Model Comparison

Grok-4.1 Fast Reasoning vs Kimi K2-Instruct-0905Which is better in 2026?

Comparing Grok-4.1 Fast Reasoning and Kimi K2-Instruct-0905 across benchmarks, pricing, and capabilities.

Verdict: Grok-4.1 Fast Reasoning vs Kimi K2-Instruct-0905 — which is better?

Grok-4.1 Fast Reasoning (by xAI) and Kimi K2-Instruct-0905 (by Moonshot AI) are two of the AI models people compare most. Here is how they stack up on benchmarks, price and capabilities, and which one to pick in 2026.

Choose Grok-4.1 Fast Reasoning if…

  • you want the most recent training data — it shipped Nov 2025

Choose Kimi K2-Instruct-0905 if…

  • you need open weights you can self-host or fine-tune

Performance Benchmarks

Comparative analysis across standard metrics

No common benchmarks found

Grok-4.1 Fast Reasoning and Kimi K2-Instruct-0905don't have any common benchmark datasets to compare. They may have been evaluated on different testing suites.

Arena Performance

Human preference votes

Context Window

Maximum input and output token capacity

Only Grok-4.1 Fast Reasoning specifies input context (2,000,000 tokens). Only Grok-4.1 Fast Reasoning specifies output context (30,000 tokens).

xAI
Grok-4.1 Fast Reasoning
Input2,000,000 tokens
Output30,000 tokens
Moonshot AI
Kimi K2-Instruct-0905
Input- tokens
Output- tokens
Tue Jul 28 2026 • llm-stats.com

Input Capabilities

Supported data types and modalities

Grok-4.1 Fast Reasoning supports multimodal inputs, whereas Kimi K2-Instruct-0905 does not.

Grok-4.1 Fast Reasoning can handle both text and other forms of data like images, making it suitable for multimodal applications.

Grok-4.1 Fast Reasoning

Text
Images
Audio
Video

Kimi K2-Instruct-0905

Text
Images
Audio
Video

License

Usage and distribution terms

Grok-4.1 Fast Reasoning is licensed under a proprietary license, while Kimi K2-Instruct-0905 uses MIT.

License differences may affect how you can use these models in commercial or open-source projects.

Grok-4.1 Fast Reasoning

Proprietary

Closed source

Kimi K2-Instruct-0905

MIT

Open weights

Release Timeline

When each model was launched

Grok-4.1 Fast Reasoning was released on 2025-11-17, while Kimi K2-Instruct-0905 was released on 2025-09-05.

Grok-4.1 Fast Reasoning is 2 months newer than Kimi K2-Instruct-0905.

Grok-4.1 Fast Reasoning

Nov 17, 2025

8 months ago

2mo newer
Kimi K2-Instruct-0905

Sep 5, 2025

10 months 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

Outputs Comparison

Notice missing or incorrect data?Start an Issue discussion→

Key Takeaways

xAI
Larger context window (2,000,000 tokens)
Supports multimodal inputs
Moonshot AI
Has open weights

Detailed Comparison

Interactive Arena

Judge for yourself.

Run your own prompts against Grok-4.1 Fast Reasoning and Kimi K2-Instruct-0905 side-by-side, then vote on the output you prefer.

Grok-4.1 Fast Reasoning
✓ Preferred
Kimi K2-Instruct-0905
Open in Playground
AI Model Comparison Table
Feature
xAI
Grok-4.1 Fast Reasoning
Moonshot AI
Kimi K2-Instruct-0905

FAQ

Common questions about Grok-4.1 Fast Reasoning vs Kimi K2-Instruct-0905.

Which is better, Grok-4.1 Fast Reasoning or Kimi K2-Instruct-0905?

Grok-4.1 Fast Reasoning (xAI) and Kimi K2-Instruct-0905 (Moonshot AI) each have strengths in different areas. Compare their benchmark scores, pricing, context windows, and capabilities above to determine which fits your needs.

How does Grok-4.1 Fast Reasoning compare to Kimi K2-Instruct-0905 in benchmarks?

Kimi K2-Instruct-0905 scores MATH-500: 97.4%, MMLU-Redux: 92.7%, IFEval: 89.8%, AutoLogi: 89.5%, MMLU: 89.5%.

What are the context window sizes for Grok-4.1 Fast Reasoning and Kimi K2-Instruct-0905?

Grok-4.1 Fast Reasoning supports 2.0M tokens and Kimi K2-Instruct-0905 supports an unknown number of tokens. A larger context window lets you process longer documents, conversations, or codebases in a single request.

What are the main differences between Grok-4.1 Fast Reasoning and Kimi K2-Instruct-0905?

Key differences include multimodal support (yes vs no), licensing (Proprietary vs MIT). See the full comparison above for benchmark-by-benchmark results.

Who makes Grok-4.1 Fast Reasoning and Kimi K2-Instruct-0905?

Grok-4.1 Fast Reasoning is developed by xAI and Kimi K2-Instruct-0905 is developed by Moonshot AI.

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