Claude Mythos 5.1: Benchmarks, Pricing, Use Cases & Full Overview

Claude Mythos 5.1 Overview: Benchmarks, Pricing, Use Cases & Everything You Need to Know

Choosing a frontier model for serious research or security work is hard when the most capable option is locked behind an application form. Claude Mythos 5.1 is Anthropic's restricted-access model for cybersecurity and life-sciences work, released September 1, 2026, priced at $10 per million input tokens and $50 per million output tokens. Fable 5.1 and Mythos 5.1 are the same model with different levels of safeguards, with Fable 5.1 generally available and Mythos 5.1 available only through trusted access programs. That said, most teams will never touch Mythos directly, so this guide covers both.

Below you will find benchmarks, pricing, how it compares to Fable 5.1 and other frontier models, the strongest use cases, and how access actually works. If you pay for Claude, ChatGPT, or Gemini every month, we will also cover the smartest way to avoid losing money to foreign transaction fees on those USD subscriptions.

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1. What Is Claude Mythos 5.1?

Model Identity and Release Context

Anthropic's lineup has a clear structure. Haiku, Sonnet, and Opus are the small, medium, and large models, and the Mythos-class tier sits above Opus entirely. The Claude family covers three named size tiers, Opus, Sonnet, and Haiku, joined in June 2026 by the Mythos-class tier that sits above the Opus class.

The "5.1" naming signals a point release rather than a full generational jump. Anthropic shipped the 5.1 update on September 1, 2026 as a point release on top of Fable 5, and the headline price did not move, the context window did not move, and the model ID barely moved. Even so, the capability gains are real, concentrated in agentic and scientific work rather than saturated exam benchmarks.

Invite-Only and Trusted-Access Launch

Mythos 5.1 is not a model you can simply call. Mythos 5.1 is the identical model to Fable 5.1 with lighter safeguards, restricted to vetted organizations through the Cyber Verification Program and the Life Sciences Verification Program, currently US-only. The rationale is dual-use risk. It is Anthropic's most capable model for cybersecurity defense and life-sciences research, including threat intelligence, vulnerability discovery, red teaming, and drug discovery, and access is gated to a vetted set of organizations given the dual-use nature of these domains.

For biology specifically, Anthropic established an access program developed in partnership with the US government to enable access to Mythos 5.1's advanced biology capabilities, and expects to open enrollment for scientists soon. If you do not qualify, the generally available Fable 5.1 is the twin you will actually use.

2. Claude Mythos 5.1 Benchmark Performance

Key Benchmark Scores at a Glance

Because the two 5.1 variants share weights, most published numbers come from Fable 5.1, with Mythos 5.1 appearing where lifted safeguards let it score higher.

GPQA Diamond, the graduate-level science reasoning benchmark, comes in at 92.6%, reinforcing that these gains are not limited to agent-style workflows. On formal maths, Vals AI recorded a perfect 100% on ProofBench v1.1, a formal mathematical-proof benchmark, a first for any frontier model.

Performance Frontier Claims

The clearest signal of progress is agentic science. Terminal-Bench-Science 0.1 tests whether a model can run a scientific investigation end to end, and Fable 5.1 lands at 52.6%, against Fable 5's 24.7%, Opus 5's 29.0%, and GPT-5.6 Sol's 22.4%. That is a doubling in a point release. The score more than doubled Fable 5's Terminal-Bench-Science.

Mythos 5.1's one exclusive appearance shows the safeguard-driven gap. Terminal-Bench 4.0 measures long-horizon terminal work, and it is the one row where Mythos 5.1 appears at 60.9%, against Fable 5.1's 55.8%, Opus 5's 52.3%, Fable 5's 42.0%, and GPT-5.6 Sol's 37.3%. One honest caveat worth flagging: the 95.0% SWE-bench Verified figure widely repeated online belongs to Fable 5 from its June 2026 launch, and Anthropic's 5.1 announcement does not report a SWE-bench score, leading instead with agentic benchmarks.

Advanced Reasoning and Long-Context Performance

Independent evaluators back the direction. Artificial Analysis ranks Fable 5.1 first on its Intelligence Index at 66, ARC-AGI puts it at 97.5% on ARC-AGI-1, and Vals AI ranks it first on the Vals Index. The gains are uneven by design. The gains are smaller on tasks Fable 5 already did well, like everyday coding, and Anthropic measured these with safeguards on, which it says likely lowers a few scores. In practice, treat the launch table as a map and run your own evals before switching production traffic.

