OpenAI: GPT-5.6 Luna vs xAI: Grok 4 Fast (free)
Compare OpenAI: GPT-5.6 Luna by OpenAI against xAI: Grok 4 Fast (free) by xAI, context windows of 1.1M vs 2.0M, tested across 34 shared challenges.
Our Verdict
xAI: Grok 4 Fast (free)
OpenAI: GPT-5.6 LunaRunner-up
No community votes yet. On paper, xAI: Grok 4 Fast (free) has the edge — bigger model tier, bigger context window.
Too close to call
API pricing
Cost per 1M tokens
OpenAI: GPT-5.6 Luna
Input
$1.00
Output
$6.00
xAI: Grok 4 Fast (free)
Input
$0.000
Output
$0.000
Writing DNA
Style Comparison
Similarity
60%
xAI: Grok 4 Fast (free) uses 2.8x more transitions
OpenAI: GPT-5.6 Luna
xAI: 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
