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Model comparison

Claude Mythos 5.1 vs Claude Opus 5

Two Anthropic tiers compared on the figures that decide which one a workload actually needs.

Recorded profile verification October 1, 2026Individual provider fields may changeEvidence confidence: Medium — see receipts below

Anthropic

Claude Mythos 5.1

Frontier

vs

Anthropic

Claude Opus 5

Frontier

AI model capability comparison
SpecificationClaude Mythos 5.1Claude Opus 5
ProviderAnthropicAnthropic
TierFrontierFrontier
Context window1M1M
Max output128K128K
Input / 1M tokens$10Winner: $5
Output / 1M tokens$50Winner: $25
WeightsClosedClosed
ParametersNot verifiedUnverifiedNot verifiedUnverified
Reasoning levelslow, medium, high, xhigh, maxlow, medium, high, xhigh, max
Modalitiestext, imagetext, image
API model idclaude-mythos-5-1claude-opus-5
ReleasedSeptember 1, 2026July 24, 2026
Humanity's Last Exam · source date Claude Opus 5: 2026-09-01Not verifiedUnverified63.6
Terminal-Bench 2.1 · source date Claude Opus 5: 2026-09-02Not verifiedUnverified89.1

Dates beside benchmark scores refer to the linked source’s publication or capture date unless that source explicitly states when it measured the result. A shared URL alone does not establish comparable runs; unverified setups are marked. Prices are USD per million tokens at standard rates, excluding batch and caching discounts. Values we could not confirm from the provider are shown as “Not verified” rather than estimated.

Benchmark receipts

Where each score comes from, and how far it can be compared across models.

Pricing tiers

Claude Mythos 5.1: 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.

Claude Opus 5: Standard $5/$25 per MTok with no long-context surcharge. Cache reads $0.50; 5-minute writes $6.25 and 1-hour writes $10. Batch $2.50/$12.50. First-party Fast research preview $10/$50, unavailable with Batch. US-only inference adds 10% where supported. Batch 300K output requires output-300k-2026-03-24; standard stays 128K. Knowledge cutoff May 2026; retirement not sooner than July 24, 2027.

FrontierRecord checked October 1, 2026

Claude Mythos 5.1

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.

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.

FrontierRecord checked October 1, 2026

Claude Opus 5

Claude Opus 5 is Anthropic's July 2026 agentic-coding model, now 'Active (legacy)'. Its successor Opus 5.5 has lower base input/output prices ($4/$20 vs $5/$25 per million tokens); performance depends on the workload and evaluation settings.

Best for

  • Complex agentic coding
  • Enterprise work
  • Long-context analysis

Watch out

Active legacy, superseded by Opus 5.5. Test migration: Opus 5.5 makes adaptive thinking always on, rejects forced tool use and the older computer-use tool, and changes thinking-block/progress behavior. Partner cloud lifecycles and prices can differ.

When the cheaper one wins

Claude Opus 5 is cheaper on output at $25 per million tokens against $50 for Claude Mythos 5.1 — about 2.0×. Use the cheaper tier for classification, extraction, summarisation, and any task where the expensive model’s extra score does not change the accepted output. The expensive one only pays if your hardest task actually fails on the cheap tier. This compares standard short-context token rates, not total cost per accepted task. Include input, cache, context tiers, schedules, retries and review in your pilot.

Run the model picker

Evidence confidence: Medium

How strong and complete the evidence behind this comparison is — not a prediction of which model is better.

  • Pricing verified on both sides — Input and output rates are verified for both models.
  • 4/5 core specs verified on both sides — Not published for at least one side: parameter count.
  • No directly aligned benchmark setup — No benchmark has matching cited sources, source dates, units, effort and recorded run details on both models.
  • Verified within the last 90 days — Older of the two catalog checks was 25 days ago.
  • Both models carry source citations — Each side has at least two catalog sources on record.

Source receipts

Each catalog figure was checked against the provider or an independent second source on the date shown.

Related comparisons

Diving deeper on one model? Claude Mythos 5.1 · Claude Opus 5

Common questions

Claude Mythos 5.1 vs Claude Opus 5

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

Is Claude Mythos 5.1 or Claude Opus 5 cheaper for input?
Claude Opus 5 is cheaper at $5 per million input tokens, against $10 for Claude Mythos 5.1 — roughly 2.0× the price. Estimate both input and output token volume for your workload. These are base rates; Claude Mythos 5.1: 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. Claude Opus 5: Standard $5/$25 per MTok with no long-context surcharge. Cache reads $0.50; 5-minute writes $6.25 and 1-hour writes $10. Batch $2.50/$12.50. First-party Fast research preview $10/$50, unavailable with Batch. US-only inference adds 10% where supported. Batch 300K output requires output-300k-2026-03-24; standard stays 128K. Knowledge cutoff May 2026; retirement not sooner than July 24, 2027.
Is Claude Mythos 5.1 or Claude Opus 5 cheaper for output?
Claude Opus 5 is cheaper at $25 per million output tokens, against $50 for Claude Mythos 5.1 — roughly 2.0× the price. Estimate both input and output token volume for your workload. These are base rates; Claude Mythos 5.1: 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. Claude Opus 5: Standard $5/$25 per MTok with no long-context surcharge. Cache reads $0.50; 5-minute writes $6.25 and 1-hour writes $10. Batch $2.50/$12.50. First-party Fast research preview $10/$50, unavailable with Batch. US-only inference adds 10% where supported. Batch 300K output requires output-300k-2026-03-24; standard stays 128K. Knowledge cutoff May 2026; retirement not sooner than July 24, 2027.
Which has the larger context window, Claude Mythos 5.1 or Claude Opus 5?
Both accept about 1M tokens of context, so document length will not decide between them.
Do Claude Mythos 5.1 and Claude Opus 5 support the same reasoning levels?
Both expose these named effort settings: "low", "medium", "high", "xhigh", "max". The same labels do not guarantee equal compute budgets or behavior across providers. Compare quality, latency and cost on your own tasks.
Should I use Claude Mythos 5.1 or Claude Opus 5?
Both are classified in our frontier tier; this editorial grouping does not imply equal capability. Claude Mythos 5.1 is a candidate for cyber-defence research; Claude Opus 5 is a candidate for complex agentic coding. Compare accuracy, tool reliability, latency and cost on representative tasks.

Next step

Choosing between them

The verified figures that separate this pair, computed from the catalog rather than restated boilerplate.

  • Input price: Claude Opus 5 costs $5 per 1M tokens versus $10 for Claude Mythos 5.1 — a 2x difference at the headline tier.
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