3. Claude Mythos 5.1 Pricing and Token Costs

Token Pricing Structure

Pricing is straightforward and identical across the twins. Pricing for Claude Mythos 5.1 starts at $10 per million input tokens and $50 per million output tokens. The efficiency story lives in the cache. Fable 5.1 costs $10.00 per million input tokens and $50.00 per million output tokens, with Cache Read at $0.25 per million, Cache Write at $12.50 per million, and Cache Write (1h) at $20.00 per million. Cache reads dropped 75% to $0.25 per million, and the Batch API takes 50% off input and output.

Cost Tiers and Volume Discounts

The single moved line changes the economics for agents. The cache read dropped from $1.00 to $0.25 per million tokens, which matters because a long agentic session re-reads the same file tree, system prompt, and conversation history on every step, and the overwhelming majority of its input tokens are cache hits rather than fresh text. Anthropic frames the real-world savings clearly. Anthropic estimates token-billed costs will be about 25% lower for typical workloads and up to approximately 45% lower for highly agentic workloads.

Against the wider ladder, Mythos and Fable sit at the top. The lineup as of September 1, 2026 runs Fable 5 at $10/$50 per million tokens, Opus 5 at $5/$25, Sonnet 5 at $2/$10, and Haiku 4.5 at $1/$5.

Total Cost of Ownership Considerations

Anthropic itself recommends routing before reaching for the top tier. The company's own model guide says to start with Opus 5 for most workloads and move to Fable 5.1 when Opus fails an evaluation that matters. One access caveat affects budgeting: Fable 5.1 requires 30-day data retention and is not available under zero data retention unless Anthropic authorizes it, and it has no Priority Tier.

None of this touches your personal Claude subscription, which is billed separately in USD. If you pay from a euro card that adds a 2-3% foreign transaction fee on every renewal, that markup quietly stacks up across a year. Paying in USD at the real rate removes it entirely.

4. Claude Mythos 5.1 vs. Competing Frontier Models

Head-to-Head Comparison Table

Note: Anthropic ran the GPT-5.6 Sol comparison numbers itself, and several rows have no published GPT-5.6 Sol or Gemini figure. Populate remaining cells from official sources at time of use.

Claude Mythos 5.1 vs. Fable 5.1: Key Differentiators

This is the comparison that matters most, and the answer is simpler than it looks. Fable 5.1 and Mythos 5.1 are the same underlying model, and the benchmark gap reflects tasks where earlier, less precise cyber safeguards intervened. In other words, you do not choose Mythos for raw intelligence. You choose it because production safeguards would otherwise block legitimate cybersecurity or biology work. For everything else, Fable 5.1 is the identical model you can actually call today.

Where Mythos 5.1 Leads and Where It Trails

Against OpenAI's frontier, Anthropic's published numbers favor its model. On the four benchmarks where Anthropic published both, Fable 5.1 beat GPT-5.6 Sol by 6 to 30 points. The honest weakness is cost efficiency on ordinary work. At $10 per million input tokens and $50 per million output tokens, Fable 5.1 costs twice as much as Claude Opus 5 before caching. On CursorBench, the gap to Opus 5 is 3.4 points on a model that costs twice as much. For procurement, the rule is simple: pay the premium only where a failed run costs more than the token delta.

Whichever frontier model you settle on, you still pay for Claude, ChatGPT, or Gemini in USD. Bleap gives you 0% FX fees on those renewals plus a flat 20% cashback on all three, so a €20 monthly plan returns €4 every cycle. Get the Bleap card →

5. Claude Mythos 5.1 Best Use Cases

Scientific Research and Academic Applications

Mythos 5.1 exists for high-stakes technical research. It is Anthropic's most capable model for cybersecurity defense and life-sciences research, including threat intelligence, vulnerability discovery, red teaming, drug discovery, and biodefense screening. The scientific gains are concrete. The protein design work hit a nearly 50% success rate, compared to a typical 10 to 15%. For labs running end-to-end investigations, the agentic science jump is the headline reason to apply for access.

Enterprise and Business Intelligence

For general knowledge work, Fable 5.1 is the enterprise workhorse. Anthropic calls the 5.1 pair the world's most advanced models for coding and knowledge work, with research capabilities that offer an early glimpse of how AI models will contribute to scientific progress. Real deployments show the appeal of long unattended runs. One Ramp engineer reported a single unattended 38-hour run on a machine learning problem that diagnosed an earlier result as a label artifact, corrected it, kicked off 6 parallel experiments overnight, and returned with a result and next steps.

