GPT-6 Astra vs GPT-6 Luna vs Claude Fable 5.1
GPT-6 Luna comes out ahead, 75 to 63 and 61 on our weighted score, and it is the cheaper option too.
OpenAIGPT-6 Astra
63/100- ECI166.5
- Price$10.00 / $50.00
- Context1.05M
- Our pickOpenAI
GPT-6 Luna
75/100- ECI—
- Price$0.10 / $0.50
- Context1.05M
AnthropicClaude Fable 5.1
61/100- ECI164.8
- Price$10.00 / $50.00
- Context1M
GPT-6 Luna is our pick
GPT-6 Luna is the better all-round choice, scoring 75/100 against GPT-6 Astra (63) and Claude Fable 5.1 (61). It leads on price. GPT-6 Astra wins on capability. The score weighs capability 50%, price 25%, inputs & features 15%, context window 10%. Capability uses the average of shared benchmarks (FrontierMath Tiers 1–3, OTIS Mock AIME 2024–2025 and SimpleQA Verified), because GPT-6 Luna has no Capabilities Index score yet.
- CapabilityGPT-6 AstraShared benchmarks: GPT-6 Astra 89.8% · Claude Fable 5.1 87.0% · GPT-6 Luna 73.1%
- Lowest priceGPT-6 LunaGPT-6 Luna $0.20 · GPT-6 Astra $20.00 · Claude Fable 5.1 $20.00 per 1M tokens (3:1 blend)
- Longest contextGPT-6 Astra and GPT-6 LunaGPT-6 Astra 1,050,000 · GPT-6 Luna 1,050,000 · Claude Fable 5.1 1,000,000 tokens
- Widest inputsSame inputsGPT-6 Astra: Text, Images, PDFs · GPT-6 Luna: Text, Images, PDFs · Claude Fable 5.1: Text, Images, PDFs
- Self-hostingNo open weightsAll three are available only through APIs
| Measure | Weight | GPT-6 Astra | GPT-6 Luna | Claude Fable 5.1 |
|---|---|---|---|---|
| CapabilityShared benchmarks | 50% | 90 | 73 | 87 |
| Price | 25% | 0 | 83 | 0 |
| Inputs & features | 15% | 80 | 80 | 80 |
| Context window | 10% | 61 | 61 | 60 |
| Overall | 100% | 63/100 | 75/100 | 61/100 |
Every spec in one table
Highlighted cells lead their row. Dashes mean the data is not published.
| Specification | GPT-6 AstraOpenAI | GPT-6 LunaOpenAI | Claude Fable 5.1Anthropic |
|---|---|---|---|
| Capability | |||
| Capabilities Index (ECI) | 166.5 (best) | — | 164.8 |
| ECI rank | #2 of 148 (best) | — | #4 of 148 |
| GPQA DiamondGraduate-level science questions | 95.8% (best) | 90.5% | — |
| FrontierMath Tiers 1–3Research-level mathematics | 93.7% (best) | 79.0% | 90.2% |
| OTIS Mock AIME 2024–2025Competition mathematics | 100% (best) | 98.9% | 100% (best) |
| SimpleQA VerifiedShort factual questions | 75.6% (best) | 41.4% | 70.8% |
| Price per million tokens | |||
| Input | $10.00 | $0.10 (best) | $10.00 |
| Output | $50.00 | $0.50 (best) | $50.00 |
| Cached input | $1.00 | $0.01 (best) | $0.25 |
| Blended (3:1) | $20.00 | $0.20 (best) | $20.00 |
| Long-context rate | Over 272K: $20.00 / $75.00 | Over 272K: $0.20 / $0.75 | Same rate |
| Price source | Official OpenAI API | Official OpenAI API | Official Anthropic API |
| Limits | |||
| Context window | 1,050,000 tokens (best) | 1,050,000 tokens (best) | 1,000,000 tokens |
| Max output | 128,000 tokens | 128,000 tokens | 128,000 tokens |
| Inputs and features | |||
| Text | Yes | Yes | Yes |
| Images | Yes | Yes | Yes |
| PDFs | Yes | Yes | Yes |
| Audio | No | No | No |
| Video | No | No | No |
| Reasoning | Yeslow · medium · high · xhigh · max | Yeslow · medium · high · xhigh · max | Yeslow · medium · high · xhigh · max |
| Tool calling | Yes | Yes | Yes |
| Structured output | Yes | Yes | Yes |
| Availability | |||
| Weights | Proprietary | Proprietary | Proprietary |
| API model ID | gpt-6-astra | gpt-6-luna | claude-fable-5-1 |
| API providers | 26 | 24 | 29 (best) |
| Released | Sep 4, 2026 | Sep 22, 2026 | Sep 1, 2026 |
| Knowledge cutoff | Apr 30, 2026 | May 18, 2026 | Jun 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 Astra$200.00
- GPT-6 Luna$2.00
- Claude Fable 5.1$200.00
Which should you choose?
