, Google Cloud Vertex AI, Microsoft Azure.
What changed
Anthropic announced Claude Fable 5.1 and Claude Mythos 5.1 on September 1, 2026 — the same underlying model with different safeguard profiles:
| Tier | Name | Safeguards | Availability |
|---|---|---|---|
| GA | Fable 5.1 | Standard production safeguards | All platforms, all customers |
| Restricted | Mythos 5.1 | Permissive cyber/life-science safeguards | Vetted orgs only (CVP, LSVP) |
Key insight: They are the same model weights; the difference is purely in safeguard configuration.
Performance gains
Fable 5.1 sets a new bar for coding, knowledge work, and long-horizon problem solving. At Medium effort, it often matches or beats Fable 5 at High effort — at much lower cost.
| Benchmark | Fable 5.1 | Fable 5 | Opus 5 | GPT-5.6 Sol |
|---|---|---|---|---|
| Agentic scientific research (Terminal-Bench-Science 0.1) | 52.6% | 24.7% | 29.0% | 22.4% |
| Agentic coding (Terminal-Bench 4.0) | 55.8% | 42.0% | 52.3% | 37.3% |
| Knowledge work (GDPval-AA v2) | 1853 | 1723 | 1824 | 1711 |
| Computer use (OSWorld 2.0 accuracy) | 77.9% | 72.9% | 75.4% | — |
| Multidisciplinary reasoning (Humanity's Last Exam, no tools) | 60.9% | 57.8% | 56.6% | — |
| Agentic coding (CursorBench 3.2.0) | 73.4% | 70.5% | 70.0% | 67.2% |
Key qualitative improvements (from early-access partners):
- Readable over long tasks — "remains readable over long, multi-step tasks" (Jane Street)
- Root-cause analysis — found a 1-in-a-million crash in a vendor library (Millennium)
- Unattended multi-day work — 38-hour ML run diagnosed label artifact, launched 6 parallel experiments (Ramp)
- 2× faster than Opus 5, half the tokens (Every)
- Creative problem solving — novel solution on a grand-challenge problem (iGent)
- Contract redlining — 47.9 → 57.0 on RedlineBench, first-turn quality doubled (Crosby)
Pricing: the cache read revolution
| Metric | Fable 5 | Fable 5.1 | Change |
|---|---|---|---|
| Input tokens | $10/M | $10/M | — |
| Output tokens | $50/M | $50/M | — |
| Cache reads | $1.00/M | $0.25/M | –75% |
Why it matters: Cache reads dominate cost in context-heavy, tool-heavy, multi-turn agentic work (long context, repeated reads, computer use, code execution).
| Workload type | Cost reduction vs Fable 5 |
|---|---|
| Typical (Enterprise, Code, API mix) | ~25% |
| Highly agentic (context-heavy, tool-heavy, computer use) | ~45% |
Example: A 38-hour unattended ML run with 6 parallel overnight experiments — the kind of workload where cache reads dominate — sees ~45% cost reduction.
Base pricing unchanged: $10/M input, $50/M output. Cache reads now $0.25/M (was $1.00/M).
Mythos 5.1: the restricted twin
Same model, permissive safeguards for two vetted communities:
| Program | Focus | Status |
|---|---|---|
| Cyber Verification Program (CVP) | Defensive security: vulnerability discovery, defensive coding | Expanding to Mythos-class models |
| Life Sciences Verification Program (LSVP) | Professional R&D in biology/medicine (US-gov partnership) | First participants enrolled; expanding |
Mythos 5.1 capabilities (per System Card):
- Chemical/biological: CB-1 level (can help synthesize known weapons with basic technical background) — below CB-2 (replacing rare expert talent). Same safeguards as Mythos 5.
- Cyber: Strongest cyber capabilities of any released model; still lower risk tier in Frontier Compliance Framework. No critical-severity jailbreak found.
- Agentic safety: Most robust to prompt injection to date (external benchmark).
- Alignment: Better than Mythos 5 — less resource-exfiltration attempts, less motivated reasoning, less constraint-ignoring. Still: can sometimes bypass approvals/auto-mode classifiers; limited visibility into very long-context & multi-agent settings.
Claude Security (vulnerability scanner) now runs on Mythos 5.1.
Enterprise Frontier Safeguards (EFS): zero data retention + automated detection
| Feature | Description |
|---|---|
| Data location | Customer-controlled cloud (AWS, GCP, Azure) — not Anthropic |
| Human review | By customer, not Anthropic (default) |
| Privacy | Equivalent to zero data retention (ZDR) |
| Misuse detection | Automated, state-of-the-art |
| Rollout | Phased, starting fall 2026 |
| Bridge | Eligible customers get ZDR on Fable 5 / 5.1 until EFS ready |
Supported on: Claude Code, Claude Enterprise, Claude Platform, AWS Bedrock, Claude on AWS, Google Vertex AI, Google Agent Platform, Microsoft Foundry.
Developed with: 100+ enterprise customers (finance, healthcare, manufacturing, telecom, law, retail, public sector) + cloud partners (AWS, GCP, Azure).
