GPT-6 Luna vs Gemini 3.8 Flash
Official OpenAI and Google API prices and LMArena scores, checked 2026-10-21. Add up to 2 more models to compare 4.
GPT-6 Luna costs less; Gemini 3.8 Flash wins more LMArena tasks.
GPT-6 Luna is 7.5x cheaper on input and output. Gemini 3.8 Flash scores 96 points higher for reasoning and math. Across 7 shared LMArena tasks, Gemini 3.8 Flash leads 7 to 0. A medium month (20M input + 5M output tokens) costs $4.50 on GPT-6 Luna and $33.75 on Gemini 3.8 Flash.
- Pick GPT-6 Luna if you want the lower bill ($4.50 vs $33.75 for a medium month) or you reuse long prompts (cached input $0.01 vs $0.075).
- Pick Gemini 3.8 Flash if you care most about reasoning and math (+96), vision (+80) and creative writing (+52).
Medium: 20M input + 5M output tokens a month at list price (short-context rate, no caching or batch discounts).
Marked side is better: lower for prices, higher for context and scores. Bars are scaled to the larger of the two.
| Spec | GPT-6 Luna | Gemini 3.8 Flash |
|---|---|---|
| Price per 1M tokens | ||
| Input | $0.10 | $0.75 |
| Cached input | $0.01 | $0.075 |
| Output | $0.50 | $3.75 |
| Blend 3:1 | $0.20 | $1.50 |
| Sample month, medium | $4.50 | $33.75 |
| Context | ||
| Context window | 1.05M tokens | 1M tokens |
| Quality (LMArena, out of 100) | ||
| Coding | 15/100 #16 of 23 | 18/100 #15 of 23 |
| Chat | 0/100 #23 of 25 | 51/100 #5 of 25 |
| Creative writing | 0/100 #23 of 25 | 52/100 #5 of 25 |
| Roleplay and characters | 0/100 #22 of 25 | 38/100 #3 of 25 |
| Reasoning and math | 0/100 #20 of 25 | 96/100 #3 of 25 |
| Vision | 0/100 #21 of 22 | 80/100 #4 of 22 |
| Long documents | Not ranked | Not ranked |
| Cheap bulk and agents | 46/100 #15 of 23 | 48/100 #11 of 23 |
| Details | ||
| Provider | OpenAI | |
| API model id | gpt-6-luna | gemini-3.8-flash |
| Notes | Prompts up to 272K. | Through Dec 31, 2026. From Jan 1, 2027: $1.50 in, $7.50 out. |
| Sources | ||
| Last checked | 2026-10-21 | 2026-10-21 |
| Official pricing | OpenAI pricing | Google pricing |
Plans from data checked hourly against each vendor's official pricing page. Vendors state limits in different units, so we quote them as published. Team and enterprise plans are per seat.
API prices in USD per 1M tokens from each provider's official pricing page. Blend 3:1 = (3 x input + output) / 4. Sample month = stated input and output tokens at the list price, no caching. Quality: relative score out of 100 on each LMArena leaderboard (100 = top model we price on that board, n/a = not ranked there), data licensed CC BY 4.0. See how we pick.
Where each one ranks on LMArena
Relative score out of 100 on each leaderboard (100 = the top model we price there), with the rank among the models from our API price table that appear on that board.
- Coding (Code Arena): GPT-6 Luna #16 of 23 (15/100), Gemini 3.8 Flash #15 of 23 (18/100)
- Chat (Text Arena): GPT-6 Luna #23 of 25 (0/100), Gemini 3.8 Flash #5 of 25 (51/100)
- Creative writing (Text Arena creative writing): GPT-6 Luna #23 of 25 (0/100), Gemini 3.8 Flash #5 of 25 (52/100)
- Roleplay and characters (Text Arena multi-turn): GPT-6 Luna #22 of 25 (0/100), Gemini 3.8 Flash #3 of 25 (38/100)
- Reasoning and math (Text Arena math): GPT-6 Luna #20 of 25 (0/100), Gemini 3.8 Flash #3 of 25 (96/100)
- Vision (Vision Arena): GPT-6 Luna #21 of 22 (0/100), Gemini 3.8 Flash #4 of 22 (80/100)
- Long documents (Document Arena): GPT-6 Luna not ranked, Gemini 3.8 Flash not ranked
- Cheap bulk and agents (Agent Arena): GPT-6 Luna #15 of 23 (46/100), Gemini 3.8 Flash #11 of 23 (48/100)
Pricing details from the official pages
GPT-6 Luna lists $0.10 input, $0.50 output, $0.01 cached input per 1M tokens on OpenAI pricing. Gemini 3.8 Flash lists $0.75 input, $3.75 output, $0.075 cached input per 1M tokens on Google pricing. Prices checked 2026-10-21.
- GPT-6 Luna: Prompts up to 272K.
- Gemini 3.8 Flash: Through Dec 31, 2026. From Jan 1, 2027: $1.50 in, $7.50 out.
Sample monthly bill at list price, no caching: Light (2M input + 0.5M output tokens): $0.45 vs $3.38. Medium (20M input + 5M output tokens): $4.50 vs $33.75. Heavy (200M input + 50M output tokens): $45 vs $337.50.
FAQ
Is GPT-6 Luna cheaper than Gemini 3.8 Flash?
GPT-6 Luna costs $0.10 input and $0.50 output per 1M tokens; Gemini 3.8 Flash costs $0.75 and $3.75. For a medium month (20M input + 5M output tokens) GPT-6 Luna is cheaper: $4.50 vs $33.75.
Which is better for coding, GPT-6 Luna or Gemini 3.8 Flash?
On LMArena's Code Arena GPT-6 Luna scores 15/100 (#16 of 23) and Gemini 3.8 Flash 18/100 (#15 of 23), so Gemini 3.8 Flash is ahead by 3 points.
Which has the bigger context window?
GPT-6 Luna: 1.05M tokens. Gemini 3.8 Flash: 1M tokens.
How much do GPT-6 Luna and Gemini 3.8 Flash cost per month?
Light (2M input + 0.5M output tokens): $0.45 vs $3.38. Medium (20M input + 5M output tokens): $4.50 vs $33.75. Heavy (200M input + 50M output tokens): $45 vs $337.50. Pick your own token counts in the plan vs API calculator.
Where do these numbers come from?
Prices come from each provider's official pricing page, checked daily. Quality is the relative score on the LMArena leaderboard dataset, licensed CC BY 4.0. Where a model is not ranked we show n/a.