Model Comparison

GPT-5.6 Luna vs Grok-4 HeavyWhich is better in 2026?

GPT-5.6 Luna significantly outperforms across most benchmarks.

Verdict: GPT-5.6 Luna vs Grok-4 Heavy — which is better?

GPT-5.6 Luna (by OpenAI) 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.

GPT-5.6 Luna outperforms in 1 benchmarks (GPQA), while Grok-4 Heavy is better at 0 benchmarks. GPT-5.6 Luna significantly outperforms across most benchmarks.

Choose GPT-5.6 Luna if…

  • you want the strongest raw capability — it leads on 1 of 1 shared benchmarks

Choose Grok-4 Heavy if…

  • you are already invested in the xAI ecosystem

Performance Benchmarks

Comparative analysis across standard metrics

1 benchmarks

GPT-5.6 Luna outperforms in 1 benchmarks (GPQA), while Grok-4 Heavy is better at 0 benchmarks.

GPT-5.6 Luna significantly outperforms across most benchmarks.

Tue Jul 21 2026 • llm-stats.com

Arena Performance

Human preference votes

Context Window

Maximum input and output token capacity

Only GPT-5.6 Luna specifies input context (1,050,000 tokens). Only GPT-5.6 Luna specifies output context (128,000 tokens).

OpenAI
GPT-5.6 Luna
Input1,050,000 tokens
Output128,000 tokens
xAI
Grok-4 Heavy
Input- tokens
Output- tokens
Tue Jul 21 2026 • llm-stats.com

Input Capabilities

Supported data types and modalities

Both GPT-5.6 Luna and Grok-4 Heavy support multimodal inputs.

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

GPT-5.6 Luna

Text
Images
Audio
Video

Grok-4 Heavy

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.

GPT-5.6 Luna

Proprietary

Closed source

Grok-4 Heavy

Proprietary

Closed source

Release Timeline

When each model was launched

GPT-5.6 Luna was released on 2026-07-09, while Grok-4 Heavy's release date is not specified.

We can confirm GPT-5.6 Luna's release timeline, but cannot make a direct age comparison without Grok-4 Heavy's release date.

GPT-5.6 Luna

Jul 9, 2026

1 weeks ago

Grok-4 Heavy

—

Knowledge Cutoff

When training data ends

GPT-5.6 Luna has a knowledge cutoff of 2026-02-16, while Grok-4 Heavy has a cutoff of 2024-12-31.

GPT-5.6 Luna has more recent training data (up to 2026-02-16), making it potentially better informed about events through that date compared to Grok-4 Heavy (2024-12-31).

GPT-5.6 Luna

Feb 2026

1.2 yr newer
Grok-4 Heavy

Dec 2024

Outputs Comparison

Notice missing or incorrect data?Start an Issue discussion→

Key Takeaways

OpenAI
Larger context window (1,050,000 tokens)
Higher GPQA score (92.3% vs 88.4%)
xAI

No standout differentiators in the data we have for this pair.

Detailed Comparison

Interactive Arena

Judge for yourself.

Run your own prompts against GPT-5.6 Luna and Grok-4 Heavy side-by-side, then vote on the output you prefer.

GPT-5.6 Luna
✓ Preferred
Grok-4 Heavy
Open in Playground
AI Model Comparison Table
Feature
OpenAI
GPT-5.6 Luna
xAI
Grok-4 Heavy

FAQ

Common questions about GPT-5.6 Luna vs Grok-4 Heavy.

Which is better, GPT-5.6 Luna or Grok-4 Heavy?

GPT-5.6 Luna significantly outperforms across most benchmarks. GPT-5.6 Luna is made by OpenAI and Grok-4 Heavy is made by xAI. The best choice depends on your use case — compare their benchmark scores, pricing, and capabilities above.

How does GPT-5.6 Luna compare to Grok-4 Heavy in benchmarks?

GPT-5.6 Luna scores Connectors: 99.9%, HealthBench Consensus: 95.1%, GPQA: 92.3%, Search and Function-Calling: 89.7%, Capture-the-Flag Challenges (Internal): 85.2%. Grok-4 Heavy scores AIME 2025: 100.0%, HMMT25: 96.7%, GPQA: 88.4%, LiveCodeBench: 79.4%, USAMO25: 61.9%.

What are the context window sizes for GPT-5.6 Luna and Grok-4 Heavy?

GPT-5.6 Luna supports 1.1M tokens and Grok-4 Heavy supports an unknown number of tokens. A larger context window lets you process longer documents, conversations, or codebases in a single request.

Who makes GPT-5.6 Luna and Grok-4 Heavy?

GPT-5.6 Luna is developed by OpenAI and Grok-4 Heavy is developed by xAI.

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