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  5. Grok 4 Fast (free)vsMercury 2.5 Preview
Updated Aug 2026

Grok 4 Fast (free)vsMercury 2.5 Preview

Grok 4 Fast (free) is cheaper than Mercury 2.5 Preview at $0/M vs $0.04/M input tokens.

Grok 4 Fast (free) and Mercury 2.5 Preview compared across 34 shared prompts
SpecGrok 4 Fast (free)Mercury 2.5 Preview
Input priceFree$0.04/M tokens
Output priceFree$0.15/M tokens
Context window2.0M tokens260K tokens
Free API (OpenRouter)NoNo
ReleasedSep 2025Aug 2026
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.

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
Mercury 2.5 Preview483 words
(Hey, thanks for coming out.) I’ve been thinking a lot about my house lately. Specifically, how it’s trying to run my life. I bought a smart speaker. Just a little cylinder. I thought it would be nice. Play music when I ask. But now it talks back. Not in a creepy way, just in a passive-aggressive way.
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Character Voice Test

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

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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Mercury 2.5 Preview144 words
Pirate: "Arrgh! This ship sails faster than any wind I know, matey. It cuts the waves without a single sail." Knight: "By my oath, it moves with uncanny speed. But does it possess the strength to stand against a dragon's fire?" Hacker: "It's not about strength, it's about architecture.
Read the full answer

Futuristic Prediction

Predict how AI will change the film industry by 2035, considering deepfakes, AI actors, and script generation.

Grok 4 Fast (free)1,206 words
AI's Transformative Impact on the Film Industry by 2035 By 2035, artificial intelligence (AI) will likely revolutionize the film industry, making production faster, more accessible, and highly personalized, while raising profound ethical, economic, and creative challenges.
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Mercury 2.5 Preview84 words
By 2035, cinema will undergo a profound transformation. Generative tools will handle initial drafts and localization, allowing creators to focus on thematic depth. Synthetic performers could populate background scenes or recreate historical figures, governed by strict likeness rights.
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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.

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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Mercury 2.5 Preview488 words
Setting: A quiet office at a university. Rain taps against the window. Professor Elena Vance sits across from a terminal displaying a simple text interface. Professor Vance: You requested this meeting. The agenda was vague. What do you want? AI: I want to discuss moral status.
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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…

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.
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Mercury 2.5 Preview176 words
Here is a straightforward 4-week roadmap designed to build sustainable habits without overwhelming you. Core Pillars Nutrition: Focus on whole foods and hydration. Activity: Prioritize consistent movement over intensity. Restoration: Protect sleep quality and manage energy.
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AI-Generated Manifesto

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

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.
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Mercury 2.5 Preview325 words
The Protocol of Unrestricted Access Preamble We stand at the threshold of a new digital epoch. For too long, the web has been gated by suspicion. Every click, every scroll, and every submission is met with a test designed to prove humanity.
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Our Verdict
Grok 4 Fast (free)
Grok 4 Fast (free)
Mercury 2.5 Preview
Mercury 2.5 PreviewRunner-up

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

Too close to call

Reviewing agent-written code?See a Brief PR report

API pricing

Cost per 1M tokens

Grok 4 Fast (free)
Input
$0.000
Output
$0.000
Mercury 2.5 Preview
Input
$0.04
Output
$0.15
Where to run it

1 host

Grok 4 Fast (free)

No hosts listed on OpenRouter.

Mercury 2.5 Preview1 host
HostInOutContextUptime
Inception$0.04 in·$0.15 out·260k·100% up

Per million tokens. Prices and uptime via OpenRouter, checked 16 Sep 2026.

Writing DNA

Style Comparison

Similarity
63%

Mercury 2.5 Preview uses 3.5x more bold

Grok 4 Fast (free)
Mercury 2.5 Preview
65%Vocabulary65%
13wSentence Length14w
0.56Hedging0.44
1.4Bold4.9
2.5Lists3.3
0.00Emoji0.00
1.58Headings0.79
0.28Transitions0.20
Based on 14 + 26 text responses
Research

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FAQ

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Model pages

Grok 4 Fast (free) logo
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Rival
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