GPT-6 Luna vs Gemini 3.8 Flash vs GLM-5.3-Flash
GPT-6 Luna comes out ahead, 84 to 79 and 75 on our weighted score, and it is the cheaper option too.
- Our pick
OpenAIGPT-6 Luna
84/100- ECI—
- Price$0.10 / $0.50
- Context1.05M
GoogleGemini 3.8 Flash
75/100- ECI156.9
- Price$0.75 / $3.75
- Context1.05M
Z.ai (Zhipu)GLM-5.3-Flash
79/100- ECI151.9
- Price$0.15 / $0.50
- Context1M
GPT-6 Luna is our pick
GPT-6 Luna is the better all-round choice, scoring 84/100 against GLM-5.3-Flash (79) and Gemini 3.8 Flash (75). It leads on capability and price. Gemini 3.8 Flash wins on inputs & features. The score weighs capability 50%, price 25%, inputs & features 15%, context window 10%. Capability uses the average of shared benchmarks (GPQA Diamond, FrontierMath Tiers 1–3 and OTIS Mock AIME 2024–2025), because GPT-6 Luna has no Capabilities Index score yet.
- CapabilityGPT-6 LunaShared benchmarks: GPT-6 Luna 89.4% · Gemini 3.8 Flash 87.6% · GLM-5.3-Flash 79.9%
- Lowest priceGPT-6 LunaGPT-6 Luna $0.20 · GLM-5.3-Flash $0.237 · Gemini 3.8 Flash $1.50 per 1M tokens (3:1 blend)
- Longest contextGPT-6 Luna and Gemini 3.8 FlashGPT-6 Luna 1,050,000 · Gemini 3.8 Flash 1,048,576 · GLM-5.3-Flash 1,000,000 tokens
- Widest inputsGemini 3.8 FlashGPT-6 Luna: Text, Images, PDFs · Gemini 3.8 Flash: Text, Images, PDFs, Audio, Video · GLM-5.3-Flash: Text, Images, PDFs, Video
- Self-hostingGLM-5.3-FlashPublishes downloadable weights
| Measure | Weight | GPT-6 Luna | Gemini 3.8 Flash | GLM-5.3-Flash |
|---|---|---|---|---|
| CapabilityShared benchmarks | 50% | 89 | 88 | 80 |
| Price | 25% | 83 | 42 | 79 |
| Inputs & features | 15% | 80 | 100 | 90 |
| Context window | 10% | 61 | 61 | 60 |
| Overall | 100% | 84/100 | 75/100 | 79/100 |
Every spec in one table
Highlighted cells lead their row. Dashes mean the data is not published.
| Specification | GPT-6 LunaOpenAI | Gemini 3.8 FlashGoogle | GLM-5.3-FlashZ.ai (Zhipu) |
|---|---|---|---|
| Capability | |||
| Capabilities Index (ECI) | — | 156.9 (best) | 151.9 |
| ECI rank | — | #15 of 148 (best) | #42 of 148 |
| GPQA DiamondGraduate-level science questions | 90.5% | 95.4% (best) | 90.2% |
| FrontierMath Tiers 1–3Research-level mathematics | 79.0% (best) | 68.4% | 55.8% |
| OTIS Mock AIME 2024–2025Competition mathematics | 98.9% (best) | 98.9% (best) | 93.9% |
| SimpleQA VerifiedShort factual questions | 41.4% | 69.7% (best) | — |
| Price per million tokens | |||
| Input | $0.10 (best) | $0.75 | $0.15 |
| Output | $0.50 (best) | $3.75 | $0.50 (best) |
| Cached input | $0.01 (best) | $0.075 | $0.03 |
| Blended (3:1) | $0.20 (best) | $1.50 | $0.237 |
| Long-context rate | Over 272K: $0.20 / $0.75 | Same rate | Same rate |
| Price source | Official OpenAI API | Official Google API | Official Z.AI API |
| Limits | |||
| Context window | 1,050,000 tokens (best) | 1,048,576 tokens | 1,000,000 tokens |
| Max output | 128,000 tokens | 65,536 tokens | 131,072 tokens (best) |
| Inputs and features | |||
| Text | Yes | Yes | Yes |
| Images | Yes | Yes | Yes |
| PDFs | Yes | Yes | Yes |
| Audio | No | Yes | No |
| Video | No | Yes | Yes |
| Reasoning | Yeslow · medium · high · xhigh · max | Yeslow · medium · high | Yeslow · high · max |
| Tool calling | Yes | Yes | Yes |
| Structured output | Yes | Yes | Yes |
| Availability | |||
| Weights | Proprietary | Proprietary | Open |
| API model ID | gpt-6-luna | gemini-3.8-flash | glm-5.3-flash |
| API providers | 24 | 21 | 65 (best) |
| Released | Sep 22, 2026 | Sep 2, 2026 | Aug 26, 2026 |
| Knowledge cutoff | May 18, 2026 | — | — |
What would a month cost?
