What is the difference between GPT-5.6 Luna and Grok 4 Fast (free)?

GPT-5.6 Luna is developed by OpenAI while Grok 4 Fast (free) is developed by xAI. GPT-5.6 Luna has a 1.1M token context window vs Grok 4 Fast (free)'s 2.0M. You can compare their actual outputs across 34 challenges on Rival to see how they differ in practice.

Which is better, GPT-5.6 Luna or Grok 4 Fast (free)?

It depends on your use case. GPT-5.6 Luna and Grok 4 Fast (free) each have strengths in different areas. Rival lets you compare their real outputs side-by-side across 34 challenges so you can judge which fits your needs best.

How much does GPT-5.6 Luna cost compared to Grok 4 Fast (free)?

GPT-5.6 Luna costs $1/M input tokens and Grok 4 Fast (free) costs $0/M input tokens. Grok 4 Fast (free) is $1.00/M cheaper per input. Check their side-by-side outputs on Rival to see if the price difference is justified by quality.

How can I compare GPT-5.6 Luna and Grok 4 Fast (free) on Rival?

This page shows a side-by-side comparison of GPT-5.6 Luna and Grok 4 Fast (free) across shared challenges. You can vote on which model produced the better output in a blind duel. Browsing and voting are free. No account is needed to look; signing in only saves your votes and likes.

What is the difference between GPT-5.6 Luna and Grok 4 Fast (free)?

GPT-5.6 Luna is developed by OpenAI while Grok 4 Fast (free) is developed by xAI. GPT-5.6 Luna has a 1.1M token context window vs Grok 4 Fast (free)'s 2.0M. You can compare their actual outputs across 34 challenges on Rival to see how they differ in practice.

Which is better, GPT-5.6 Luna or Grok 4 Fast (free)?

It depends on your use case. GPT-5.6 Luna and Grok 4 Fast (free) each have strengths in different areas. Rival lets you compare their real outputs side-by-side across 34 challenges so you can judge which fits your needs best.

How much does GPT-5.6 Luna cost compared to Grok 4 Fast (free)?

GPT-5.6 Luna costs $1/M input tokens and Grok 4 Fast (free) costs $0/M input tokens. Grok 4 Fast (free) is $1.00/M cheaper per input. Check their side-by-side outputs on Rival to see if the price difference is justified by quality.

How can I compare GPT-5.6 Luna and Grok 4 Fast (free) on Rival?

This page shows a side-by-side comparison of GPT-5.6 Luna and Grok 4 Fast (free) across shared challenges. You can vote on which model produced the better output in a blind duel. Browsing and voting are free. No account is needed to look; signing in only saves your votes and likes.

GPT-5.6 Luna vs Grok 4 Fast (free): Which Is Better?

Updated Jul 2026

Our Verdict

Grok 4 Fast (free)

GPT-5.6 LunaRunner-up

No community votes yet. On paper, Grok 4 Fast (free) has the edge — bigger model tier, bigger context window.

Too close to call

What do GPT-5.6 Luna and Grok 4 Fast (free) say about your brand?

10 models, one question, verbatim receipts.

or peek at a demo: Notion Linear Vercel

API pricing

Cost per 1M tokens

GPT-5.6 Luna

Input

$1.00

Output

$6.00

Grok 4 Fast (free)

Input

$0.000

Output

$0.000

Writing DNA

Style Comparison

Similarity

60%

Grok 4 Fast (free) uses 2.8x more transitions

GPT-5.6 Luna

Grok 4 Fast (free)

59%Vocabulary65%

19wSentence Length13w

0.84Hedging0.56

2.7Bold1.4

4.5Lists2.5

0.00Emoji0.00

1.20Headings1.58

0.10Transitions0.28

Based on 27 + 14 text responses

Research

What we learned reading every model

FAQ

Common questions

Keep exploring

More comparisons

Grok 4 Fast (free) vs MiniMax M3New provider

Grok 4 Fast (free) vs Ox AlphaNew provider

Grok 4 Fast (free) vs Claude Sonnet 4New provider

Updated Jul 2026

Our Verdict

Grok 4 Fast (free)

GPT-5.6 LunaRunner-up

No community votes yet. On paper, Grok 4 Fast (free) has the edge — bigger model tier, bigger context window.

Too close to call

What do GPT-5.6 Luna and Grok 4 Fast (free) say about your brand?

10 models, one question, verbatim receipts.

or peek at a demo: Notion Linear Vercel

API pricing

Cost per 1M tokens

GPT-5.6 Luna

Input

$1.00

Output

$6.00

Grok 4 Fast (free)

Input

$0.000

Output

$0.000

Writing DNA

Style Comparison

Similarity

60%

Grok 4 Fast (free) uses 2.8x more transitions

GPT-5.6 Luna

Grok 4 Fast (free)

59%Vocabulary65%

19wSentence Length13w

0.84Hedging0.56

2.7Bold1.4

4.5Lists2.5

0.00Emoji0.00

1.20Headings1.58

0.10Transitions0.28

Based on 27 + 14 text responses

Research

What we learned reading every model

FAQ

Common questions

Keep exploring

More comparisons

Grok 4 Fast (free) vs MiniMax M3New provider

Grok 4 Fast (free) vs Ox AlphaNew provider

Grok 4 Fast (free) vs Claude Sonnet 4New provider