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  5. GPT-6 LunavsGrok 4 Fast (free)
Updated Sep 2026

GPT-6 LunavsGrok 4 Fast (free)

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

GPT-6 Luna and Grok 4 Fast (free) compared across 2 shared prompts
SpecGPT-6 LunaGrok 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.

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 Luna330 words
A simple 1-month starter plan Choose three foundations: food, movement, and sleep. Aim for steady habits, not perfection. If you have a health condition, significant pain, or concerns about changing your activity or diet, check with a healthcare professional first.
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

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 Luna113 words
Crispy Parmesan Chickpeas A crunchy, savory snack with a cheesy finish Ingredients (7): 1 can chickpeas, drained and dried 1 tablespoon olive oil ½ teaspoon salt ½ teaspoon paprika ¼ teaspoon garlic powder ¼ cup grated Parmesan 1 tablespoon lemon juice Prep time: 5 minutes Cook time: 30–35 minutes Instructions: Heat…
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
Our Verdict
GPT-6 Luna
GPT-6 Luna
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
Input
$0.10
Output
$0.50
Grok 4 Fast (free)
Input
$0.000
Output
$0.000
Where to run it

3 hosts, cheapest first

GPT-6 Luna3 hosts
HostInOutContextUptime
Azure AI Foundry$0.10 in·$0.50 out·1.1M·88.4% upOpenAI$0.10 in·$0.50 out·1.1M·99.9% upAmazon Bedrock$0.11 in·$0.55 out·1.1M·99.8% 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 is developed by OpenAI while Grok 4 Fast (free) is developed by xAI. GPT-6 Luna 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 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 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 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.

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Model pages

GPT-6 Luna logo
GPT-6 Luna20 outputs, specs and price
Grok 4 Fast (free) logo
Grok 4 Fast (free)35 outputs, specs and price
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