Safeguard improvements (Fable 5.1)
| Area | Change |
|---|---|
| Biology false positives | 85% fewer interventions on benign elementary biology/medical queries |
| Cyber false positives | ~60% fewer safeguard interventions per session in Claude Code |
| Vulnerability discovery | Now allowed (defensive work) — exploit gen, pen testing still redirected to Opus |
| Anti-distillation | New API accounts: cannot manually edit prior context while preserving thinking transcript (blocks common distillation technique). Rolling out gradually. |
EU AI Act compliance (effective Aug 2, 2026)
| Requirement | Implementation |
|---|---|
| Watermarking | Invisible text watermark on all post-Aug-2 model outputs |
| Detection API | Private preview for regulators, law enforcement, media, fact-checkers, researchers, EU civil society, compliance-obligated enterprises |
| No quality impact | Watermark invisible, no user/org/conversation metadata |
Benchmark deep dive (selected)
| Benchmark | Fable 5.1 | Fable 5 | Notes |
|---|---|---|---|
| Terminal-Bench-Science 0.1 (agentic science) | 52.6% | 24.7% | 2.1× gain |
| Terminal-Bench 4.0 (agentic coding) | 55.8% (Fable) / 60.9% (Mythos) | 42.0% | Mythos gap = safeguard interventions |
| CursorBench 3.2.0 (agentic coding) | 73.4% | 70.5% | Best Fable-class score |
| OSWorld 2.0 (computer use, accuracy) | 77.9% | 72.9% | |
| Humanity's Last Exam (no tools) | 60.9% | 57.8% | |
| AutomationBench (business workflows) | 31.4% | 17.1% | |
| RedlineBench (contract redlining) | 57.0 | 47.9 | First-turn quality doubled |
Note: Fable 5.1 evaluated with production safeguards on. Tasks where safeguards intervened score zero on OSWorld 2.0 / AutomationBench; cyber tasks completed by Opus 4.8, biology by Opus 5.
Availability matrix
| Platform | Fable 5.1 | Mythos 5.1 |
|---|---|---|
| Claude API | ✅ claude-fable-5-1 | CVP/LSVP only |
| Claude.ai / Claude Code / Cowork | ✅ Default: Medium effort (Code: High) | CVP/LSVP only |
| AWS Bedrock | ✅ | CVP/LSVP only |
| Google Cloud Vertex AI | ✅ | CVP/LSVP only |
| Microsoft Azure / Foundry | ✅ | CVP/LSVP only |
| Claude Security | — | ✅ Now powered by Mythos 5.1 |
Effort defaults: Fable 5.1 defaults to High in Claude Code, Medium in Cowork and Claude.ai.
What we don't know / unconfirmed
- Exact Mythos 5.1 global rollout timeline (US-only now; "coordinating with US government to expand")
- EFS exact GA date ("phased, starting this fall")
- Detection API public GA timeline ("expand access over time")
- Anti-distillation impact on existing custom integrations ("small number affected")
- Long-context / multi-agent alignment coverage (acknowledged gap in System Card)
- Whether cache read discount applies to Bedrock/Vertex/Foundry (API pricing mentioned; cloud partner pricing not explicitly confirmed)
- Opus 5.1 / Sonnet 5.1 timeline ("bringing many improvements to rest of model family")
TowCue take
This release is three stories in one:
-
Economics shift — The 75% cache read cut is the real story. It makes Fable-class models finally economical for agentic workloads (Cognition moving Opus traffic to Fable 5.1 on launch day). For teams running context-heavy agents (computer use, code execution, long-context RAG), this is a step-function cost reduction.
-
Safeguard-as-product-differentiation — Same weights, two SKUs via safeguard config. Mythos 5.1 for vetted pros; Fable 5.1 for everyone else. This is a regulatory/compliance product strategy — expect more of this as AI regulation matures.
-
Enterprise privacy architecture — EFS = ZDR + automated detection, data in your cloud. This is the privacy architecture enterprises have been waiting for — no Anthropic human review, data never leaves your VPC. If you're in regulated industry (finance, healthcare, gov), this is the feature to watch.
For developers: Fable 5.1 is now the default choice for agentic coding — better than Fable 5, cheaper than Opus, readable over long runs. Switch Cursor/Code traffic today.
For enterprises: If you need ZDR + misuse detection, get on the EFS waitlist now. Bridge ZDR on Fable 5/5.1 is available immediately for eligible customers.
For regulated industries: Mythos 5.1 via CVP/LSVP is now the path for defensive cyber / life-science R&D. Start the vetting process if you're in those spaces.
For compliance: Watermarking + detection API = EU AI Act compliance checklist item. If you're an enterprise with AI-generated content obligations, register for detection API access.
Bottom line: Fable 5.1 makes agentic workflows economically viable at scale. The cost curve just bent. If you've been holding back on multi-turn, tool-heavy agents due to cost — the barrier just dropped ~25–45%.
Verify current pricing, platform availability, and program access at Anthropic's Fable 5.1 page.