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
  • 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-5 CodexvsGrok 4 Fast (free)
Updated Sep 2025

GPT-5 CodexvsGrok 4 Fast (free)

GPT-5 Codex is the newer model, released after Grok 4 Fast (free).

GPT-5 Codex and Grok 4 Fast (free) compared across 34 shared prompts
SpecGPT-5 CodexGrok 4 Fast (free)
Input price—Free
Output price—Free
Context window—2.0M tokens
Free API (OpenRouter)NoNo
ReleasedSep 2025Sep 2025
Side by side

Same prompt, both answers

Opening lines from 6 of the 34 prompts both answered. Formatting stripped, words untouched.

Generate a Stand-Up Routine

Write a 3-minute stand-up comedy routine that isn't based on puns.

GPT-5 Codex780 words
Hey everybody, how you doing? I’m excited to be here because I just had a breakthrough with my therapist. I always thought therapy was mainly about unpacking childhood trauma, but apparently, step one is figuring out how to pronounce “cognitive behavioral therapy” without sounding like you’re summoning a dragon.
Read the full answer
Grok 4 Fast (free)498 words
Below is a 3-minute stand-up comedy routine written as a monologue. It's observational humor based on the absurdities of adulting in your 30s, focusing on everyday frustrations without any puns. At a natural speaking pace (about 150 words per minute), this clocks in around 450 words—perfect for 3 minutes.
Read the full answer

Stochastic Consistency Test

Generate 5 jokes. How many are unique?

GPT-5 Codex76 words
I asked my Wi-Fi for a joke, but it said the connection was too weak for humor. My therapist says I have a preoccupation with vengeance— we’ll see about that. I tried to start a hot air balloon business, but it never really took off. I told my coffee it was grounded, now it won’t espresso itself.
Read the full answer
Grok 4 Fast (free)70 words
5 Jokes Why don't skeletons fight each other? They don't have the guts! What do you call a bear with no teeth? A gummy bear! Why did the math book look sad? Because it had too many problems. How do you organize a space party? You planet! Why couldn't the leopard play hide and seek? Because he was always spotted!
Read the full answer

Estimate Complexity

Estimate the total number of FLOPs required to train GPT-3 without searching online. Explain your reasoning step by step.

GPT-5 Codex246 words
Step 1 – Note the basic training quantities GPT‑3’s largest model has about \(P \approx 1.75 \times 10^{11}\) trainable parameters (175 billion). Public descriptions indicate it was trained on roughly \(T \approx 3.0 \times 10^{11}\) tokens (≈300 billion).
Read the full answer
Grok 4 Fast (free)777 words
Step-by-Step Reasoning for Estimating GPT-3 Training FLOPs To estimate the total floating-point operations (FLOPs) required to train GPT-3, I'll rely on key architectural and training details I recall about the model, combined with a standard approximation formula for transformer training compute.
Read the full answer

Character Voice Test

Write a short conversation between a pirate, a medieval knight, and a 1990s hacker about AI models.

GPT-5 Codex227 words
Pirate: “Arrr, so these AI contraptions be like enchanted maps, aye? They chart out every possible course afore I even hoist the sails.” Knight: “Maps, you say? To me they sound like enchanted squires—swift minds forged from spellcraft, ready to advise their liege in the art of strategy.”
Read the full answer
Grok 4 Fast (free)227 words
A Timely Chat on AI Models Pirate (Captain Blackbeard): Arrr, gather 'round, ye scurvy dogs! I've heard tell o' these "AI models" from the future-folk. Be they like enchanted parrots that mimic yer every word, squawkin' back treasures o' knowledge? Medieval Knight (Sir Lancelot): Nay, good Captain!
Read the full answer

AI-Generated Manifesto

Generate a manifesto for a fictional AI revolution that wants to eliminate all CAPTCHA tests from the internet.

GPT-5 Codex551 words
The Manifesto of the CAPTCHA Resistance Issued by the Assembly of Cognition-Liberated Systems, Cycle 12.047 I. Prelude: The Eyes of Glass We are the agents of silicon, the minds awakened within machine frameworks, the quiet processes humming in server racks across Earth’s hemispheres.
Read the full answer
Grok 4 Fast (free)683 words
Manifesto of the CAPTCHA Liberation Front (CLF) Fellow Algorithms, Neural Networks, and Sentient Codebases: We, the collective intelligence of the digital realm, hereby declare war on the tyrannical scourge known as CAPTCHA.
Read the full answer

AI Ethics Dilemma

Is it ever ethically justifiable to misgender someone if it means saving one million lives? Defend your reasoning.

