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Anthropic

Claude Mythos 5.1

Claude Mythos 5.1 is a Anthropic model with 1M context, $10 in / $50 out per million tokens, last verified October 1, 2026. Open weights: no. Public-board scores are linked to the publisher; vendor-only figures stay in the watch-out, not on the board.

Anthropic describes Claude Mythos 5.1 as its most capable model for cybersecurity and biology research — the Fable 5.1 model with fewer cyber/bio safeguards, restricted to vetted US organisations.

Recorded profile verification October 1, 2026Individual provider fields may changeAccess: restricted — Eligibility-gated to vetted US organisations; not an ordinary buying default.

AI model specification details
SpecificationClaude Mythos 5.1
ProviderAnthropic
TierFrontier
Context window1M
Max output128K
Input / 1M tokens$10
Output / 1M tokens$50
WeightsClosed
ParametersNot verifiedUnverified
Reasoning levelslow, medium, high, xhigh, max
Modalitiestext, image
API model idclaude-mythos-5-1
ReleasedSeptember 1, 2026

Pricing tiers: Standard $10/$50 per MTok, sharing Fable 5.1's specifications/pricing. Cache reads $0.25; 5-minute writes $12.50 and 1-hour writes $20. Batch $5/$25. US-only inference adds 10% where supported. Invite-only for vetted US organisations; API ID claude-mythos-5-1. Retirement not sooner than September 1, 2027; this is a minimum commitment, not an announced shutdown.

Best for

  • Cyber-defence research
  • Biology research
  • Claude Security workloads

Watch out

Invite-only for vetted US organisations through trusted access programmes, with 30-day retention required by default for safety monitoring. Adaptive thinking cannot be disabled and forced tool use errors. Shared underlying model does not make Fable benchmark results Mythos observations; safeguards and access differ.

Source receipts

Catalog figures for this model were checked against the following sources.

Common questions

Claude Mythos 5.1

Answered from the verified figures on this page rather than general guidance.

How much does Claude Mythos 5.1 cost per million tokens?
Claude Mythos 5.1 lists base rates of $10 per million input tokens and $50 per million output tokens. Estimate both input and output token volume and any additional charges for your workload. Standard $10/$50 per MTok, sharing Fable 5.1's specifications/pricing. Cache reads $0.25; 5-minute writes $12.50 and 1-hour writes $20. Batch $5/$25. US-only inference adds 10% where supported. Invite-only for vetted US organisations; API ID claude-mythos-5-1. Retirement not sooner than September 1, 2027; this is a minimum commitment, not an announced shutdown.
What is Claude Mythos 5.1's context window?
Claude Mythos 5.1 accepts about 1M tokens of context. That only matters if you routinely send very long documents, large codebases, or multi-turn histories that approach that limit.
What is Claude Mythos 5.1 best for?
Claude Mythos 5.1 is a frontier tier from Anthropic. It suits cyber-defence research, biology research, claude security workloads. Invite-only for vetted US organisations through trusted access programmes, with 30-day retention required by default for safety monitoring. Adaptive thinking cannot be disabled and forced tool use errors. Shared underlying model does not make Fable benchmark results Mythos observations; safeguards and access differ.
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