Enter your expected volume in millions of tokens. List prices only; caching and batch discounts would lower these.
- GPT-6 Luna$2.00
- Gemini 3.8 Flash$15.00
- GLM-5.3-Flash$2.50
Which should you choose?
Which is better: GPT-6 Luna, Gemini 3.8 Flash or GLM-5.3-Flash?
GPT-6 Luna is the better all-round choice, scoring 84/100 against GLM-5.3-Flash (79) and Gemini 3.8 Flash (75). It leads on capability and price. Gemini 3.8 Flash wins on inputs & features. The score weighs capability 50%, price 25%, inputs & features 15%, context window 10%. Capability uses the average of shared benchmarks (GPQA Diamond, FrontierMath Tiers 1–3 and OTIS Mock AIME 2024–2025), because GPT-6 Luna has no Capabilities Index score yet.
Which is cheaper, GPT-6 Luna, Gemini 3.8 Flash or GLM-5.3-Flash?
GPT-6 Luna is cheaper at $0.10 input / $0.50 output per million tokens (official OpenAI API price). GLM-5.3-Flash costs $0.15 input / $0.50 output per million tokens (official Z.AI API price); Gemini 3.8 Flash costs $0.75 input / $3.75 output per million tokens (official Google API price). At a typical mix of three input tokens to one output token, that is $0.20 per million tokens for GPT-6 Luna versus $0.237 for GLM-5.3-Flash (1.2× as much) and $1.50 for Gemini 3.8 Flash (7.5× as much).
Which scores higher on benchmarks?
Not every model here has a Capabilities Index score, so we compare the average of shared benchmarks (GPQA Diamond, FrontierMath Tiers 1–3 and OTIS Mock AIME 2024–2025): GPT-6 Luna 89.4%, Gemini 3.8 Flash 87.6% and GLM-5.3-Flash 79.9%. On individual benchmarks: GPQA Diamond — Gemini 3.8 Flash 95.4%, GPT-6 Luna 90.5%, GLM-5.3-Flash 90.2%; FrontierMath Tiers 1–3 — GPT-6 Luna 79.0%, Gemini 3.8 Flash 68.4%, GLM-5.3-Flash 55.8%; OTIS Mock AIME 2024–2025 — GPT-6 Luna 98.9%, Gemini 3.8 Flash 98.9%, GLM-5.3-Flash 93.9%.
Which is better for coding?
There are no published SWE-bench Verified results for GPT-6 Luna, Gemini 3.8 Flash and GLM-5.3-Flash yet, so there is no like-for-like coding score. On overall capability, GPT-6 Luna leads, which tends to carry over to coding, but test on your own codebase. All three support tool calling for agent workflows.
Which has the bigger context window?
GPT-6 Luna and Gemini 3.8 Flash have the largest context windows (1,050,000 and 1,048,576 tokens), against 1,000,000 for GLM-5.3-Flash. Maximum output per response: GPT-6 Luna up to 128,000, Gemini 3.8 Flash up to 65,536, GLM-5.3-Flash up to 131,072 tokens.
Which can read images, PDFs, audio or video?
GPT-6 Luna accepts text, images and PDFs; Gemini 3.8 Flash accepts text, images, PDFs, audio and video; GLM-5.3-Flash accepts text, images, PDFs and video. Gemini 3.8 Flash handles the widest range of inputs.
Are any of these open source?
GLM-5.3-Flash publishes its weights and can be self-hosted; GPT-6 Luna and Gemini 3.8 Flash is proprietary.
Which is newer?
GPT-6 Luna is the newest, released Sep 22, 2026. Gemini 3.8 Flash came out Sep 2, 2026; GLM-5.3-Flash came out Aug 26, 2026. Knowledge cutoff: GPT-6 Luna May 18, 2026.
How do you decide the winner?
Each model gets a 0–100 score on capability (50%, independent benchmark results); price (25%, blended price per million tokens (3 input : 1 output), log scale); inputs & features (15%, image, PDF, audio and video input, tool calling, structured output and reasoning); context window (10%, maximum tokens per request, log scale). Dimensions missing for any model are dropped and the remaining weights rescaled, so every model is judged on the same evidence. Specs and prices come from public model listings and the labs’ own API pages; capability scores come from independent benchmark runs. Data updated Oct 4, 2026.
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