Claude Mythos 5.1
Anthropic · Released Sep 2026
Same weights as Claude Fable 5.1 at the same $10/$50 rate, with cyber and life-sciences safeguards loosened for vetted users. The catch is access: Mythos 5.1 ships only through Anthropic's trusted-access programs (Cyber Verification and Life Sciences Verification), so there is no self-serve API and no way to buy your way in. Conditional for the same reason as Mythos 5: elite capability behind a vetting gate. Everyone else should use Fable 5.1.
Is it right for you?
Good for
- Vulnerability discovery and offensive-security research — cyber safeguards are loosened relative to Fable 5.1
- Agentic terminal work at the frontier: 60.9% Terminal-Bench 4.0, 5.1 points above Fable 5.1
- Life-sciences work that trips Fable 5.1's biology classifiers, for organizations inside the Life Sciences Verification Program
Not good for
- Any team not admitted to a verification program — there is no public API and no self-serve path
- General coding and knowledge work — identical price, identical weights, and Fable 5.1 is the one you can actually call
- Organizations outside the US — Anthropic says access is currently limited to a set of US organizations, with international expansion coordinated with the US government but undated
How it performs by task
Cybersecurity research
Loosened cyber safeguards on frontier weights; the reason the model exists
Agentic coding
60.9% Terminal-Bench 4.0, the highest published score in the 5.1 release
General knowledge work
Same weights as Fable 5.1, but no published index scores and no reason to prefer it here
Pricing
Input
$10 / 1M tokens
Output
$50 / 1M tokens
Context
1M context, $0.25/1M cache; vetted access
Benchmarks
| Benchmark | Score | Source |
|---|---|---|
| Terminal-Bench 4.0 | 60.9% | Source |
No verdict changes yet
The clock starts day one — changes land here as our verdict evolves.
Sources
Verification log
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