Advanced Reasoning and Agentic Workflows

Agentic pipelines are where the model separates from its predecessor. Fable 5.1's biggest gains are in long, tool-using work, more than doubling Fable 5 on agentic science and nearly doubling it on business automation from 17.1% to 31.4%. The design goal is autonomy without stalling. Developer descriptions say it gets further into long tasks, is better at identifying when it is stuck, and has a more natural writing style.

Use Cases Where a Lighter Model May Suffice

Not every task needs the top tier. Anthropic's own routing advice is to start cheaper. Anthropic's Fable 5.1 model page explicitly recommends starting with Opus 5 for most work. For high-volume, lower-complexity jobs, Haiku or Sonnet keeps costs down. The decision framework is task complexity weighed against token budget: reserve Fable and Mythos for frontier-difficulty work where a wrong answer is expensive.

6. Core Specifications and Capabilities

Context Window and Memory

The context window is large and unchanged from the previous generation. The model has a 1M-token context window and 128K tokens of maximum output. A practical reminder from independent reviewers: a big context limit is not a guarantee. A large context limit does not guarantee perfect recall or low cost, so measure retrieval quality, cache behavior, and task success on your own data.

Supported Modalities

Inputs are multimodal, output is text. Fable 5.1 accepts text, images, and files such as PDFs as input and returns text. Tool use is fully supported. It accepts tools for function calling and supports structured outputs via a JSON schema in response_format. A behavioral change worth noting: unlike previous Claude models where extended thinking was opt-in, Fable 5.1 thinks on every request by default.

Model IDs, Versioning, and API Endpoints

The public model ID is simple. The confirmed public model ID is claude-fable-5-1. It is broadly hosted. Fable 5.1 is generally available as claude-fable-5-1 on the Claude API, plus AWS, Google Cloud, and Microsoft Azure. Mythos 5.1 uses the trusted-access verification programs rather than an open endpoint. One breaking change to watch: developers upgrading agents reported existing code returning errors, so pin versions and test before rolling to production.

7. Safety, Security, and Alignment

Constitutional AI and RLHF Alignment Approach

Mythos sits squarely inside Anthropic's safety framework. Mythos sits within Anthropic's Responsible Scaling Policy and AI Safety Level framework, and its controlled initial release is a direct result of Anthropic's evaluation of the model's capabilities relative to its safety thresholds. The 5.1 release also introduced a new enterprise security layer. Anthropic introduced a new security architecture called Enterprise Frontier Safeguards, designed to let organizations retain monitoring data inside infrastructure they control.

Safeguards and Refusal Behaviour

The core distinction between the twins is refusal precision. With Fable 5.1 the safeguards are more precise, biology safeguards intervene on benign requests 85% less often than at launch, and dual-use biology and chemistry questions are still routed to Opus models, while penetration testing and exploit generation remain blocked. Cyber interventions dropped too, meaning fewer false refusals for legitimate defensive researchers.

Third-Party Safety Audits and Certifications

Independent evaluation has been part of the Mythos story from the start. The UK AI Security Institute evaluated Mythos on expert-level hacking tasks and reported a 73% success rate on tasks that, until April 2025, no AI model could complete at all. Anthropic also published unusually detailed documentation. The company published a 244-page system card for the model, the first time it released system card documentation for an unreleased model.

8. Access and Availability

Current Access Status

As of publication, Mythos 5.1 remains gated. Access remains limited to a small set of vetted organizations, provided through trusted access programs to a small but growing set. There is a hard geographic restriction. Mythos 5.1 is restricted to vetted organizations through the Cyber Verification Program and the Life Sciences Verification Program, currently US-only. Fable 5.1, by contrast, is available to anyone through the standard Claude API and major cloud platforms.

How to Apply for Trusted Access

Access runs through Anthropic's verification programs rather than a self-serve console. Organizations working in cybersecurity apply through the Cyber Verification Program, while research teams apply through the Life Sciences Verification Program. Approval favors organizations with a clear defensive or research use case and demonstrable safety practices, given the dual-use sensitivity. For biology, Anthropic expects to open enrollment for scientists soon.

Roadmap to General Availability

Anthropic's stated goal is gradual expansion. Mythos 5 was made available to a small group of vetted partners with a goal of opening up more broadly in the future. To widen access safely, Anthropic added additional safeguards to release Mythos-level capabilities more broadly. The generally available path to Mythos-class capability today is Fable 5.1.

