Model comparison

Claude Mythos 5.1 vs Claude Sonnet 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 Sonnet 5

Balanced

AI model capability comparison
Specification	Claude Mythos 5.1	Claude Sonnet 5
Provider	Anthropic	Anthropic
Tier	Frontier	Balanced
Context window	Not verifiedUnverified	1M
Max output	Not verifiedUnverified	128K
Input / 1M tokens	$10	Winner: $2
Output / 1M tokens	$50	Winner: $10
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-sonnet-5
Released	September 1, 2026	June 30, 2026
Artificial Analysis Intelligence Index (2026-08-14)	Not verifiedUnverified	55
Terminal-Bench 2.1 (2026-06-30)	Not verifiedUnverified	80.4
Humanity's Last Exam (2026-06-30)	Not verifiedUnverified	57.4
SWE-bench Verified (2026-09-01)	Not verifiedUnverified	79.6
LiveBench (2026-09-05)	Not verifiedUnverified	76

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

2026-09-05: LiveBench official leaderboard (benchmark-owned), xhigh effortContamination-resistant general capability across reasoning, coding, math, data analysis, and language, with monthly question refreshes. Comparability: comparable with caveat — Rolling question set: observations months apart measure different question mixes. Record the measurement date and compare within ~1 month windows.
2026-09-01: vals.ai SWE-bench Verified leaderboard (independent, mini-swe-agent bash-only); not published by Anthropic; a 72.7 CosmicJS claim is unverifiedReal GitHub issue resolution: does the model's patch pass the hidden tests. Comparability: comparable with caveat — Post-audit vendor claims and pre-audit scores sit on different task trust levels; scaffolding (agent harness, compute budget) also dominates results. Never aggregate across scaffolds.
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).
2026-06-30: Anthropic announcement (vendor, with 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.
2026-06-30: Anthropic announcement table (vendor, default effort; via Vellum transcription)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.

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 Sonnet 5: Standard $2/$10 per MTok; cache reads 10% of base input. Knowledge cutoff Jan 2026.

FrontierRecord checked September 3, 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.

BalancedRecord checked September 3, 2026

Claude Sonnet 5

Claude Sonnet 5 is Anthropic's best speed/intelligence balance for high-volume agentic work at $2/$10.

Best for

Everyday generation
High-volume agents
Cost-sensitive work

Watch out

A step below Opus 5 on the hardest tasks; pick Opus when quality is paramount.

When the cheaper one wins

Claude Sonnet 5 is cheaper on output at $10 per million tokens against $50 for Claude Mythos 5.1 — about 5.0×. 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.

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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.
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 Sonnet 5: Anthropic — Claude Sonnet 5 (accessed 2026-08-29)
Claude Sonnet 5: Anthropic news — Claude Sonnet 5 (accessed 2026-08-29)

Related comparisons

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

Common questions

Claude Mythos 5.1 vs Claude Sonnet 5

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

Is Claude Mythos 5.1 or Claude Sonnet 5 cheaper for input?

Claude Sonnet 5 is cheaper at $2 per million input tokens, against $10 for Claude Mythos 5.1 — roughly 5.0× 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 Sonnet 5 has tiered pricing: Standard $2/$10 per MTok; cache reads 10% of base input. Knowledge cutoff Jan 2026.

Is Claude Mythos 5.1 or Claude Sonnet 5 cheaper for output?

Claude Sonnet 5 is cheaper at $10 per million output tokens, against $50 for Claude Mythos 5.1 — roughly 5.0× 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 Sonnet 5 has tiered pricing: Standard $2/$10 per MTok; cache reads 10% of base input. Knowledge cutoff Jan 2026.

Do Claude Mythos 5.1 and Claude Sonnet 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 Sonnet 5?

Claude Mythos 5.1 is the frontier tier and Claude Sonnet 5 the balanced tier. The useful question is whether your hardest task actually fails on the cheaper one — most production volume such as classification, extraction and summarisation does not.

Next step

Choosing between them

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

Input price: Claude Sonnet 5 costs $2 per 1M tokens versus $10 for Claude Mythos 5.1 — a 5x difference at the headline tier.
Positioning: Claude Mythos 5.1 sits in the frontier tier, Claude Sonnet 5 in the balanced tier — most production volume (classification, extraction, summarisation) does not need the pricier tier.

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