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AI Choice Engine

Model comparison

Claude Fable 5.1 vs Step 5 Preview

Anthropic against StepFun, compared on context, price, and verified benchmark results.

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

Anthropic

Claude Fable 5.1

Frontier

vs

StepFun

Step 5 Preview

Frontier

AI model capability comparison
SpecificationClaude Fable 5.1Step 5 Preview
ProviderAnthropicStepFun
TierFrontierFrontier
Context window1M1M
Max output128KNot verifiedUnverified
Input / 1M tokens$10Winner: $1
Output / 1M tokens$50Winner: $2.70
WeightsClosedClosed
ParametersNot disclosedUnverified600B total / 27B active MoE (StepFun)
Reasoning levelsadaptiveNot verifiedUnverified
Modalitiestext, imagetext, image
API model idclaude-fable-5-1step-5-preview
ReleasedSeptember 1, 2026September 18, 2026
Artificial Analysis Intelligence Index [max] (2026-09-26)53.4Not verifiedUnverified
Artificial Analysis Intelligence Index (2026-09-26)Not verifiedUnverified43.7
Humanity's Last Exam (2026-09-01)65Not verifiedUnverified
Terminal-Bench 2.1 (2026-09-26)91.4Not verifiedUnverified
Terminal-Bench 4.0 [max] (2026-09-26)52Not verifiedUnverified
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.

Step 5 Preview: $1.00/$2.70 per MTok on the StepFun platform (cache-hit input $0.05); StepFun lists it under Multimodal Reasoning Models and says output tokens include reasoning. No max-output figure is published. StepFun says open weights follow on 2026-10-15. Release date per Artificial Analysis's listing (StepFun's launch page is undated).

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

Step 5 Preview

Step 5 Preview is StepFun's new frontier reasoning model — a 1M-context, vision-input API preview that lands near Kimi K3 and GLM 5.3 on the Artificial Analysis index at a lower price.

Best for

  • Long-context work on a budget
  • Chinese + English
  • Early testing before the open-weight drop

Watch out

Preview: weights are promised for 2026-10-15 but not yet released, and its max output is unpublished and the launch benchmarks are vendor-run.

When the cheaper one wins

Step 5 Preview is cheaper on output at $2.70 per million tokens against $50 for Claude Fable 5.1 — about 19×. 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.
  • 2/5 core specs verified on both sides — Not published for at least one side: max output, parameter count, reasoning levels.
  • 1 shared named benchmark with differing scores — Measured on: Artificial Analysis Intelligence Index.
  • 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 · Step 5 Preview

Common questions

Claude Fable 5.1 vs Step 5 Preview

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

Is Claude Fable 5.1 or Step 5 Preview cheaper for input?
Step 5 Preview is cheaper at $1 per million input tokens, against $10 for Claude Fable 5.1 — roughly 10× 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. Step 5 Preview has tiered pricing: $1.00/$2.70 per MTok on the StepFun platform (cache-hit input $0.05); StepFun lists it under Multimodal Reasoning Models and says output tokens include reasoning. No max-output figure is published. StepFun says open weights follow on 2026-10-15. Release date per Artificial Analysis's listing (StepFun's launch page is undated).
Is Claude Fable 5.1 or Step 5 Preview cheaper for output?
Step 5 Preview is cheaper at $2.70 per million output tokens, against $50 for Claude Fable 5.1 — roughly 19× 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. Step 5 Preview has tiered pricing: $1.00/$2.70 per MTok on the StepFun platform (cache-hit input $0.05); StepFun lists it under Multimodal Reasoning Models and says output tokens include reasoning. No max-output figure is published. StepFun says open weights follow on 2026-10-15. Release date per Artificial Analysis's listing (StepFun's launch page is undated).
Which has the larger context window, Claude Fable 5.1 or Step 5 Preview?
Both accept about 1M tokens of context, so document length will not decide between them.
Should I use Claude Fable 5.1 or Step 5 Preview?
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; Step 5 Preview suits long-context work on a budget.

Next step

Choosing between them

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

  • Input price: Step 5 Preview costs $1 per 1M tokens versus $10 for Claude Fable 5.1 — a 10x difference at the headline tier.
  • Measured capability: Claude Fable 5.1 leads Artificial Analysis Intelligence Index 53.4 to 43.7 (measured 2026-09-26).
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