At a Glance
| Vendor | Anthropic |
| Family | Claude 5 series |
| Launched | Anthropic announced Claude Mythos 5.1 on September 1, 2026, together with Claude Fable 5.1. Anthropic states that the two are the same underlying model with different levels of safeguards. Mythos 5.1 carries the lighter safeguard profile and is offered only to vetted organizations through trusted access programs. |
| Context window | The same model as Fable 5.1, which lists a 1,000,000-token context window and 128,000-token maximum output. Anthropic does not publish a separate specification sheet for Mythos 5.1. |
| Pricing | Anthropic does not publish general list pricing for Mythos 5.1 because it is not self-serve. Its pricing notes confirm that cache reads on both Fable 5.1 and Mythos 5.1 are billed at 2.5 percent of the base input price, down from the usual 10 percent on other Claude models. |
| Access channels | Access runs through two programs. The Cyber Verification Program covers defensive security work. The Life Sciences Verification Program, built with the US government, gives vetted researchers access to Mythos 5.1's biology capabilities, with first participants enrolled and broader enrollment for scientists expected soon. Both are currently limited to US organizations. |
Notable Benchmarks
Anthropic reports Mythos 5.1 at 60.9 percent on Terminal-Bench 4.0, against 55.8 percent for Fable 5.1 on the same benchmark. The gap reflects what the lighter safeguards allow the model to attempt rather than a different model. Beyond that, the release material focuses on access programs rather than a separate benchmark table.
Strengths
Security research and life sciences work that the general safeguards on Fable 5.1 would block or interrupt. Anthropic positions the model as an early look at how frontier models will contribute to scientific research.
Limitations
Not available to most organizations, not available outside the US at launch, and gated behind verification. For almost every marketing, content, or product use case, Fable 5.1 or Opus 5 is the model that actually applies.
Brand-Visibility Implications
Mythos 5.1 will not answer many consumer brand questions directly, but it matters for two groups. Security vendors and biotech companies are exactly the brands whose buyers are most likely to be verified Mythos users, so their representation in Claude answers now splits across two safeguard profiles of the same model. And because Fable 5.1 and Mythos 5.1 share weights, what each knows about a brand is identical; only what it is willing to discuss differs. Track Fable 5.1 as the public baseline and treat Mythos as a specialist surface. See Claude Mythos 5 brief and Claude citation patterns.
How Presenc AI Tracks This Model
Presenc AI monitors brand visibility on Anthropic's Claude 5 series as part of continuous multi-platform AI visibility tracking. We sample Claude Mythos 5.1 across representative prompt sets daily, compare against competitor performance on the same prompts, and flag material mention-rate changes so brand teams can respond quickly when AI representation shifts.