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.
Catalog record checked September 3, 2026Individual provider fields may changeEvidence confidence: Medium — see receipts below
Anthropic
Claude Mythos 5.1
Frontier
Anthropic
Claude Opus 5
Frontier
| Specification | Claude Mythos 5.1 | Claude Opus 5 |
|---|---|---|
| Provider | Anthropic | Anthropic |
| Tier | Frontier | Frontier |
| Context window | Not verifiedUnverified | 1M |
| Max output | Not verifiedUnverified | 128K |
| Input / 1M tokens | $10 | Winner: $5 |
| Output / 1M tokens | $50 | Winner: $25 |
| Weights | Closed | Closed |
| Parameters | cyber/bio research variant of Fable 5.1 | Not disclosedUnverified |
| Reasoning levels | adaptive | adaptive |
| Modalities | text, image | text, image |
| API model id | claude-mythos-5 | claude-opus-5 |
| Released | September 1, 2026 | July 24, 2026 |
| Artificial Analysis Intelligence Index [max] (2026-08-14) | Not verifiedUnverified | 63 |
| Humanity's Last Exam (2026-09-01) | Not verifiedUnverified | 63.6 |
| Terminal-Bench 2.1 (2026-09-03) | Not verifiedUnverified | 86.7 |
Prices are USD per million tokens at standard rates, excluding batch and caching discounts. Bold indicates the better figure where one is objectively better. Values we could not confirm from the provider are shown as “Not verified” rather than estimated.
Benchmark receipts
- 2026-09-03: Meta Muse Spark 1.3 launch table (independent cross-vendor run; conflicts with Google's 89.1 — both preserved)Agentic terminal work: multi-step tasks executed in a sandboxed shell environment. Comparability: comparable with caveat — Scores across Terminal-Bench major versions (v1 vs v2) are NOT comparable; harness configuration (container, time limits) also shifts results.
- 2026-09-02: Google Gemini 3.8 Flash launch eval table (independent cross-vendor run)Agentic terminal work: multi-step tasks executed in a sandboxed shell environment. Comparability: comparable with caveat — Scores across Terminal-Bench major versions (v1 vs v2) are NOT comparable; harness configuration (container, time limits) also shifts results.
- 2026-09-01: Anthropic Fable 5.1 comparison table (vendor, with tools; 56.6 no tools)Frontier-knowledge ceiling: extremely hard multi-domain questions written to be near-impossible without deep expertise. Comparability: comparable with caveat — Subset/tool configurations (text-only vs with-tools) differ between vendors and materially change scores; must match configuration to compare.
- 2026-08-14: Artificial AnalysisComposite index blending reasoning, knowledge, and coding evals into one 0–100 score. Comparability: directly comparable — AA occasionally rebaselines the index scale between snapshots — a score captured on one date is only comparable to same-snapshot scores (check measuredAt).
Pricing tiers: Claude Mythos 5.1: Starts at $10/$50 per MTok — priced with Fable 5.1, which shares the same underlying model. API id claude-mythos-5 replaced the deprecated claude-mythos-preview. · Claude Opus 5: Standard $5/$25 per MTok; cache reads 10% of base input. Knowledge cutoff May 2026.
Claude Mythos 5.1
Claude Mythos 5.1 is Anthropic's 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 (vetted US organisations, verification programmes); 30-day data retention applies by default. Same underlying model as Fable 5.1 — most teams should just use Fable.
Claude Opus 5
Claude Opus 5 is Anthropic's top model for complex agentic coding and enterprise work — superseded at the top of the Artificial Analysis Index by Fable 5.1 (2026-09-01) but stronger on price.
Best for
- Complex agentic coding
- Enterprise work
- Long-context analysis
Watch out
Output costs 5x input; long generations dominate the bill.
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. These are standard-tier API rates, excluding batch and cache discounts.
Run the model pickerEvidence 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.
- 2/5 core specs verified on both sides — Not published for at least one side: context window, max output, parameter count.
- No shared named benchmark — No benchmark has been measured on both models.
- Verified within the last 90 days — Newest catalog check was 8 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.
- Claude Mythos 5.1: Anthropic — Claude Mythos (accessed 2026-09-03)
- Claude Mythos 5.1: Anthropic — model deprecations (claude-mythos-5) (accessed 2026-09-03)
- Claude Opus 5: Anthropic — Claude Opus 5 (accessed 2026-08-29)
- Claude Opus 5: Anthropic news — Claude Opus 5 (accessed 2026-08-29)
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- Claude Fable 5.1 vs Claude Mythos 5.1
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. Output tokens usually dominate a real bill, so weigh the output rate more heavily than the input rate. These are base rates; Claude Mythos 5.1 has tiered pricing: Starts at $10/$50 per MTok — priced with Fable 5.1, which shares the same underlying model. API id claude-mythos-5 replaced the deprecated claude-mythos-preview. Claude Opus 5 has tiered pricing: Standard $5/$25 per MTok; cache reads 10% of base input. Knowledge cutoff May 2026.
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. Output tokens usually dominate a real bill, so weigh the output rate more heavily than the input rate. These are base rates; Claude Mythos 5.1 has tiered pricing: Starts at $10/$50 per MTok — priced with Fable 5.1, which shares the same underlying model. API id claude-mythos-5 replaced the deprecated claude-mythos-preview. Claude Opus 5 has tiered pricing: Standard $5/$25 per MTok; cache reads 10% of base input. Knowledge cutoff May 2026.
Do Claude Mythos 5.1 and Claude Opus 5 support the same reasoning levels?
Yes — both accept the same effort settings: "adaptive". Higher effort costs more and takes longer, so start low and raise it only where output quality actually improves.
Should I use Claude Mythos 5.1 or Claude Opus 5?
Both sit in the frontier tier, so the choice usually comes down to price and context rather than capability. Claude Mythos 5.1 suits cyber-defence research; Claude Opus 5 suits complex agentic coding.
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.
Choosing a harness rather than a model — Cursor, Copilot, Claude Code, Windsurf, or Lovable? Compare agentic harnesses · Latest releases.