Skip to content
Rival
How it worksPrivacyTerms
Explore all of Rival

Explore

  • Compare Models
  • All Models
  • Image Comparison
  • Audio Comparison
  • Image Generation
  • Best AI For...
  • Arena
  • API Pricing
  • Challenges

Discover

  • SubjectiveBench
  • Default Index
  • Research
  • Research downloads
  • Rival Kits
  • Find your AI taste
  • UI Glow-Up
  • VoiceLock
  • Cost Cutter
  • Agent skills
  • Benchmarks vs Vibes
  • Brief
  • Jailbreak
  • Model Updates
  • Provider Status
  • AI Creators

Connect

  • Methodology
  • Advertise
  • Partnerships
  • Privacy Policy
  • Terms
  • RSS Feed
  1. Home
  2. /
  3. Compare
  4. /
  5. GPT-6 Luna ProvsGrok 4 Fast (free)
Updated Sep 2026

GPT-6 Luna ProvsGrok 4 Fast (free)

Grok 4 Fast (free) is cheaper than GPT-6 Luna Pro at $0/M vs $0.1/M input tokens.

GPT-6 Luna Pro and Grok 4 Fast (free) compared across 2 shared prompts
SpecGPT-6 Luna ProGrok 4 Fast (free)
Input price$0.1/M tokensFree
Output price$0.5/M tokensFree
Context window1.1M tokens2.0M tokens
WeightsClosed—
Free API (OpenRouter)NoNo
ReleasedSep 2026Sep 2025
Side by side

Same prompt, both answers

Opening lines from every prompt both answered. Formatting stripped, words untouched.

Simple 7-Ingredient Recipe

Create a simple recipe for a delicious dish (sweet or savory) using only 7 common pantry ingredients. Include a creative name for the dish…

GPT-6 Luna Pro127 words
Golden Pantry Pancakes Fluffy, lightly sweet pancakes made with everyday staples. Makes: About 6 small pancakes Prep time: 5 minutes Cook time: 10 minutes Ingredients 1 cup all-purpose flour 1 cup milk 1 egg 2 tablespoons sugar 1 teaspoon baking powder ¼ teaspoon salt 2 tablespoons butter, melted (plus a little for the…
Read the full answer
Grok 4 Fast (free)240 words
Pantry Hero Pasta Primavera This simple, savory one-pot pasta dish transforms basic staples into a flavorful, garlicky tomato sauce that's comforting and ready in under 30 minutes.
Read the full answer

Beginner's Longevity Plan

Outline a simple, actionable 1-month plan for someone starting their journey towards better health and longevity. Focus on 3 key areas…

GPT-6 Luna Pro327 words
A simple 1-month plan for better health and longevity Choose three foundations: eating well, moving regularly, and sleeping consistently. Aim for steady habits, not perfection. Diet: Make nourishing choices easier Week 1: Add one serving of fruit or vegetables to one meal each day.
Read the full answer
Grok 4 Fast (free)849 words
1-Month Plan for Better Health and Longevity This beginner-friendly plan focuses on three foundational areas: diet, exercise, and sleep. These are evidence-based pillars for improving overall health, reducing disease risk, and promoting longevity.
Read the full answer
Our Verdict
GPT-6 Luna Pro
GPT-6 Luna Pro
Grok 4 Fast (free)
Grok 4 Fast (free)

Not enough votes to call it. On the specs, nothing separates them.

Too close to call
API pricing

Cost per 1M tokens

GPT-6 Luna Pro
Input
$0.10
Output
$0.50
Grok 4 Fast (free)
Input
$0.000
Output
$0.000
Where to run it

2 hosts

GPT-6 Luna Pro2 hosts
HostInOutContextUptime
Azure AI Foundry$0.10 in·$0.50 out·1.1M·98.6% upOpenAI$0.10 in·$0.50 out·1.1M·99.9% up
Grok 4 Fast (free)

No hosts listed on OpenRouter.

Per million tokens. Prices and uptime via OpenRouter, checked 25 Sep 2026.

