Model comparison

Claude Fable 5.1 vs Claude Mythos 5.1

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 Fable 5.1

Frontier

Anthropic

Claude Mythos 5.1

Frontier

AI model capability comparison
Specification	Claude Fable 5.1	Claude Mythos 5.1
Provider	Anthropic	Anthropic
Tier	Frontier	Frontier
Context window	1M	Not verifiedUnverified
Max output	128K	Not verifiedUnverified
Input / 1M tokens	$10	$10
Output / 1M tokens	$50	$50
Weights	Closed	Closed
Parameters	Not disclosedUnverified	cyber/bio research variant of Fable 5.1
Reasoning levels	adaptive	adaptive
Modalities	text, image	text, image
API model id	claude-fable-5-1	claude-mythos-5
Released	September 1, 2026	September 1, 2026
Artificial Analysis Intelligence Index (2026-08-14)	66	Not verifiedUnverified
Humanity's Last Exam (2026-09-01)	65	Not verifiedUnverified
Terminal-Bench 2.1 (2026-09-05)	91.4	Not verifiedUnverified
DeepSWE 1.1 (2026-09-03)	67.4	Not verifiedUnverified

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: Artificial Analysis Terminal-Bench v2.1 (independent, Terminus 2, max effort — current AA leader)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.
2026-09-03: OpenAI GPT-6 Astra launch chart (independent cross-vendor; Fable 5.1 not on the official Datacurve leaderboard)Agentic software engineering: pass@1 on real repo tasks with reported cost per completed task. Comparability: directly comparable
2026-09-01: Anthropic announcement comparison table (vendor, with tools; 60.9 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 Fable 5.1: Priced with Fable 5 at $10/$50 per MTok; cache reads cut 75% to $0.25/MTok. Adaptive thinking always on (default effort high); knowledge cutoff Jun 2026. Sibling Mythos 5.1 launched the same day. · 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.

FrontierRecord checked September 3, 2026

Claude Fable 5.1

Claude Fable 5.1 is Anthropic's September 2026 flagship refresh, debuting at the top of LiveBench and the Artificial Analysis Intelligence Index.

Best for

Frontier agents
Hardest reasoning work
Fable 5 workloads at the same price

Watch out

Early-life model: leaderboard position is from launch-week boards; recheck before committing spend.

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.

When the cheaper one wins

Both charge $50 per million output tokens at standard rates, so output price is not the split. Prefer the row whose “best for” line matches the job, then confirm the live vendor rate.

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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 Fable 5.1: Anthropic pricing — Fable 5.1/Mythos 5.1 $10/$50 (accessed 2026-09-03)
Claude Fable 5.1: Anthropic models overview — Fable 5.1 (1M context, 128K output) (accessed 2026-09-03)
Claude Fable 5.1: Artificial Analysis — Fable 5.1 Intelligence Index 66 #1 (accessed 2026-09-01)
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)

Related comparisons

Diving deeper on one model? Claude Fable 5.1 · Claude Mythos 5.1

Common questions

Claude Fable 5.1 vs Claude Mythos 5.1

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

Is Claude Fable 5.1 or Claude Mythos 5.1 cheaper for input?

Both cost $10 per million input tokens at standard rates, so input price is not a deciding factor between them.

Is Claude Fable 5.1 or Claude Mythos 5.1 cheaper for output?

Both cost $50 per million output tokens at standard rates, so output price is not a deciding factor between them.

Do Claude Fable 5.1 and Claude Mythos 5.1 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 Fable 5.1 or Claude Mythos 5.1?

Both sit in the frontier tier, so the choice usually comes down to price and context rather than capability. Claude Fable 5.1 suits frontier agents; Claude Mythos 5.1 suits cyber-defence research.

Next step

Choosing between them

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

These two models match on every verified catalog figure — the decision comes down to provider preference, availability, and workload fit rather than published numbers.

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