Mythos 5.1: What the Model Is and How to Request Access
Fable 5.1 is open to everyone, Mythos 5.1 is application-only. What differs, how to apply, and why most people don't need it.
Evgenii Arsentev · PhDReviewed for accuracy by Evgenii Arsentev, PhD · 2026-09-02
Anthropic released Claude Fable 5.1 and Claude Mythos 5.1, and described the difference in a single line: they are the same model with different levels of safeguards. Fable 5.1 is available to everyone, everywhere. Mythos 5.1 isn't sold at all — it is reachable only through trusted access programs, and its safeguards are loosened for two fields: cybersecurity and the life sciences.
What Fable 5.1 gives you today
It already runs on every surface — Claude Code, Claude Enterprise, the API, plus Amazon Bedrock, Google Cloud and Microsoft Foundry. Pricing is $10 per million input tokens and $50 per million output, while cache reads cost $0.25 per million, 75% below Fable 5. Translated into real work, Anthropic puts the saving at roughly 25% on typical workloads and up to 45% on highly agentic ones, where a model walks the same context again and again.
The other half of the release is about false refusals, which annoyed people more than the price did. Anthropic reports that safeguards now fire 85% less often on benign elementary biology and medical questions, and that cybersecurity interventions dropped by around 60% per session. On science work the jump is large: 52.6% on Terminal-Bench-Science against 24.7% for Fable 5.
claude-fable-5-1
The model identifier in the Claude API. No special access required — Fable 5.1 is generally available.
How Mythos differs
Not in capability — the weights are the same. The difference is in the fuses: Mythos 5.1 loosens them for professional work where a request looks dangerous on its face but is defensive in substance. A security engineer takes an exploit apart to close the hole; a researcher works with biological sequences because that is the lab's job. The general model blocks those requests, and rightly so — which is why a separate door with verification at the entrance had to exist.
How to apply
Cyber Verification Program (CVP) — the door for defenders
- 1Open portal.anthropic.com/programs/cvp — the entry point for Claude.ai, Claude Code and Claude API users. The program is free.
- 2Sign in with your organization's work email: access is granted to an organization and scoped to its API key, not to an individual.
- 3Complete identity verification — it is mandatory, and applications aren't reviewed without it.
- 4Describe your work and which tasks hit the blocks: the program exists precisely for defensive work that falls under the dual-use label.
- 5Wait for the decision email — Anthropic states a two-business-day target.
- 6On Microsoft Foundry the application goes through claude.com/form/cyber-use-case, and on AWS through portal.anthropic.com/link?account_source=aws&program=cvp.
For the life sciences there's a second door — the Life Sciences Verification Program, which Anthropic runs together with the US government and is only now beginning to enroll. Both programs are free: you pay for tokens, not for status.
I've sent my own application and I'm waiting on the answer — the stated target is two business days. I'd suggest doing the same: the program is free, the form takes minutes, and a rejection costs nothing. Anthropic is expanding the circle together with the US government, so an application on file is also a place in the queue.
What to do next
Apply and find out. The door is open, no public list of eligible job titles narrows it, and the only way to learn your answer is to get it from Anthropic rather than guess it on their behalf. In the meantime put Fable 5.1 to work: it's available to everyone today, costs less than its predecessor, and refuses normal questions far less often.
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Evgenii Arsentev
PhD · Chief Executive Officer, digital health
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