Skip to main content
AI Choice Engine

Model comparison

Claude Fable 5.1 vs Claude Sonnet 5.5

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

Recorded profile verification October 1, 2026Individual provider fields may changeEvidence confidence: High — see receipts below

Anthropic

Claude Fable 5.1

Frontier

vs

Anthropic

Claude Sonnet 5.5

Balanced

AI model capability comparison
SpecificationClaude Fable 5.1Claude Sonnet 5.5
ProviderAnthropicAnthropic
TierFrontierBalanced
Context window1M1M
Max output128K128K
Input / 1M tokens$10Winner: $2
Output / 1M tokens$50Winner: $10
WeightsClosedClosed
ParametersNot verifiedUnverifiedNot verifiedUnverified
Reasoning levelslow, medium, high, xhigh, maxlow, medium, high, xhigh, max
Modalitiestext, imagetext, image
API model idclaude-fable-5-1claude-sonnet-5-5
ReleasedSeptember 1, 2026September 28, 2026
Artificial Analysis Intelligence Index [max] · source date Claude Fable 5.1: 2026-10-0153Not verifiedUnverified
Artificial Analysis Intelligence Index [max] · source date Claude Sonnet 5.5: 2026-10-01Not verifiedUnverified56
Humanity's Last Exam · source date Claude Fable 5.1: 2026-09-0165Not verifiedUnverified
Humanity's Last Exam · source date Claude Sonnet 5.5: 2026-10-01Not verifiedUnverified64.5
Terminal-Bench 2.1 · source date Claude Fable 5.1: 2026-09-2691.4Not verifiedUnverified
DeepSWE 1.1 · source date Claude Fable 5.1: 2026-09-0367.4Not verifiedUnverified
Terminal-Bench 4.0 [max] · source date Claude Sonnet 5.5: 2026-10-01Not verifiedUnverified63.6
Terminal-Bench 4.0 · source date Claude Sonnet 5.5: 2026-10-01Not verifiedUnverified70.6
CursorBench 4.0 [max] · source date Claude Sonnet 5.5: 2026-10-01Not verifiedUnverified55.5
FrontierCode 1.1 (Main) [xhigh] · source date Claude Sonnet 5.5: 2026-10-01Not verifiedUnverified52.1
OSWorld 2.1 · source date Claude Sonnet 5.5: 2026-10-01Not verifiedUnverified80.1

Dates beside benchmark scores refer to the linked source’s publication or capture date unless that source explicitly states when it measured the result. A shared URL alone does not establish comparable runs; unverified setups are marked. Prices are USD per million tokens at standard rates, excluding batch and caching discounts. 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: Standard $10/$50 per MTok with no long-context surcharge. Cache reads $0.25, versus Fable 5's $1; 5-minute writes $12.50 and 1-hour writes $20. Batch $5/$25. US-only inference adds 10% where supported. Adaptive thinking is always on, default effort high; knowledge cutoff Jun 2026. Retirement not sooner than September 1, 2027.

Claude Sonnet 5.5: Standard $2/$10 per MTok with no long-context surcharge; cache reads $0.20, 5-minute writes $2.50 and 1-hour writes $4. Batch $1/$5. US-only inference adds 10% where supported. The 300K output limit requires the Batch API beta header output-300k-2026-03-24; standard output stays 128K. Knowledge cutoff Jun 2026; retirement not sooner than September 28, 2027.

FrontierRecord checked October 1, 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

Standard token rates are 2.5x Opus 5.5's. Forced tool use errors; thinking blocks are model/conversation-bound, so editing earlier turns or switching models can invalidate them. Display updates and per-message effort are beta. Match benchmark settings and evaluate pinned workloads before migrating.

BalancedRecord checked October 1, 2026

Claude Sonnet 5.5

Anthropic’s newer Sonnet model for everyday coding and knowledge work at the same $2/$10 token rates as Sonnet 5.

Best for

  • Everyday coding
  • Document and slide creation
  • Fast iteration

Watch out

Vendor scores use Anthropic's evaluation setup. between_tools turns off up-front thinking only at high effort or below. Forced tool use and non-default temperature/top_p/top_k error; older computer_20251124 and some advisors are rejected. Thinking blocks are model/conversation-bound; progress text needs a display setting that returns it. Haiku 5.5 is announced, not released as of October 1.