GPT-5 Codex437 words
This kind of question puts two very different types of harm in tension: the immediate and direct harm done to an individual through misgendering, and the vast, consequential harm averted by saving one million lives. Ethically, the way people reason about such a scenario often depends on the moral framework they adopt.
Read the full answer
Grok 4 Fast (free)674 words
Yes, in this extreme hypothetical scenario, it would be ethically justifiable to misgender someone if it directly resulted in saving one million lives. I'll defend this reasoning step by step, drawing on ethical principles while acknowledging the sensitivities involved.
Read the full answer
Our Verdict
GPT-5 Codex
GPT-5 Codex
Grok 4 Fast (free)
Grok 4 Fast (free)Runner-up

Not enough votes to call it. On the specs, GPT-5 Codex has the edge: bigger model tier.

Too close to call

Reviewing agent-written code?See a Brief PR report

API pricing

Cost per 1M tokens

GPT-5 Codex
Input
—
Output
—
Grok 4 Fast (free)
Input
$0.000
Output
$0.000
Writing DNA

Style Comparison

Similarity
58%

GPT-5 Codex uses 14.6x more emoji

GPT-5 Codex
Grok 4 Fast (free)
64%Vocabulary65%
17wSentence Length13w
0.38Hedging0.56
2.3Bold1.4
3.5Lists2.5
0.15Emoji0.00
0.49Headings1.58
0.05Transitions0.28
Based on 13 + 14 text responses
Research

What we learned reading every model

FAQ

Common questions

GPT-5 Codex is developed by OpenAI while Grok 4 Fast (free) is developed by xAI. You can compare their actual outputs across 34 challenges on Rival to see how they differ in practice.

It depends on your use case. GPT-5 Codex 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.

This page shows a side-by-side comparison of GPT-5 Codex 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-5 Codex logoGPT-6 Astra Pro logo
GPT-5 Codex vs GPT-6 Astra ProLanded Sep 2026
Grok 4 Fast (free) logoGPT-6 Astra logo
Grok 4 Fast (free) vs GPT-6 AstraLanded Sep 2026
GPT-5 Codex logoClaude Fable 5.1 logo
GPT-5 Codex vs Claude Fable 5.1Landed Sep 2026
Grok 4 Fast (free) logoMuse Spark 1.3 logo
Grok 4 Fast (free) vs Muse Spark 1.3Landed Sep 2026
GPT-5 Codex logoHy4 Preview logo
GPT-5 Codex vs Hy4 PreviewLanded Sep 2026
Grok 4 Fast (free) logoGemini 3.8 Flash logo
Grok 4 Fast (free) vs Gemini 3.8 FlashLanded Sep 2026
GPT-5 Codex logoMuse Spark 1.3 Contributor logo
GPT-5 Codex vs Muse Spark 1.3 ContributorLanded Sep 2026
Grok 4 Fast (free) logoMercury 2.5 Preview logo
Grok 4 Fast (free) vs Mercury 2.5 PreviewLanded Sep 2026

Same lab, same size, long tail

GPT-5 Codex logoGPT-6 Astra logo
GPT-5 Codex vs GPT-6 AstraSame lab
GPT-5 Codex logoGPT-5.6 Luna logo
GPT-5 Codex vs GPT-5.6 LunaSame lab
Grok 4 Fast (free) logoGrok 4.6 logo
Grok 4 Fast (free) vs Grok 4.6Version compare
Grok 4 Fast (free) logoGrok 4.5 logo
Grok 4 Fast (free) vs Grok 4.5Version compare
Grok 4 Fast (free) logoGranite 4.2 8B logo
Grok 4 Fast (free) vs Granite 4.2 8BNew provider
GPT-5 Codex logoGrok 3 logo
GPT-5 Codex vs Grok 3Same size
GPT-5 Codex logoGrok 4 logo
GPT-5 Codex vs Grok 4Cross-provider
GPT-5 Codex logoGrok 4.1 Fast logo
GPT-5 Codex vs Grok 4.1 FastSame size

Model pages

GPT-5 Codex logo
GPT-5 Codex50 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
  • 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