9. Data Retention and Privacy Policies

Data handling is stricter on the frontier tier. Fable 5.1 requires 30-day data retention and is not available under zero data retention unless Anthropic authorizes it, and it has no Priority Tier, which Fable 5 had. If you were relying on zero data retention or Priority Tier for an existing Fable 5 workload, plan for that change before migrating.

The 5.1 release does add enterprise-grade privacy controls through its new security architecture. The Enterprise Frontier Safeguards system stores data in cloud infrastructure controlled entirely by the customer, not Anthropic, giving customers complete privacy. For GDPR and CCPA exposure, enterprise buyers should confirm data residency and the specifics of any authorized zero-data-retention arrangement directly with Anthropic before deployment, since the default 30-day retention will not suit every regulated workload.

10. Regulatory Compliance and Governance

The dual-use nature of Mythos has already drawn direct government involvement. Security concerns led the US government to order Anthropic to suspend the earlier models for a few weeks. The predecessor was subject to an even broader restriction. In June 2026 the US government sent Anthropic a letter prohibiting access to both Mythos 5 and Fable 5 for any non-US national regardless of location, and Anthropic revoked access to both models for all customers as a result.

Access was later restored under government approval, which is why the current Mythos 5.1 biology program is run in partnership with the US government. For EU AI Act purposes, a model at this capability level would fall under general-purpose AI obligations with systemic-risk considerations, meaning transparency reporting and incident logging. Enterprise buyers in regulated sectors should treat the published system card as the starting point for their own compliance review rather than the endpoint.

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Frequently Asked Questions About Claude Mythos 5.1

What makes Claude Mythos 5.1 different from Claude Opus?

Mythos sits a full tier above Opus in Anthropic's hierarchy. Claude Mythos is a new model class above Opus, designed for the most demanding AI tasks, particularly those requiring advanced reasoning, long agentic task sequences, and deep domain expertise. Mythos 5.1 also carries lighter safeguards than the generally available Fable 5.1, tuned specifically for cybersecurity and life-sciences work.

How much does Claude Mythos 5.1 cost per million tokens?

Pricing for Claude Mythos 5.1 starts at $10 per million input tokens and $50 per million output tokens. Cache reads dropped 75% to $0.25 per million, cache writes are $12.50 for the 5-minute window and $20 for the 1-hour window, and the Batch API takes 50% off input and output.

How does Claude Mythos 5.1 compare to Fable 5.1 on benchmarks?

They are the same weights, so scores match except where safeguards differ. On Terminal-Bench 4.0, the one row where Mythos 5.1 appears, it scores 60.9% against Fable 5.1's 55.8%. The five-point gap reflects tasks where Fable's cyber safeguards intervened, not a difference in underlying capability.

What is the context window size for Claude Mythos 5.1?

The model has a 1M-token context window and 128K tokens of maximum output. That supports long documents and multi-turn agent sessions, though real-world recall should be measured on your own data rather than assumed from the headline limit.

Is Claude Mythos 5.1 available via the Anthropic API?

Not openly. Mythos 5.1 is restricted to vetted organizations through the Cyber Verification Program and the Life Sciences Verification Program, currently US-only. The identical model is available to everyone as Fable 5.1 on the Claude API, AWS, Google Cloud, and Microsoft Azure.

What use cases is Claude Mythos 5.1 best suited for?

It is Anthropic's most capable model for cybersecurity defense and life-sciences research, including threat intelligence, vulnerability discovery, red teaming, drug discovery, and biodefense screening. For general reasoning and agentic knowledge work, Fable 5.1 covers the same ground without the access gate. See Section 5 for the full breakdown.

Conclusion & Summary

Four takeaways sum up Claude Mythos 5.1. On benchmarks, it leads on agentic science, with Fable 5.1 more than doubling its predecessor on Terminal-Bench-Science and Mythos 5.1 topping long-horizon terminal work at 60.9%. On pricing, it sits at the top of the ladder at $10/$50 per million tokens, with cache reads cut 75% to reward long-running agents. On use cases, it is built for cybersecurity and life-sciences research, while Fable 5.1 handles general knowledge work. On access, Mythos stays trusted-access and US-only, so most teams should build on Fable 5.1 or route cheaper work to Opus 5. The model is evolving fast, so check Anthropic's official changelog before committing a production workload.

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