Claude Fable 5.1 vs GPT-6 Astra vs Claude Sonnet 5.5
Claude Sonnet 5.5 comes out ahead, 72 to 68 and 66 on our weighted score, and it is the cheaper option too.
AnthropicClaude Fable 5.1
66/100- ECI164.8
- Price$10.00 / $50.00
- Context1M
OpenAIGPT-6 Astra
68/100- ECI166.5
- Price$10.00 / $50.00
- Context1.05M
- Our pickAnthropic
Claude Sonnet 5.5
72/100- ECI165.2
- Price$2.00 / $10.00
- 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 · Claude Fable 5.1 $20.00 · GPT-6 Astra $20.00 per 1M tokens (3:1 blend)
- Longest contextGPT-6 AstraGPT-6 Astra 1,050,000 · Claude Fable 5.1 1,000,000 · Claude Sonnet 5.5 1,000,000 tokens
- Widest inputsSame inputsClaude Fable 5.1: Text, Images, PDFs · GPT-6 Astra: Text, Images, PDFs · Claude Sonnet 5.5: Text, Images, PDFs
- Self-hostingNo open weightsAll three are available only through APIs
| Measure | Weight | Claude Fable 5.1 | GPT-6 Astra | Claude Sonnet 5.5 |
|---|---|---|---|---|
| CapabilityCapabilities Index (ECI) | 50% | 97 | 99 | 97 |
| Price | 25% | 0 | 0 | 21 |
| Inputs & features | 15% | 80 | 80 | 80 |
| Context window | 10% | 60 | 61 | 60 |
| Overall | 100% | 66/100 | 68/100 | 72/100 |
Every spec in one table
Highlighted cells lead their row. Dashes mean the data is not published.
| Specification | Claude Fable 5.1Anthropic | GPT-6 AstraOpenAI | Claude Sonnet 5.5Anthropic |
|---|---|---|---|
| Capability | |||
| Capabilities Index (ECI) | 164.8 | 166.5 (best) | 165.2 |
| ECI rank | #4 of 148 | #2 of 148 (best) | #3 of 148 |
| GPQA DiamondGraduate-level science questions | — | 95.8% (best) | 95.6% |
| FrontierMath Tiers 1–3Research-level mathematics | 90.2% | 93.7% (best) | 88.8% |
| OTIS Mock AIME 2024–2025Competition mathematics | 100% | 100% | 100% |
| SimpleQA VerifiedShort factual questions | 70.8% | 75.6% (best) | 46.5% |
| Price per million tokens | |||
| Input | $10.00 | $10.00 | $2.00 (best) |
| Output | $50.00 | $50.00 | $10.00 (best) |
| Cached input | $0.25 | $1.00 | $0.20 (best) |
| Blended (3:1) | $20.00 | $20.00 | $4.00 (best) |
| Long-context rate | Same rate | Over 272K: $20.00 / $75.00 | Same rate |
| Price source | Official Anthropic API | Official OpenAI API | Official Anthropic API |
| Limits | |||
| Context window | 1,000,000 tokens | 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 | claude-fable-5-1 | gpt-6-astra | claude-sonnet-5-5 |
| API providers | 29 (best) | 26 | 23 |
| Released | Sep 1, 2026 | Sep 4, 2026 | Sep 28, 2026 |
| Knowledge cutoff | Jun 2026 | Apr 30, 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.
- Claude Fable 5.1$200.00
- GPT-6 Astra$200.00
- Claude Sonnet 5.5$40.00
Which should you choose?
Which is better: Claude Fable 5.1, GPT-6 Astra or Claude Sonnet 5.5?
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, Claude Fable 5.1, GPT-6 Astra or Claude Sonnet 5.5?
Claude Sonnet 5.5 is cheaper at $2.00 input / $10.00 output per million tokens (official Anthropic API price). Claude Fable 5.1 costs $10.00 input / $50.00 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 $4.00 per million tokens for Claude Sonnet 5.5 versus $20.00 for Claude Fable 5.1 (5× as much) and $20.00 for GPT-6 Astra (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 — Claude Fable 5.1 100%, GPT-6 Astra 100%, Claude Sonnet 5.5 100%; 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 Claude Fable 5.1, GPT-6 Astra and Claude Sonnet 5.5 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 Fable 5.1 and 1,000,000 for Claude Sonnet 5.5. Maximum output per response: Claude Fable 5.1 up to 128,000, GPT-6 Astra up to 128,000, Claude Sonnet 5.5 up to 128,000 tokens.
Which can read images, PDFs, audio or video?
Claude Fable 5.1 accepts text, images and PDFs; GPT-6 Astra accepts text, images and PDFs; Claude Sonnet 5.5 accepts text, images and PDFs. They handle the same number of input types.
Are any of these open source?
No. Claude Fable 5.1, GPT-6 Astra and Claude Sonnet 5.5 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: Claude Fable 5.1 Jun 2026, GPT-6 Astra Apr 30, 2026, Claude Sonnet 5.5 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.