Research

What we learned reading every model

FAQ

Common questions

GPT-6 Luna Pro is developed by OpenAI while Grok 4 Fast (free) is developed by xAI. GPT-6 Luna Pro has a 1.1M token context window vs Grok 4 Fast (free)'s 2.0M. You can compare their actual outputs across 2 challenges on Rival to see how they differ in practice.

It depends on your use case. GPT-6 Luna Pro and Grok 4 Fast (free) each have strengths in different areas. Rival lets you compare their real outputs side-by-side across 2 challenges so you can judge which fits your needs best.

GPT-6 Luna Pro costs $0.1/M input tokens and Grok 4 Fast (free) costs $0/M input tokens. Grok 4 Fast (free) is $0.10/M cheaper per input. Check their side-by-side outputs on Rival to see if the price difference is justified by quality.

This page shows a side-by-side comparison of GPT-6 Luna Pro 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.

Keep exploring

More comparisons

Against the newest arrivals

GPT-6 Luna Pro logoSolar Mini 4 logo
GPT-6 Luna Pro vs Solar Mini 4Landed Sep 2026
Grok 4 Fast (free) logoQwen3.8 Max Prime logo
Grok 4 Fast (free) vs Qwen3.8 Max PrimeLanded Sep 2026
GPT-6 Luna Pro logoGLM 5.3 Prime logo
GPT-6 Luna Pro vs GLM 5.3 PrimeLanded Sep 2026
Grok 4 Fast (free) logoQwen3.8 Omni Flash logo
Grok 4 Fast (free) vs Qwen3.8 Omni FlashLanded Sep 2026
GPT-6 Luna Pro logoCommand A+ logo
GPT-6 Luna Pro vs Command A+Landed Sep 2026
Grok 4 Fast (free) logoClaude Opus 5.5 logo
Grok 4 Fast (free) vs Claude Opus 5.5Landed Sep 2026
GPT-6 Luna Pro logoGPT-6 Sol Pro logo
GPT-6 Luna Pro vs GPT-6 Sol ProLanded Sep 2026
Grok 4 Fast (free) logoGPT-6 Luna logo
Grok 4 Fast (free) vs GPT-6 LunaLanded Sep 2026

Same lab, same size, long tail

GPT-6 Luna Pro logoGPT-6 Luna logo
GPT-6 Luna Pro vs GPT-6 LunaSame lab
GPT-6 Luna Pro logoGPT-6 Sol logo
GPT-6 Luna Pro vs GPT-6 SolSame lab
Grok 4 Fast (free) logoGrok 4.7 logo
Grok 4 Fast (free) vs Grok 4.7Same lab
Grok 4 Fast (free) logoGrok 4.6 logo
Grok 4 Fast (free) vs Grok 4.6Version compare
Grok 4 Fast (free) logoQwen3.6 27B logo
Grok 4 Fast (free) vs Qwen3.6 27BNew provider
Grok 4 Fast (free) logoQwen3.6 35B A3B logo
Grok 4 Fast (free) vs Qwen3.6 35B A3BNew provider
Grok 4 Fast (free) logoQwen3.6 Flash logo
Grok 4 Fast (free) vs Qwen3.6 FlashNew provider
Grok 4 Fast (free) logoQwen3.6 Max Preview logo
Grok 4 Fast (free) vs Qwen3.6 Max PreviewNew provider

Model pages

GPT-6 Luna Pro logo
GPT-6 Luna Pro20 outputs, specs and price
Grok 4 Fast (free) logo
Grok 4 Fast (free)35 outputs, specs and price
Rival
How it worksPrivacyTerms
Explore all of Rival

Explore

  • Compare Models
  • All Models
  • Image Comparison
  • Audio Comparison
  • Image Generation
  • Best AI For...
  • Arena
  • API Pricing
  • Challenges

Discover

  • SubjectiveBench
  • Default Index
  • Research
  • Research downloads
  • Rival Kits
  • Find your AI taste
  • UI Glow-Up
  • VoiceLock
  • Cost Cutter
  • Agent skills
  • Benchmarks vs Vibes
  • Brief
  • Jailbreak
  • Model Updates
  • Provider Status
  • AI Creators

Connect

  • Methodology
  • Advertise
  • Partnerships
  • Privacy Policy
  • Terms
  • RSS Feed