Anthropic Mythos: What Developers Need to Know About the Newest Model Line

Anthropic Mythos is a new tier of AI models above the Claude Opus line, designed for high-stakes cybersecurity, biology, and scientific research. Mythos 5, the latest version, is only available to vetted US partners. The public version, Claude Fable 5, ships with the same intelligence but adds safety classifiers that route sensitive queries to a lower-tier model.

What Is Anthropic Mythos?

Mythos is a new model class that sits above Anthropic's existing Claude lineup. While Claude Opus handles general-purpose tasks, Mythos targets domains where mistakes have serious consequences: cybersecurity defense, drug discovery, and scientific research.

The first Mythos model, called Mythos Preview, launched in April 2026 through Project Glasswing. This is a collaboration between Anthropic and the US government focused on defending critical software infrastructure. Mythos Preview was restricted to a small group of security researchers and government labs.

The second release, Mythos 5, launched on June 26, 2026. It improves on Mythos Preview across cybersecurity, biology, and healthcare benchmarks. The US government, through Commerce Secretary Lutnick, granted Anthropic permission to release Mythos 5 to a list of "trusted partners" that includes federal agencies and vetted private companies.

How Mythos Differs from Claude

Anthropic's model lineup now has four tiers:

• Claude Haiku 4.5: Fast and cheap ($1/$5 per million tokens). Best for high-volume, simple tasks.
• Claude Sonnet 5: Balanced performance ($3/$15 per million tokens). Best for everyday development work.
• Claude Opus 4.8: Frontier intelligence ($5/$25 per million tokens). Best for complex coding and reasoning.
• Mythos 5 / Fable 5: Above-Opus capability with domain-specific safeguards. Restricted access.

The key difference between Mythos 5 and Claude Opus is the removal of safety classifiers for cybersecurity and biology. Mythos 5 can fully engage with offensive security research, vulnerability analysis, and biological modeling without guardrails redirecting its responses.

Claude Fable 5: The Public Version of Mythos

Most developers cannot access Mythos 5 directly. Instead, Anthropic released Claude Fable 5, which uses the same underlying model but adds safeguards.

Fable 5 ships with classifiers that detect cybersecurity and biology queries. When it detects these topics, it routes the response to Claude Opus 4.8 instead of answering directly with Mythos-level capabilities. For all other topics, Fable 5 responds with the full Mythos intelligence.

This means Fable 5 performs within 1 to 3 percentage points of Mythos 5 on standard benchmarks. On cybersecurity and biology benchmarks, the gap is larger because of the routing safeguards.

Fable 5 is available through the standard Anthropic API and through Eden AI with the model string anthropic/claude-fable-5.

Mythos 5 Capabilities and Benchmarks

Anthropic has published limited benchmark data for Mythos-class models. Here is what we know:

Cybersecurity

Mythos Preview demonstrated exceptional offensive security capabilities. In red-team testing against open-source repositories, it developed 181 working exploits and achieved register-level control on 29 additional targets. Anthropic grades crashes on a five-tier severity scale, from basic crashes (tier 1) to complete control flow hijack (tier 5). Mythos Preview regularly reached the highest tiers.

Mythos 5 improves on these results. It is designed to help security teams find and fix vulnerabilities before attackers can exploit them.

Biology and Healthcare

Mythos 5 shows strong performance on drug design and biological research tasks. Anthropic plans to expand trusted access to biology researchers after the initial cybersecurity rollout.

General Benchmarks

On standard benchmarks (MMLU, HumanEval, MATH), Fable 5 scores within 1 to 3 points of Mythos 5. This puts it at or above Claude Opus 4.8 level on general tasks. Specific numbers are not yet public.

The Trusted Partners Program

Mythos 5 access is controlled through a "trusted partners" program. Here is how it works:

• Anthropic submits a list of partner organizations to the US government.
• The Commerce Department reviews the list and issues an export license for approved entities.
• Approved partners receive API access to Mythos 5 with cybersecurity safeguards lifted.
• Foreign nationals on Anthropic's staff and non-US organizations are currently excluded.

The initial list, announced June 26, 2026, includes US federal agencies, defense contractors, and vetted technology companies. Anthropic says it plans to expand access to a broader trusted access program "in the coming weeks."

How to Access Fable 5 Through Eden AI

While Mythos 5 remains restricted, any developer can use Claude Fable 5 through Eden AI. The model string is anthropic/claude-fable-5.

Here is an example using the OpenAI-compatible endpoint:

import os
from openai import OpenAI

client = OpenAI(
    api_key=os.environ["EDENAI_API_KEY"],
    base_url="https://api.edenai.run/v3",
)

response = client.chat.completions.create(
    model="anthropic/claude-fable-5",
    messages=[{"role": "user", "content": "Explain the key differences between Mythos 5 and Fable 5."}],
    max_tokens=500,
)

print(response.choices[0].message.content)

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You can also add a fallback to Claude Opus 4.8 in case Fable 5 is rate-limited or unavailable:

import os, urllib.request, json

headers = {
    "Authorization": f"Bearer {os.environ['EDENAI_API_KEY']}",
    "Content-Type": "application/json",
}

models = [
    "anthropic/claude-fable-5",
    "anthropic/claude-opus-4-8",
    "anthropic/claude-sonnet-5",
]

for model in models:
    payload = json.dumps({
        "model": model,
        "messages": [{"role": "user", "content": "Analyze this code for vulnerabilities."}],
        "max_tokens": 1000,
    }).encode()
    req = urllib.request.Request(
        "https://api.edenai.run/v3/chat/completions",
        data=payload,
        headers=headers,
    )
    try:
        with urllib.request.urlopen(req, timeout=60) as resp:
            result = json.loads(resp.read())
            print(f"Success with {model}")
            break
    except Exception as e:
        print(f"{model} failed: {e}, trying next...")

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What This Means for Developers

Mythos 5 represents a new direction for frontier AI models: capability tiers that go beyond general intelligence into domain-specific expertise. Here is what matters for developers today:

• Fable 5 is available now. It gives you near-Mythos intelligence through any standard API. Use it for complex analysis, research, and development tasks.
• Mythos 5 access will expand. Anthropic plans to broaden the trusted partners program. Security teams and researchers should watch for application windows.
• Multi-provider routing matters. Whether you use Fable 5, Opus 4.8, or Sonnet 5, having fallbacks across models keeps your application running. Eden AI handles this with one API key.
• Expect a premium price tier. When Mythos 5 eventually opens wider, it will likely cost more than Opus 4.8. Budget accordingly.

They share the same underlying model. Mythos 5 has cybersecurity and biology safeguards lifted, giving it full capability in those domains. Fable 5 adds classifiers that route sensitive cybersecurity and biology queries to Claude Opus 4.8 instead. On standard benchmarks, they perform within 1 to 3 percentage points of each other.

Conclusion

Anthropic Mythos represents a new tier of AI models built for high-stakes domains. While Mythos 5 remains restricted to trusted partners, Claude Fable 5 gives every developer access to near-Mythos intelligence through the standard API.

FAQs - Anthropic Mythos