GPT-6 Astra vs Claude Sonnet 5.5 vs Claude Fable 5.1
Claude Sonnet 5.5 comes out ahead, 72 to 68 and 66 on our weighted score, and it is the cheaper option too.
OpenAIGPT-6 Astra
68/100- ECI166.5
- Price$10.00 / $50.00
- Context1.05M
- Our pick
AnthropicClaude Sonnet 5.5
72/100- ECI165.2
- Price$2.00 / $10.00
- Context1M
- Anthropic
Claude Fable 5.1
66/100- ECI164.8
- Price$10.00 / $50.00
- Context1M
Claude Sonnet 5.5 is our pick
Claude Sonnet 5.5 is the better all-round choice, scoring 72/100 against GPT-6 Astra (68) and Claude Fable 5.1 (66). It leads on price. GPT-6 Astra wins on capability. The score weighs capability 50%, price 25%, inputs & features 15%, context window 10%.
- CapabilityGPT-6 AstraCapabilities Index (ECI): GPT-6 Astra 166.5 · Claude Sonnet 5.5 165.2 · Claude Fable 5.1 164.8
- Lowest priceClaude Sonnet 5.5Claude Sonnet 5.5 $4.00 · GPT-6 Astra $20.00 · Claude Fable 5.1 $20.00 per 1M tokens (3:1 blend)
- Longest contextGPT-6 AstraGPT-6 Astra 1,050,000 · Claude Sonnet 5.5 1,000,000 · Claude Fable 5.1 1,000,000 tokens
- Widest inputsSame inputsGPT-6 Astra: Text, Images, PDFs · Claude Sonnet 5.5: 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 | Claude Sonnet 5.5 | Claude Fable 5.1 |
|---|---|---|---|---|
| CapabilityCapabilities Index (ECI) | 50% | 99 | 97 | 97 |
| Price | 25% | 0 | 21 | 0 |
| Inputs & features | 15% | 80 | 80 | 80 |
| Context window | 10% | 61 | 60 | 60 |
| Overall | 100% | 68/100 | 72/100 | 66/100 |
Every spec in one table
Highlighted cells lead their row. Dashes mean the data is not published.
| Specification | GPT-6 AstraOpenAI | Claude Sonnet 5.5Anthropic | Claude Fable 5.1Anthropic |
|---|---|---|---|
| Capability | |||
| Capabilities Index (ECI) | 166.5 (best) | 165.2 | 164.8 |
| ECI rank | #2 of 148 (best) | #3 of 148 | #4 of 148 |
| GPQA DiamondGraduate-level science questions | 95.8% (best) | 95.6% | — |
| FrontierMath Tiers 1–3Research-level mathematics | 93.7% (best) | 88.8% | 90.2% |
| OTIS Mock AIME 2024–2025Competition mathematics | 100% | 100% | 100% |
| SimpleQA VerifiedShort factual questions | 75.6% (best) | 46.5% | 70.8% |
| Price per million tokens | |||
| Input | $10.00 | $2.00 (best) | $10.00 |
| Output | $50.00 | $10.00 (best) | $50.00 |
| Cached input | $1.00 | $0.20 (best) | $0.25 |
| Blended (3:1) | $20.00 | $4.00 (best) | $20.00 |
| Long-context rate | Over 272K: $20.00 / $75.00 | Same rate | Same rate |
| Price source | Official OpenAI API | Official Anthropic API | Official Anthropic API |
| Limits | |||
| Context window | 1,050,000 tokens (best) | 1,000,000 tokens | 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 | claude-sonnet-5-5 | claude-fable-5-1 |
| API providers | 26 | 23 | 29 (best) |
| Released | Sep 4, 2026 | Sep 28, 2026 | Sep 1, 2026 |
| Knowledge cutoff | Apr 30, 2026 | Jun 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
- Claude Sonnet 5.5$40.00
- Claude Fable 5.1$200.00
Which should you choose?
Which is better: GPT-6 Astra, Claude Sonnet 5.5 or Claude Fable 5.1?
Claude Sonnet 5.5 is the better all-round choice, scoring 72/100 against GPT-6 Astra (68) and Claude Fable 5.1 (66). It leads on price. GPT-6 Astra wins on capability. The score weighs capability 50%, price 25%, inputs & features 15%, context window 10%.
Which is cheaper, GPT-6 Astra, Claude Sonnet 5.5 or Claude Fable 5.1?
Claude Sonnet 5.5 is cheaper at $2.00 input / $10.00 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); 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 $4.00 per million tokens for Claude Sonnet 5.5 versus $20.00 for GPT-6 Astra (5× as much) and $20.00 for Claude Fable 5.1 (5× as much).
Which scores higher on benchmarks?
GPT-6 Astra scores higher on the Capabilities Index (ECI): GPT-6 Astra 166.5 (#2 of 148), Claude Sonnet 5.5 165.2 (#3 of 148) and Claude Fable 5.1 164.8 (#4 of 148). The confidence ranges of the top two overlap (163.1–171.1 vs 161.8–169.4), so treat the gap as small. On individual benchmarks: FrontierMath Tiers 1–3 — GPT-6 Astra 93.7%, Claude Fable 5.1 90.2%, Claude Sonnet 5.5 88.8%; OTIS Mock AIME 2024–2025 — GPT-6 Astra 100%, Claude Sonnet 5.5 100%, Claude Fable 5.1 100%; SimpleQA Verified — GPT-6 Astra 75.6%, Claude Fable 5.1 70.8%, Claude Sonnet 5.5 46.5%.
Which is better for coding?
There are no published SWE-bench Verified results for GPT-6 Astra, Claude Sonnet 5.5 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 has the largest context window at 1,050,000 tokens, against 1,000,000 for Claude Sonnet 5.5 and 1,000,000 for Claude Fable 5.1. Maximum output per response: GPT-6 Astra up to 128,000, Claude Sonnet 5.5 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; Claude Sonnet 5.5 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, Claude Sonnet 5.5 and Claude Fable 5.1 are proprietary and only available through APIs and apps.
Which is newer?
Claude Sonnet 5.5 is the newest, released Sep 28, 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, Claude Sonnet 5.5 Jun 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.
