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AI Choice Engine

Model comparison

Claude Fable 5.1 vs Claude Opus 5.5

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

Catalog record checked September 26, 2026Individual provider fields may changeEvidence confidence: High — see receipts below

Anthropic

Claude Fable 5.1

Frontier

vs

Anthropic

Claude Opus 5.5

Frontier

AI model capability comparison
SpecificationClaude Fable 5.1Claude Opus 5.5
ProviderAnthropicAnthropic
TierFrontierFrontier
Context window1M1M
Max output128K128K
Input / 1M tokens$10Winner: $4
Output / 1M tokens$50Winner: $20
WeightsClosedClosed
ParametersNot disclosedUnverifiedNot disclosedUnverified
Reasoning levelsadaptiveadaptive
Modalitiestext, imagetext, image
API model idclaude-fable-5-1claude-opus-5-5
ReleasedSeptember 1, 2026September 22, 2026
Artificial Analysis Intelligence Index [max] (2026-09-26)53.4Winner: 57.6
Humanity's Last Exam (2026-09-01)65Not verifiedUnverified
Humanity's Last Exam [max] (2026-09-26)Not verifiedUnverified61.4
Terminal-Bench 2.1 (2026-09-26)91.4Not verifiedUnverified
Terminal-Bench 4.0 [max] (2026-09-26)52Winner: 59.6
DeepSWE 1.1 (2026-09-03)67.4Not 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

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

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 Opus 5.5: $4/$20 per MTok, no long-context surcharge. Cache reads $0.20; 5-minute cache writes $5, 1-hour writes $8. Batch $2/$10; Fast mode $8/$40. Batches API allows 300K output with the output-300k-2026-03-24 beta header. Knowledge cutoff Jun 2026; retirement not before 2027-09-22.

FrontierRecord checked September 26, 2026

Claude Fable 5.1

Claude Fable 5.1 is Anthropic's model for the most demanding reasoning and long-horizon agentic work at $10/$50 — Anthropic now recommends Opus 5.5 as the default for most workloads.

Best for

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

Watch out

Costs 2.5x Opus 5.5 per token and trails it on the Artificial Analysis Intelligence Index (53.4 vs 57.6, v4.3.2); reserve it for the hardest long-horizon work.

FrontierRecord checked September 26, 2026

Claude Opus 5.5

Claude Opus 5.5 is Anthropic's default model for agentic coding and knowledge work — Anthropic says it performs near Fable 5.1 on most work at about 40% lower running cost than Opus 5, and it leads the Artificial Analysis Intelligence Index v4.3.2.

Best for

  • Complex agentic coding
  • Enterprise knowledge work
  • Default frontier model

Watch out

Breaking changes from Opus 5: adaptive thinking cannot be disabled (effort runs low to max, default medium) and forced tool use returns an error. Verbose at max effort, so cap output on cost-sensitive routes.

When the cheaper one wins

Claude Opus 5.5 is cheaper on output at $20 per million tokens against $50 for Claude Fable 5.1 — about 2.5×. 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 4.0.
  • Verified within the last 90 days — Newest catalog check was 2 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 Fable 5.1 · Claude Opus 5.5

Common questions

Claude Fable 5.1 vs Claude Opus 5.5

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

Is Claude Fable 5.1 or Claude Opus 5.5 cheaper for input?
Claude Opus 5.5 is cheaper at $4 per million input tokens, against $10 for Claude Fable 5.1 — roughly 2.5× 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 Opus 5.5 has tiered pricing: $4/$20 per MTok, no long-context surcharge. Cache reads $0.20; 5-minute cache writes $5, 1-hour writes $8. Batch $2/$10; Fast mode $8/$40. Batches API allows 300K output with the output-300k-2026-03-24 beta header. Knowledge cutoff Jun 2026; retirement not before 2027-09-22.
Is Claude Fable 5.1 or Claude Opus 5.5 cheaper for output?
Claude Opus 5.5 is cheaper at $20 per million output tokens, against $50 for Claude Fable 5.1 — roughly 2.5× 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 Opus 5.5 has tiered pricing: $4/$20 per MTok, no long-context surcharge. Cache reads $0.20; 5-minute cache writes $5, 1-hour writes $8. Batch $2/$10; Fast mode $8/$40. Batches API allows 300K output with the output-300k-2026-03-24 beta header. Knowledge cutoff Jun 2026; retirement not before 2027-09-22.
Which has the larger context window, Claude Fable 5.1 or Claude Opus 5.5?
Both accept about 1M tokens of context, so document length will not decide between them.
Do Claude Fable 5.1 and Claude Opus 5.5 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 Opus 5.5?
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 Opus 5.5 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 5.5 costs $4 per 1M tokens versus $10 for Claude Fable 5.1 — a 2.5x difference at the headline tier.
  • Measured capability: Claude Opus 5.5 leads Artificial Analysis Intelligence Index 57.6 to 53.4 (measured 2026-09-26).
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