Model Comparison
GPT-5.6 Luna vs Grok 4 FastWhich is better in 2026?
GPT-5.6 Luna significantly outperforms across most benchmarks. Grok 4 Fast is 8.2x cheaper per token.
Verdict: GPT-5.6 Luna vs Grok 4 Fast — which is better?
GPT-5.6 Luna (by OpenAI) and Grok 4 Fast (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.
GPT-5.6 Luna outperforms in 2 benchmarks (BrowseComp, GPQA), while Grok 4 Fast is better at 0 benchmarks. GPT-5.6 Luna significantly outperforms across most benchmarks.
On price, Grok 4 Fast is roughly 8.2x cheaper per token on a blended 3:1 input/output basis, which adds up quickly at production volume.
Grok 4 Fast also accepts a larger context window (2,000,000 input tokens), making it the stronger choice for long documents and large codebases.
Choose GPT-5.6 Luna if…
- you want the strongest raw capability — it leads on 2 of 2 shared benchmarks
- you want the most recent training data — it shipped Jul 2026
Choose Grok 4 Fast if…
- cost matters — it's about 8.2x cheaper per token
- you process long inputs — it offers a 2,000,000 token context window
Performance Benchmarks
Comparative analysis across standard metrics
GPT-5.6 Luna outperforms in 2 benchmarks (BrowseComp, GPQA), while Grok 4 Fast is better at 0 benchmarks.
GPT-5.6 Luna significantly outperforms across most benchmarks.
Arena Performance
Human preference votes
Pricing Analysis
Price comparison per million tokens
For input processing, GPT-5.6 Luna ($1.00/1M tokens) is 5.0x more expensive than Grok 4 Fast ($0.20/1M tokens).
For output processing, GPT-5.6 Luna ($6.00/1M tokens) is 12.0x more expensive than Grok 4 Fast ($0.50/1M tokens).
In conclusion, GPT-5.6 Luna is more expensive than Grok 4 Fast.*
* Using a 3:1 ratio of input to output tokens
Context Window
Maximum input and output token capacity
Grok 4 Fast accepts 2,000,000 input tokens compared to GPT-5.6 Luna's 1,050,000 tokens. GPT-5.6 Luna can generate longer responses up to 128,000 tokens, while Grok 4 Fast is limited to 30,000 tokens.
Input Capabilities
Supported data types and modalities
Both GPT-5.6 Luna and Grok 4 Fast support multimodal inputs.
They are both capable of processing various types of data, offering versatility in application.
GPT-5.6 Luna
Grok 4 Fast
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
GPT-5.6 Luna was released on 2026-07-09, while Grok 4 Fast was released on 2025-08-28.
GPT-5.6 Luna is 11 months newer than Grok 4 Fast.
Jul 9, 2026
1 weeks ago
10mo newerAug 28, 2025
10 months ago
Knowledge Cutoff
When training data ends
GPT-5.6 Luna has a documented knowledge cutoff of 2026-02-16, while Grok 4 Fast's cutoff date is not specified.
We can confirm GPT-5.6 Luna's training data extends to 2026-02-16, but cannot make a direct comparison without Grok 4 Fast's cutoff date.
Feb 2026
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Provider Availability
GPT-5.6 Luna is available from OpenAI. Grok 4 Fast is available from xAI.
GPT-5.6 Luna
Grok 4 Fast
Outputs Comparison
Key Takeaways
GPT-5.6 Luna
View detailsOpenAI
Detailed Comparison
Interactive Arena
Judge for yourself.
Run your own prompts against GPT-5.6 Luna and Grok 4 Fast side-by-side, then vote on the output you prefer.
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FAQ
Common questions about GPT-5.6 Luna vs Grok 4 Fast.