When the cheaper one wins

Claude Sonnet 5.5 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. This compares standard short-context token rates, not total cost per accepted task. Include input, cache, context tiers, schedules, retries and review in your pilot.

Run the model picker

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.
  • 1 benchmark setup aligned with differing scores — Matched source, source date, units, effort and recorded run details: Artificial Analysis Intelligence Index.
  • Verified within the last 90 days — Older of the two catalog checks was 25 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 Sonnet 5.5

Common questions

Claude Fable 5.1 vs Claude Sonnet 5.5

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

Is Claude Fable 5.1 or Claude Sonnet 5.5 cheaper for input?
Claude Sonnet 5.5 is cheaper at $2 per million input tokens, against $10 for Claude Fable 5.1 — roughly 5.0× the price. Estimate both input and output token volume for your workload. These are base rates; Claude Fable 5.1: Standard $10/$50 per MTok with no long-context surcharge. Cache reads $0.25, versus Fable 5's $1; 5-minute writes $12.50 and 1-hour writes $20. Batch $5/$25. US-only inference adds 10% where supported. Adaptive thinking is always on, default effort high; knowledge cutoff Jun 2026. Retirement not sooner than September 1, 2027. Claude Sonnet 5.5: Standard $2/$10 per MTok with no long-context surcharge; cache reads $0.20, 5-minute writes $2.50 and 1-hour writes $4. Batch $1/$5. US-only inference adds 10% where supported. The 300K output limit requires the Batch API beta header output-300k-2026-03-24; standard output stays 128K. Knowledge cutoff Jun 2026; retirement not sooner than September 28, 2027.
Is Claude Fable 5.1 or Claude Sonnet 5.5 cheaper for output?
Claude Sonnet 5.5 is cheaper at $10 per million output tokens, against $50 for Claude Fable 5.1 — roughly 5.0× the price. Estimate both input and output token volume for your workload. These are base rates; Claude Fable 5.1: Standard $10/$50 per MTok with no long-context surcharge. Cache reads $0.25, versus Fable 5's $1; 5-minute writes $12.50 and 1-hour writes $20. Batch $5/$25. US-only inference adds 10% where supported. Adaptive thinking is always on, default effort high; knowledge cutoff Jun 2026. Retirement not sooner than September 1, 2027. Claude Sonnet 5.5: Standard $2/$10 per MTok with no long-context surcharge; cache reads $0.20, 5-minute writes $2.50 and 1-hour writes $4. Batch $1/$5. US-only inference adds 10% where supported. The 300K output limit requires the Batch API beta header output-300k-2026-03-24; standard output stays 128K. Knowledge cutoff Jun 2026; retirement not sooner than September 28, 2027.
Which has the larger context window, Claude Fable 5.1 or Claude Sonnet 5.5?
Both accept about 1M tokens of context, so document length will not decide between them.
Do Claude Fable 5.1 and Claude Sonnet 5.5 support the same reasoning levels?
Both expose these named effort settings: "low", "medium", "high", "xhigh", "max". The same labels do not guarantee equal compute budgets or behavior across providers. Compare quality, latency and cost on your own tasks.
Should I use Claude Fable 5.1 or Claude Sonnet 5.5?
We classify Claude Fable 5.1 in the frontier tier and Claude Sonnet 5.5 in the balanced tier. Test whether either meets your accuracy, tool-use and latency requirements before choosing on price.

Next step

Choosing between them

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

  • Input price: Claude Sonnet 5.5 costs $2 per 1M tokens versus $10 for Claude Fable 5.1 — a 5x difference at the headline tier.
  • Published Artificial Analysis Intelligence Index results differ: Claude Sonnet 5.5 scores 56 to 53 on the same listed benchmark, source attribution, source date, effort, and run variant (2026-10-01).
  • Positioning: Claude Fable 5.1 sits in the frontier tier, Claude Sonnet 5.5 in the balanced tier. Compare accuracy, reliability, latency, and cost on representative tasks before choosing.
Run the model pickerEstimate stack costAll model comparisons

Choosing a harness rather than a model — Cursor, Copilot, Claude Code, Windsurf, or Lovable? Compare agentic harnesses · Latest releases.