MythosLM

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Claude Mythos System Card: What It Is and Its Status

Specs & benchmarks informational Last reviewed Updated

What is a system card?

A system card is a structured document a model developer publishes to describe how a model behaves, how it was evaluated, and what safety measures surround it. It is the authoritative, vendor-produced reference, distinct from community leaderboards or marketing pages.

What a Mythos system card would document

Because Mythos-class is a Covered Model, its system card would be expected to cover:

  • Capabilities and intended use, including the 1M context window and 128k output limit.
  • Evaluation results, the official benchmark numbers that are not yet published.
  • Safety and safeguards, such as Fable 5's classifiers for cyber, bio/chem, and distillation risks, and how flagged requests reroute to Opus 4.8.
  • The Fable 5 vs Mythos 5 distinction, since they share a model but differ in safeguard configuration.
  • Risk testing and limitations, including red-teaming and known failure modes.

Status as of mid-2026

As of June 30, 2026, treat the official Mythos/Fable system card as not yet fully published, or just emerging. The models launched on June 9, 2026, and access was suspended by a US export-control directive on June 12, which has affected the normal documentation rollout.

How to use this page

Do not rely on third-party documents claiming to be "the system card." When the official card appears, it, and Anthropic's own announcements, are the source of truth for benchmarks and safety claims. Until then, this page describes the concept, not invented contents.

Frequently asked questions

Has Anthropic published the Mythos system card?

As of June 30, 2026, the official Mythos/Fable system card is not yet fully published or is just emerging. Check Anthropic directly for the latest.

What does a system card contain?

Typically capabilities, intended use, evaluation and benchmark results, safety measures and safeguards, and risk testing such as red-teaming findings.

Why does a Covered Model need a system card?

Covered Models carry additional safety and compliance expectations, so thorough public documentation of capabilities, risks, and safeguards is especially important.

Where should I look for the official card?

Anthropic's own site and announcements. Be wary of third-party documents claiming to be the official system card.

Sources & further reading

Facts on this page link to their source. Quotes are kept under 15 words and attributed; figures labelled unofficial are third-party until Anthropic publishes system-card numbers.