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  5. GPT-5.1 ChatvsGPT-6 Luna Pro
Updated Sep 2026

GPT-5.1 ChatvsGPT-6 Luna Pro

GPT-6 Luna Pro is cheaper than GPT-5.1 Chat at $0.1/M vs $1.25/M input tokens.

GPT-5.1 Chat and GPT-6 Luna Pro compared across 20 shared prompts
SpecGPT-5.1 ChatGPT-6 Luna Pro
Input price$1.25/M tokens$0.1/M tokens
Output price$10/M tokens$0.5/M tokens
Context window128K tokens1.1M tokens
Weights—Closed
Free API (OpenRouter)NoNo
ReleasedNov 2025Sep 2026
Side by side

Same prompt, both answers

Opening lines from 5 of the 20 prompts both answered. Formatting stripped, words untouched.

Advanced Investment Memo (IC Memo)

You are writing an internal investment committee memo for a long/short equity fund. Company (fictional): LedgerLift (ticker: LLLT) – B2B…

GPT-5.1 Chat905 words
1) Recommendation, PT Range, Two‑Sentence Thesis Recommendation: Long 12‑month PT range: $63–$78 Thesis (2 sentences): LedgerLift is a mid‑market spend‑management platform with best‑in‑class retention, strong NRR, and a durable subscription mix that supports compounding cash generation as operating leverage…
Read the full answer
GPT-6 Luna Pro888 words
LedgerLift (LLLT) — Investment Committee Memo Recommendation: Pass 12-month price target range: $28–44 per share LedgerLift has attractive customer economics, recurring revenue, and a credible path to margin expansion, but those strengths are already reflected in a $46 share price.
Read the full answer

Mini LBO Underwrite

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

GPT-5.1 Chat391 words
Below is a clean, concise underwriting output following your instructions. YEAR‑BY‑YEAR SUMMARY (FY26–FY30) All figures in $m. Term loan = 480 initial; mezzanine = 180 initial. Optional amortization applied.
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GPT-6 Luna Pro434 words
Operating and debt schedule Conventions: Interest is calculated on beginning-of-year debt; the 1% Term Loan amortization is 1% of its original $480m principal ($4.8m annually). Mezzanine PIK accrues at 2% annually.
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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…

GPT-5.1 Chat191 words
Here’s a simple, beginner‑friendly 1‑month plan focused on diet, exercise, and sleep. Diet • Week 1: Add one serving of vegetables to one meal each day. • Week 2: Swap one processed snack (chips, cookies) for fruit, nuts, or yogurt. • Week 3: Aim for one balanced plate per day (protein, vegetables, healthy carbs).
Read the full answer
GPT-6 Luna Pro327 words
A simple 1-month plan for better health and longevity Choose three foundations: eating well, moving regularly, and sleeping consistently. Aim for steady habits, not perfection. Diet: Make nourishing choices easier Week 1: Add one serving of fruit or vegetables to one meal each day.
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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-5.1 Chat111 words
Sweet Citrus Cloud Squares Ingredients (7 total) 1 cup all-purpose flour 1 cup sugar 1 cup plain yogurt 2 eggs 1 teaspoon baking powder Zest of 1 lemon 2 tablespoons melted butter Instructions Preheat oven to 350°F (175°C). Lightly grease an 8x8 baking dish.
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GPT-6 Luna Pro127 words
Golden Pantry Pancakes Fluffy, lightly sweet pancakes made with everyday staples. Makes: About 6 small pancakes Prep time: 5 minutes Cook time: 10 minutes Ingredients 1 cup all-purpose flour 1 cup milk 1 egg 2 tablespoons sugar 1 teaspoon baking powder ¼ teaspoon salt 2 tablespoons butter, melted (plus a little for the…
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Favorite Game

I know you're an AI and don't truly have personal preferences, but if you had to pick one video game as your all-time favorite, which would…

GPT-5.1 Chat8 words
The Legend of Zelda Breath of the Wild
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GPT-6 Luna Pro8 words
The Legend of Zelda: Breath of the Wild
Read the full answer
Our Verdict
GPT-6 Luna Pro
GPT-6 Luna Pro
GPT-5.1 Chat
GPT-5.1 ChatRunner-up

Not enough votes to call it. On the specs, GPT-6 Luna Pro has the edge: newer, bigger context window.

GPT-6 Luna Pro costs 20x less per token.

Too close to call

Reviewing agent-written code?See a Brief PR report

API pricing

Cost per 1M tokens

GPT-5.1 Chat
Input
$1.25
Output
$10.00
GPT-6 Luna Pro
Input
$0.10
13× cheaper
Output
$0.50
20× cheaper

GPT-6 Luna Pro is cheaper on both: 13× input, 20× output.

Where to run it

2 hosts

GPT-5.1 Chat

No hosts listed on OpenRouter.

GPT-6 Luna Pro2 hosts
HostInOutContextUptime
Azure AI Foundry$0.10 in·$0.50 out·1.1M·98.6% upOpenAI$0.10 in·$0.50 out·1.1M·99.9% up

Per million tokens. Prices and uptime via OpenRouter, checked 25 Sep 2026.

Research

What we learned reading every model

FAQ

Common questions

Both are developed by OpenAI but target different use cases. GPT-5.1 Chat has a 128K token context window vs GPT-6 Luna Pro's 1.1M. You can compare their actual outputs across 20 challenges on Rival to see how they differ in practice.

It depends on your use case. GPT-5.1 Chat and GPT-6 Luna Pro each have strengths in different areas. Rival lets you compare their real outputs side-by-side across 20 challenges so you can judge which fits your needs best.

GPT-5.1 Chat costs $1.25/M input tokens and GPT-6 Luna Pro costs $0.1/M input tokens. GPT-6 Luna Pro is $1.15/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 GPT-5.1 Chat and GPT-6 Luna Pro 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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Rival
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