Which is better: GPT-6 Astra, GPT-6 Luna or Claude Fable 5.1?
GPT-6 Luna is the better all-round choice, scoring 75/100 against GPT-6 Astra (63) and Claude Fable 5.1 (61). It leads on price. GPT-6 Astra wins on capability. The score weighs capability 50%, price 25%, inputs & features 15%, context window 10%. Capability uses the average of shared benchmarks (FrontierMath Tiers 1–3, OTIS Mock AIME 2024–2025 and SimpleQA Verified), because GPT-6 Luna has no Capabilities Index score yet.
Which is cheaper, GPT-6 Astra, GPT-6 Luna or Claude Fable 5.1?
GPT-6 Luna is cheaper at $0.10 input / $0.50 output per million tokens (official OpenAI API price). GPT-6 Astra costs $10.00 input / $50.00 output per million tokens (official OpenAI API price); Claude Fable 5.1 costs $10.00 input / $50.00 output per million tokens (official Anthropic 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 $20.00 for GPT-6 Astra (100× as much) and $20.00 for Claude Fable 5.1 (100× as much).
Which scores higher on benchmarks?
Not every model here has a Capabilities Index score, so we compare the average of shared benchmarks (FrontierMath Tiers 1–3, OTIS Mock AIME 2024–2025 and SimpleQA Verified): GPT-6 Astra 89.8%, Claude Fable 5.1 87.0% and GPT-6 Luna 73.1%. On individual benchmarks: FrontierMath Tiers 1–3 — GPT-6 Astra 93.7%, Claude Fable 5.1 90.2%, GPT-6 Luna 79.0%; OTIS Mock AIME 2024–2025 — GPT-6 Astra 100%, Claude Fable 5.1 100%, GPT-6 Luna 98.9%; SimpleQA Verified — GPT-6 Astra 75.6%, Claude Fable 5.1 70.8%, GPT-6 Luna 41.4%.
Which is better for coding?
There are no published SWE-bench Verified results for GPT-6 Astra, GPT-6 Luna and Claude Fable 5.1 yet, so there is no like-for-like coding score. On overall capability, GPT-6 Astra 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 Astra and GPT-6 Luna have the largest context windows (1,050,000 and 1,050,000 tokens), against 1,000,000 for Claude Fable 5.1. Maximum output per response: GPT-6 Astra up to 128,000, GPT-6 Luna up to 128,000, Claude Fable 5.1 up to 128,000 tokens.
Which can read images, PDFs, audio or video?
GPT-6 Astra accepts text, images and PDFs; GPT-6 Luna accepts text, images and PDFs; Claude Fable 5.1 accepts text, images and PDFs. They handle the same number of input types.
Are any of these open source?
No. GPT-6 Astra, GPT-6 Luna and Claude Fable 5.1 are proprietary and only available through APIs and apps.
Which is newer?
GPT-6 Luna is the newest, released Sep 22, 2026. GPT-6 Astra came out Sep 4, 2026; Claude Fable 5.1 came out Sep 1, 2026. Knowledge cutoff: GPT-6 Astra Apr 30, 2026, GPT-6 Luna May 18, 2026, Claude Fable 5.1 Jun 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.
