Model comparison

Claude Fable 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: High — see receipts below

Anthropic

Claude Fable 5.1

Frontier

Anthropic

Claude Sonnet 5

Balanced

AI model capability comparison
Specification	Claude Fable 5.1	Claude Sonnet 5
Provider	Anthropic	Anthropic
Tier	Frontier	Balanced
Context window	1M	1M
Max output	128K	128K
Input / 1M tokens	$10	Winner: $2
Output / 1M tokens	$50	Winner: $10
Weights	Closed	Closed
Parameters	Not disclosedUnverified	Not disclosedUnverified
Reasoning levels	adaptive	adaptive
Modalities	text, image	text, image
API model id	claude-fable-5-1	claude-sonnet-5
Released	September 1, 2026	June 30, 2026
Artificial Analysis Intelligence Index (2026-08-14)	Winner: 66	55
Humanity's Last Exam (2026-09-01)	Winner: 65	57.4
Terminal-Bench 2.1 (2026-09-05)	Winner: 91.4	80.4
DeepSWE 1.1 (2026-09-03)	67.4	Not verifiedUnverified
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.
2026-09-05: 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: 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.
2026-09-01: 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).
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 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 Sonnet 5: Standard $2/$10 per MTok; cache reads 10% of base input. Knowledge cutoff Jan 2026.

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.

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 Fable 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: High

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.
3 shared named benchmarks with differing scores — Measured on: Artificial Analysis Intelligence Index, Humanity's Last Exam, Terminal-Bench 2.1.
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 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 Fable 5.1 · Claude Sonnet 5

Common questions

Claude Fable 5.1 vs Claude Sonnet 5

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

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

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

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

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

Which has the larger context window, Claude Fable 5.1 or Claude Sonnet 5?

Both accept about 1M tokens of context, so document length will not decide between them.

Do Claude Fable 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 Fable 5.1 or Claude Sonnet 5?

Claude Fable 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 Fable 5.1 — a 5x difference at the headline tier.
Measured capability: Claude Fable 5.1 leads Artificial Analysis Intelligence Index 66 to 55 (measured 2026-08-14).
Positioning: Claude Fable 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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