Model Comparison
Grok 4 Fast vs Grok-4 HeavyWhich is better in 2026?
Grok-4 Heavy shows notably better performance in the majority of benchmarks.
Verdict: Grok 4 Fast vs Grok-4 Heavy — which is better?
Grok 4 Fast (by xAI) and Grok-4 Heavy (by xAI) 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.
Grok 4 Fast outperforms in 1 benchmarks (LiveCodeBench), while Grok-4 Heavy is better at 3 benchmarks (AIME 2025, GPQA, Humanity's Last Exam). Grok-4 Heavy shows notably better performance in the majority of benchmarks.
Choose Grok 4 Fast if…
- you want predictable pricing at $0.20/M input and $0.50/M output
Choose Grok-4 Heavy if…
- you want the strongest raw capability — it leads on 3 of 4 shared benchmarks
Performance Benchmarks
Comparative analysis across standard metrics
Grok 4 Fast outperforms in 1 benchmarks (LiveCodeBench), while Grok-4 Heavy is better at 3 benchmarks (AIME 2025, GPQA, Humanity's Last Exam).
Grok-4 Heavy shows notably better performance in the majority of benchmarks.
Arena Performance
Human preference votes
Context Window
Maximum input and output token capacity
Only Grok 4 Fast specifies input context (2,000,000 tokens). Only Grok 4 Fast specifies output context (30,000 tokens).
Input Capabilities
Supported data types and modalities
Both Grok 4 Fast and Grok-4 Heavy support multimodal inputs.
They are both capable of processing various types of data, offering versatility in application.
Grok 4 Fast
Grok-4 Heavy
License
Usage and distribution terms
Both models are licensed under proprietary licenses.
Both models have usage restrictions defined by their respective organizations.
Proprietary
Closed source
Proprietary
Closed source
Release Timeline
When each model was launched
Grok 4 Fast was released on 2025-08-28, while Grok-4 Heavy's release date is not specified.
We can confirm Grok 4 Fast's release timeline, but cannot make a direct age comparison without Grok-4 Heavy's release date.
Aug 28, 2025
10 months ago
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Knowledge Cutoff
When training data ends
Grok-4 Heavy has a documented knowledge cutoff of 2024-12-31, while Grok 4 Fast's cutoff date is not specified.
We can confirm Grok-4 Heavy's training data extends to 2024-12-31, but cannot make a direct comparison without Grok 4 Fast's cutoff date.
—
Dec 2024
Outputs Comparison
Key Takeaways
Detailed Comparison
Interactive Arena
Judge for yourself.
Run your own prompts against Grok 4 Fast and Grok-4 Heavy side-by-side, then vote on the output you prefer.
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FAQ
Common questions about Grok 4 Fast vs Grok-4 Heavy.