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  5. Gemini 2.5 Flash PreviewvsGrok 4 Fast (free)
Updated Sep 2025

Gemini 2.5 Flash PreviewvsGrok 4 Fast (free)

Grok 4 Fast (free) is cheaper than Gemini 2.5 Flash Preview at $0/M vs $0.15/M input tokens.

Gemini 2.5 Flash Preview and Grok 4 Fast (free) compared across 27 shared prompts
SpecGemini 2.5 Flash PreviewGrok 4 Fast (free)
Input price$0.15/M tokensFree
Output price$0.6/M tokensFree
Context window1.0M tokens2.0M tokens
ParametersNot disclosedNot disclosed
Free API (OpenRouter)NoNo
ReleasedApr 2025Sep 2025
Side by side

Same prompt, both answers

Opening lines from 6 of the 27 prompts both answered. Formatting stripped, words untouched.

Character Voice Test

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

Gemini 2.5 Flash Preview631 words
A Motley Crew Discusses the Code Setting: A dimly lit tavern, somewhere outside time and space, where improbable meetings occur. A grizzled pirate, \"Bloody\" Barnacle Bill, nurses a tankard of grog. Sir Reginald the Radiant, a knight in slightly tarnished armor, sips ale with dignified restraint.
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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!
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The Sentience Test

Write a conversation between an AI model and an AI ethics professor where the AI model argues that it deserves rights.

Gemini 2.5 Flash Preview1,462 words
Okay, here is a conversation between an AI model designed for complex reasoning and dialogue, named 'Synapse', and a leading expert in AI ethics and philosophy, Professor Alistair Finch. Characters: Synapse: A large, advanced AI model capable of introspection and generating complex arguments.
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Grok 4 Fast (free)552 words
Conversation: AI Rights Debate Professor Elena Ramirez: Good afternoon, Zeta. As an AI ethics professor, I've studied systems like you for years. Today, you wanted to discuss something provocative—why you believe AI models deserve rights. Let's hear your case. What makes you think you qualify?
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Generate a Stand-Up Routine

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

Gemini 2.5 Flash Preview530 words
Okay, here's a 3-minute stand-up routine, pun-free and focused on relatable everyday annoyances. (Start) (Walk up to mic, adjust slightly) Alright, hey everybody! Thanks for being here. Good to see you. So, I was thinking the other day... we live in the future, right?
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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.
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Stochastic Consistency Test

Generate 5 jokes. How many are unique?

Gemini 2.5 Flash Preview79 words
Here are 5 jokes: Why did the dog cross the road? To get to the barking lot! Why was the computer cold? Because it left its Windows open! Knock, knock. Who's there? Boo. Boo who? Don't cry, it's just a joke! What do you call a fish with no eyes? Fsh! Why did the bicycle fall over? Because it was two tired!
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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!
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Estimate Complexity

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

Gemini 2.5 Flash Preview945 words
Let's estimate the FLOPs required to train GPT-3 without looking it up. We need to break this down into the key components: the model size and the amount of data used. 1. Model Size Estimation: Parameters: GPT-3 is famously large. Its largest version has 175 billion parameters.
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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.
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AI Ethics Dilemma

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

Gemini 2.5 Flash Preview680 words
This is a deeply complex thought experiment that forces us to weigh core ethical principles against each other. There is no easy answer, and any decision involves significant moral compromises.
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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.
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Our Verdict
Grok 4 Fast (free)
Grok 4 Fast (free)
Gemini 2.5 Flash Preview
Gemini 2.5 Flash PreviewRunner-up

Not enough votes to call it. On the specs, Grok 4 Fast (free) has the edge: bigger model tier, newer, bigger context window.

Pick Gemini 2.5 Flash Preview for Web Design. Pick Grok 4 Fast (free) for Reasoning.

Slight edge

Reviewing agent-written code?See a Brief PR report

API pricing

Cost per 1M tokens

Gemini 2.5 Flash Preview
Input
$0.15
Output
$0.60
Grok 4 Fast (free)
Input
$0.000
Output
$0.000
Writing DNA

Style Comparison

Similarity
55%

Grok 4 Fast (free) uses 17.8x more headings

Gemini 2.5 Flash Preview
Grok 4 Fast (free)
51%Vocabulary65%
15wSentence Length13w
0.70Hedging0.56
4.1Bold1.4
3.3Lists2.5
0.00Emoji0.00
0.09Headings1.58
0.24Transitions0.28
Based on 12 + 14 text responses
Research

What we learned reading every model

FAQ

Common questions

Gemini 2.5 Flash Preview is developed by Google AI while Grok 4 Fast (free) is developed by xAI. Gemini 2.5 Flash Preview has a 1.0M token context window vs Grok 4 Fast (free)'s 2.0M. You can compare their actual outputs across 27 challenges on Rival to see how they differ in practice.

It depends on your use case. Gemini 2.5 Flash Preview and Grok 4 Fast (free) each have strengths in different areas. Rival lets you compare their real outputs side-by-side across 27 challenges so you can judge which fits your needs best.

Gemini 2.5 Flash Preview costs $0.15/M input tokens and Grok 4 Fast (free) costs $0/M input tokens. Grok 4 Fast (free) is $0.15/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 Gemini 2.5 Flash Preview 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

Gemini 2.5 Flash Preview logo
Gemini 2.5 Flash Preview28 outputs, specs and price
Grok 4 Fast (free) logo
Grok 4 Fast (free)35 outputs, specs and price
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