Claude Opus 5.5 vs GPT-6 Luna vs GPT-6 Astra
GPT-6 Luna comes out ahead, 78 to 64 and 64 on our weighted score, and it is the cheaper option too.
AnthropicClaude Opus 5.5
64/100- ECI167.4
- Price$4.00 / $20.00
- Context1M
- Our pick
OpenAIGPT-6 Luna
78/100- ECI—
- Price$0.10 / $0.50
- Context1.05M
- OpenAI
GPT-6 Astra
64/100- ECI166.5
- Price$10.00 / $50.00
- Context1.05M
GPT-6 Luna is our pick
GPT-6 Luna is the better all-round choice, scoring 78/100 against Claude Opus 5.5 (64) and GPT-6 Astra (64). 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 (GPQA Diamond, 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 91.3% · Claude Opus 5.5 88.5% · GPT-6 Luna 77.4%
- Lowest priceGPT-6 LunaGPT-6 Luna $0.20 · Claude Opus 5.5 $8.00 · GPT-6 Astra $20.00 per 1M tokens (3:1 blend)
- Longest contextGPT-6 Luna and GPT-6 AstraGPT-6 Luna 1,050,000 · GPT-6 Astra 1,050,000 · Claude Opus 5.5 1,000,000 tokens
- Widest inputsSame inputsClaude Opus 5.5: Text, Images, PDFs · GPT-6 Luna: Text, Images, PDFs · GPT-6 Astra: Text, Images, PDFs
- Self-hostingNo open weightsAll three are available only through APIs
| Measure | Weight | Claude Opus 5.5 | GPT-6 Luna | GPT-6 Astra |
|---|---|---|---|---|
| CapabilityShared benchmarks | 50% | 89 | 77 | 91 |
| Price | 25% | 7 | 83 | 0 |
| Inputs & features | 15% | 80 | 80 | 80 |
| Context window | 10% | 60 | 61 | 61 |
| Overall | 100% | 64/100 | 78/100 | 64/100 |
Every spec in one table
Highlighted cells lead their row. Dashes mean the data is not published.
| Specification | Claude Opus 5.5Anthropic | GPT-6 LunaOpenAI | GPT-6 AstraOpenAI |
|---|---|---|---|
| Capability | |||
| Capabilities Index (ECI) | 167.4 (best) | — | 166.5 |
| ECI rank | #1 of 148 (best) | — | #2 of 148 |
| GPQA DiamondGraduate-level science questions | 90.6% | 90.5% | 95.8% (best) |
| FrontierMath Tiers 1–3Research-level mathematics | 91.2% | 79.0% | 93.7% (best) |
| OTIS Mock AIME 2024–2025Competition mathematics | 100% (best) | 98.9% | 100% (best) |
| SimpleQA VerifiedShort factual questions | 72.2% | 41.4% | 75.6% (best) |
| Price per million tokens | |||
| Input | $4.00 | $0.10 (best) | $10.00 |
| Output | $20.00 | $0.50 (best) | $50.00 |
| Cached input | $0.20 | $0.01 (best) | $1.00 |
| Blended (3:1) | $8.00 | $0.20 (best) | $20.00 |
| Long-context rate | Same rate | Over 272K: $0.20 / $0.75 | Over 272K: $20.00 / $75.00 |
| Price source | Official Anthropic API | Official OpenAI API | Official OpenAI API |
| Limits | |||
| Context window | 1,000,000 tokens | 1,050,000 tokens (best) | 1,050,000 tokens (best) |
| 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 | claude-opus-5-5 | gpt-6-luna | gpt-6-astra |
| API providers | 28 (best) | 24 | 26 |
| Released | Sep 22, 2026 | Sep 22, 2026 | Sep 4, 2026 |
| Knowledge cutoff | Jun 2026 | May 18, 2026 | Apr 30, 2026 |
What would a month cost?
Enter your expected volume in millions of tokens. List prices only; caching and batch discounts would lower these.
- Claude Opus 5.5$80.00
- GPT-6 Luna$2.00
- GPT-6 Astra$200.00
Which should you choose?
Which is better: Claude Opus 5.5, GPT-6 Luna or GPT-6 Astra?
GPT-6 Luna is the better all-round choice, scoring 78/100 against Claude Opus 5.5 (64) and GPT-6 Astra (64). 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 (GPQA Diamond, 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, Claude Opus 5.5, GPT-6 Luna or GPT-6 Astra?
GPT-6 Luna is cheaper at $0.10 input / $0.50 output per million tokens (official OpenAI API price). Claude Opus 5.5 costs $4.00 input / $20.00 output per million tokens (official Anthropic API price); GPT-6 Astra costs $10.00 input / $50.00 output per million tokens (official OpenAI 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 $8.00 for Claude Opus 5.5 (40× as much) and $20.00 for GPT-6 Astra (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 (GPQA Diamond, FrontierMath Tiers 1–3, OTIS Mock AIME 2024–2025 and SimpleQA Verified): GPT-6 Astra 91.3%, Claude Opus 5.5 88.5% and GPT-6 Luna 77.4%. On individual benchmarks: GPQA Diamond — GPT-6 Astra 95.8%, Claude Opus 5.5 90.6%, GPT-6 Luna 90.5%; FrontierMath Tiers 1–3 — GPT-6 Astra 93.7%, Claude Opus 5.5 91.2%, GPT-6 Luna 79.0%; OTIS Mock AIME 2024–2025 — Claude Opus 5.5 100%, GPT-6 Astra 100%, GPT-6 Luna 98.9%; SimpleQA Verified — GPT-6 Astra 75.6%, Claude Opus 5.5 72.2%, GPT-6 Luna 41.4%.
Which is better for coding?
There are no published SWE-bench Verified results for Claude Opus 5.5, GPT-6 Luna and GPT-6 Astra 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 Luna and GPT-6 Astra have the largest context windows (1,050,000 and 1,050,000 tokens), against 1,000,000 for Claude Opus 5.5. Maximum output per response: Claude Opus 5.5 up to 128,000, GPT-6 Luna up to 128,000, GPT-6 Astra up to 128,000 tokens.
Which can read images, PDFs, audio or video?
Claude Opus 5.5 accepts text, images and PDFs; GPT-6 Luna accepts text, images and PDFs; GPT-6 Astra accepts text, images and PDFs. They handle the same number of input types.
Are any of these open source?
No. Claude Opus 5.5, GPT-6 Luna and GPT-6 Astra are proprietary and only available through APIs and apps.
Which is newer?
Claude Opus 5.5 is the newest, released Sep 22, 2026. GPT-6 Luna came out Sep 22, 2026; GPT-6 Astra came out Sep 4, 2026. Knowledge cutoff: Claude Opus 5.5 Jun 2026, GPT-6 Luna May 18, 2026, GPT-6 Astra Apr 30, 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.
