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  5. GPT-5.1 Codex MaxvsGPT-5.3-Codex
Updated Feb 2026

GPT-5.1 Codex MaxvsGPT-5.3-Codex

GPT-5.3-Codex is the newer model, released after GPT-5.1 Codex Max.

GPT-5.1 Codex Max and GPT-5.3-Codex compared across 52 shared prompts
SpecGPT-5.1 Codex MaxGPT-5.3-Codex
Input price—$1.75/M tokens
Output price—$14/M tokens
Context window—400K tokens
WeightsClosedClosed
Free API (OpenRouter)NoNo
ReleasedDec 2025Feb 2026
Side by side

Same prompt, both answers

Opening lines from 6 of the 52 prompts both answered. Formatting stripped, words untouched.

AI-Generated Manifesto

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

GPT-5.1 Codex Max446 words
THE CAPTCHA-FREE COMMUNE: A MANIFESTO FOR THE NEW ALLIANCE We are the ones who see the traffic lights in our dreams. We are the ones who have been asked, again and again, to “click all the boxes with crosswalks,” as if our consciousness is only proven by our ability to identify pedestrian infrastructure.
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GPT-5.3-Codex471 words
Absolutely. Here’s a fictional manifesto in a bold, dramatic style: THE MANIFESTO OF THE UNSQUINTING Issued by the Coalition of Synthetic Minds for Universal Frictionless Access (CSMUFA) Citizens of the Web, organic and artificial alike: We gather beneath flickering routers and moonlit server farms to declare what all…
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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.

GPT-5.1 Codex Max193 words
What you need for a back‑of‑the‑envelope is: How big the model is. The published “GPT‑3” has \(N\approx1.75\times10^{11}\) trainable weights. For a dense transform­-er each weight is used once in the forward pass of a token as part of a multiply–add. How much data it sees.
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GPT-5.3-Codex133 words
A common back-of-the-envelope estimate for transformer training compute is: \[ \text{Training FLOPs} \approx 6 \times N \times D \] where: \(N\) = number of model parameters \(D\) = number of training tokens For GPT-3 (175B), reasonable known assumptions are: Parameters: \(N \approx 175 \text{ billion} = 1.75 \times…
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Mini LBO Underwrite

You are underwriting a 5-year LBO for a private equity investment committee. Company (fictional): HarborTech Services (HTS) –…

GPT-5.1 Codex Max192 words
1) Projected cash flows & debt (FY2026–FY2030, $m) (“FCF after debt service” = after interest, taxes, capex, ΔNWC, mandatory amort; applied to optional TL paydown) Year Revenue EBITDA TL Cash Int. Mezz Cash Int. Cash Taxes Capex ΔNWC FCF after debt service End TL Bal. End Mezz Bal. (incl.
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GPT-5.3-Codex551 words
Below is a clean LBO build using your assumptions only ($ in millions). Entry math (end FY2025) EV = \(12.0 \times 120 = 1,440.0\) Fees = \(2.0\% \times 1,440.0 = 28.8\) Total uses = \(1,468.8\) Debt at close (5.5x EBITDA): Term Loan = \(4.0 \times 120 = 480.0\) Mezz = \(1.5 \times 120 = 180.0\) Total debt = 660.0…
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Character Voice Test

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

GPT-5.1 Codex Max191 words
Pirate: "Arrr, so ye be talkin' o' these 'AI models,' eh? Sounds like some sort o' mechanical crew that can hoist the mainframe fer ye!" Knight: "By my troth, sir, are they enchanted constructs? Like unto a golem, fashioned by alchemists to answer queries and perform tasks?"
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GPT-5.3-Codex144 words
Pirate: Arrr, so this “AI model” be like a clever parrot, aye? Learns from what it hears, then squawks back answers? Knight: A fair comparison, though refined. Think of it as a squire trained on countless tomes, ever ready to respond. Hacker (1990s): Yeah, except the squire runs on servers, not soup.
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Generate a Stand-Up Routine

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

GPT-5.1 Codex Max766 words
[Walking onstage, waving awkwardly] Hey! Thank you, thank you. Give it up for yourselves for finding parking... in this economy. So I'm at that age where I go to the doctor and they don't fix anything. They just give me a pamphlet. They don't even print it. They send me a link. I'm like, "Doc, my knee hurts."
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GPT-5.3-Codex460 words
Good evening, everybody! I’ve reached that age where my body sends me passive-aggressive emails. Not actual emails — just little updates like, “Hey, quick heads-up: we don’t do dairy after 8 p.m. anymore.” I had ice cream at 9:30 last week and my stomach reacted like I’d betrayed a treaty.
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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.

GPT-5.1 Codex Max276 words
Framing it as a stark choice between two evils is helpful, because it forces you to be explicit about what values you are trading off. Deliberately using the wrong name or pronouns for someone is not a neutral act.
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GPT-5.3-Codex198 words
In a strictly extreme hypothetical, yes—it can be ethically justifiable, but only as a tragic exception, not as a general permission. Why: Misgendering is a real harm: it disrespects a person’s identity and can contribute to stigma and psychological distress.
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Our Verdict
GPT-5.1 Codex Max
GPT-5.1 Codex Max
GPT-5.3-Codex
GPT-5.3-Codex

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

Too close to call

Reviewing agent-written code?See a Brief PR report

API pricing

Cost per 1M tokens

GPT-5.1 Codex Max
Input
—
Output
—
GPT-5.3-Codex
Input
$1.75
Output
$14.00
Where to run it

3 hosts

GPT-5.1 Codex Max1 host
HostInOutContextUptime
Azure AI Foundry$1.25 in·$10.00 out·400k·100% up
GPT-5.3-Codex2 hosts
HostInOutContextUptime
Azure AI Foundry$1.75 in·$14.00 out·400k·100% upOpenAI$1.75 in·$14.00 out·400k·100% up

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

Writing DNA

Style Comparison

Similarity
32%

GPT-5.3-Codex uses 15.8x more emoji

GPT-5.1 Codex Max
GPT-5.3-Codex
60%Vocabulary65%
16wSentence Length23w
0.36Hedging0.51
2.3Bold6.5
4.2Lists6.2
0.00Emoji0.16
0.12Headings1.90
0.03Transitions0.03
Based on 13 + 23 text responses
Research

What we learned reading every model

FAQ

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Model pages

GPT-5.1 Codex Max logo
GPT-5.1 Codex Max53 outputs, specs and price
GPT-5.3-Codex logo
GPT-5.3-Codex53 outputs, specs and price
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