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Claude Fable 5.1 vs Grok-4 Fast Reasoning

Comparing Claude Fable 5.1 and Grok-4 Fast Reasoning across benchmarks, pricing, and capabilities.

Anthropic · xAI · Updated for 2026

Which is better?

Claude Fable 5.1 and Grok-4 Fast Reasoning trade strengths across price, capabilities, and technical limits. The better choice depends on the workload.

On price, Grok-4 Fast Reasoning is roughly 72.7x cheaper per token on a blended 3:1 input/output basis, which adds up quickly at production volume.

Grok-4 Fast Reasoning also accepts a larger context window (2,000,000 input tokens), making it the stronger choice for long documents and large codebases.

Based on current LLM Stats indexes, shared benchmarks, pricing, and model metadata for 2026.

Choose Claude Fable 5.1

  • you want the most recent training data — it shipped Sep 2026

Choose Grok-4 Fast Reasoning

  • cost matters — it's about 72.7x cheaper per token
  • you process long inputs — it offers a 2,000,000 token context window

At a glance

The differences that matter most.

Claude Fable 5.1
Anthropic
Grok-4 Fast Reasoning
xAI
Benchmark wins
—
—
Input price
$10.00 / M
$0.20 / M
Output price
$50.00 / M
$0.50 / M
Context window
1,000,000
2,000,000

Individual benchmarks

7 reported for Claude Fable 5.1 · 0 for Grok-4 Fast Reasoning

No common benchmarks found

Claude Fable 5.1 and Grok-4 Fast Reasoningdon't have any common benchmark datasets to compare. They may have been evaluated on different testing suites.

Human preference

Blind head-to-head votes and playground preference scores

Pricing Analysis

Price comparison per million tokens

Grok-4 Fast Reasoning costs less

For input processing, Claude Fable 5.1 ($10.00/1M tokens) is 50.0x more expensive than Grok-4 Fast Reasoning ($0.20/1M tokens).

For output processing, Claude Fable 5.1 ($50.00/1M tokens) is 100.0x more expensive than Grok-4 Fast Reasoning ($0.50/1M tokens).

In conclusion, Claude Fable 5.1 is more expensive than Grok-4 Fast Reasoning.*

* Using a 3:1 ratio of input to output tokens

Lowest available price from all providers
Fri Sep 04 2026 • llm-stats.com
Anthropic
Claude Fable 5.1
Input tokens$10.00
Output tokens$50.00
Best providerAnthropic
xAI
Grok-4 Fast Reasoning
Input tokens$0.20
Output tokens$0.50
Best providerxAI
Notice missing or incorrect data?Start an Issue→

Context Window

Maximum input and output token capacity

Grok-4 Fast Reasoning accepts 2,000,000 input tokens compared to Claude Fable 5.1's 1,000,000 tokens. Claude Fable 5.1 can generate longer responses up to 128,000 tokens, while Grok-4 Fast Reasoning is limited to 30,000 tokens.

Anthropic
Claude Fable 5.1
Input1,000,000 tokens
Output128,000 tokens
xAI
Grok-4 Fast Reasoning
Input2,000,000 tokens
Output30,000 tokens
Fri Sep 04 2026 • llm-stats.com

Input capabilities

Documented input modalities across available providers

Both Claude Fable 5.1 and Grok-4 Fast Reasoning support multimodal inputs.

They are both capable of processing various types of data, offering versatility in application.

Claude Fable 5.1

Text
Images
Audio
Video

Grok-4 Fast Reasoning

Text
Images
Audio
Video

License

Usage and distribution terms

Both models are licensed under proprietary licenses.

Both models have usage restrictions defined by their respective organizations.

Claude Fable 5.1

Proprietary

Closed source

Grok-4 Fast Reasoning

Proprietary

Closed source

Release Timeline

When each model was launched

Claude Fable 5.1 was released on 2026-09-01, while Grok-4 Fast Reasoning was released on 2025-08-28.

Claude Fable 5.1 is 12 months newer than Grok-4 Fast Reasoning.

Claude Fable 5.1

Sep 1, 2026

2 days ago

1.0yr newer
Grok-4 Fast Reasoning

Aug 28, 2025

1.0 years ago

Knowledge Cutoff

When training data ends

Neither model specifies a knowledge cutoff date.

Unable to compare the recency of their training data.

No cutoff dates available

Provider Availability

Claude Fable 5.1 is available from Anthropic. Grok-4 Fast Reasoning is available from xAI.

Claude Fable 5.1

anthropic logo
Anthropic
Input Price:Input: $10.00/1MOutput Price:Output: $50.00/1M

Grok-4 Fast Reasoning

xai logo
xAI
Input Price:Input: $0.20/1MOutput Price:Output: $0.50/1M
* Prices shown are per million tokens

Outputs Comparison

Notice missing or incorrect data?Start an Issue discussion→

Judge for yourself.

Run your own prompts against Claude Fable 5.1 and Grok-4 Fast Reasoning side-by-side, then vote on the output you prefer.

Claude Fable 5.1
✓ Preferred
Grok-4 Fast Reasoning
Open in Playground

FAQ

Common questions about Claude Fable 5.1 vs Grok-4 Fast Reasoning.

Which is better, Claude Fable 5.1 or Grok-4 Fast Reasoning?

Claude Fable 5.1 (Anthropic) and Grok-4 Fast Reasoning (xAI) each have strengths in different areas. Compare their benchmark scores, pricing, context windows, and capabilities above to determine which fits your needs.

How does Claude Fable 5.1 compare to Grok-4 Fast Reasoning in benchmarks?

Claude Fable 5.1 scores OSWorld 2.0: 77.9%, CursorBench v3.2: 73.4%, Humanity's Last Exam: 65.0%, GDPval-AA: 61.8%, Terminal-Bench 4.0: 55.8%.

Is Claude Fable 5.1 cheaper than Grok-4 Fast Reasoning?

Grok-4 Fast Reasoning is 50.0x cheaper for input tokens. Claude Fable 5.1 costs $10.00/M input and $50.00/M output via anthropic. Grok-4 Fast Reasoning costs $0.20/M input and $0.50/M output via xai.

What are the context window sizes for Claude Fable 5.1 and Grok-4 Fast Reasoning?

Claude Fable 5.1 supports 1.0M tokens and Grok-4 Fast Reasoning supports 2.0M tokens. A larger context window lets you process longer documents, conversations, or codebases in a single request.

What are the main differences between Claude Fable 5.1 and Grok-4 Fast Reasoning?

Key differences include context window (1.0M vs 2.0M), input pricing ($10.00 vs $0.20/M). See the full comparison above for benchmark-by-benchmark results.

Who makes Claude Fable 5.1 and Grok-4 Fast Reasoning?

Claude Fable 5.1 is developed by Anthropic and Grok-4 Fast Reasoning is developed by xAI.

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