Core capabilities
- Frontier reasoning for the hardest tasks
- Long context with adaptive thinking
- Knowledge-work and research orientation
- Limited access through Claude surfaces
← Model indexMODEL PROFILE / claude-mythos-5-1
Anthropic / preview
Claude Mythos 5.1
Anthropic's frontier preview model released alongside Fable 5.1.
Context1M
ModalityText · Image
AccessAPI · Cloud
ReasoningAdaptive
Where it fits.
Targets the hardest reasoning and knowledge-work tiers, exposed ahead of general availability for evaluation. Released September 1, 2026 with Fable 5.1, Mythos 5.1 is the preview-tier frontier model intended for evaluation rather than production traffic.
Best-fit workflows
- Frontier reasoning research
- Worst-case evaluation sets
- Specialised knowledge work
04 / OPERATIONSFrom evaluation
From evaluation
to production.
Deployment notes
Use only for benchmarking and ahead-of-availability exploration. Keep Opus 5.5 as the production target and re-run evals whenever the preview build changes.
Watch before choosing
Preview status means pricing, limits and behaviour can shift before general availability. Do not place production SLAs on a preview model; keep a supported tier as fallback.
05
Family & versions
- Claude Opus 5.5Flagship
- Claude Fable 5.1Coding flagship
- Claude Sonnet 5.5Balanced
- Claude Haiku 5.5Fast tier
Specifications and positioning checked against provider documentation.
Open official source↗