Claude Opus 5.5 vs Claude Fable 5.1 vs GPT-6 Astra
Too close to call on our weighted score (Claude Opus 5.5 70, GPT-6 Astra 68, Claude Fable 5.1 66). The right pick depends on what you value most.
AnthropicClaude Opus 5.5
70/100- ECI167.4
- Price$4.00 / $20.00
- Context1M
- Anthropic
Claude 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
Too close to call
It is close. Our weighted score puts them within 2 points (Claude Opus 5.5 70/100, GPT-6 Astra 68/100, Claude Fable 5.1 66/100), so choose by what matters most for your work: Claude Opus 5.5 for raw capability and GPT-6 Astra for long inputs. The score weighs capability 50%, price 25%, inputs & features 15%, context window 10%.
- CapabilityClaude Opus 5.5Capabilities Index (ECI): Claude Opus 5.5 167.4 · GPT-6 Astra 166.5 · Claude Fable 5.1 164.8
- Lowest priceClaude Opus 5.5Claude Opus 5.5 $8.00 · 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 Opus 5.5 1,000,000 · Claude Fable 5.1 1,000,000 tokens
- Widest inputsSame inputsClaude Opus 5.5: Text, Images, PDFs · Claude Fable 5.1: 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 | Claude Fable 5.1 | GPT-6 Astra |
|---|---|---|---|---|
| CapabilityCapabilities Index (ECI) | 50% | 100 | 97 | 99 |
| Price | 25% | 7 | 0 | 0 |
| Inputs & features | 15% | 80 | 80 | 80 |
| Context window | 10% | 60 | 60 | 61 |
| Overall | 100% | 70/100 | 66/100 | 68/100 |
Every spec in one table
Highlighted cells lead their row. Dashes mean the data is not published.
| Specification | Claude Opus 5.5Anthropic | Claude Fable 5.1Anthropic | GPT-6 AstraOpenAI |
|---|---|---|---|
| Capability | |||
| Capabilities Index (ECI) | 167.4 (best) | 164.8 | 166.5 |
| ECI rank | #1 of 148 (best) | #4 of 148 | #2 of 148 |
| GPQA DiamondGraduate-level science questions | 90.6% | — | 95.8% (best) |
| FrontierMath Tiers 1–3Research-level mathematics | 91.2% | 90.2% | 93.7% (best) |
| OTIS Mock AIME 2024–2025Competition mathematics | 100% | 100% | 100% |
| SimpleQA VerifiedShort factual questions | 72.2% | 70.8% | 75.6% (best) |
| Price per million tokens | |||
| Input | $4.00 (best) | $10.00 | $10.00 |
| Output | $20.00 (best) | $50.00 | $50.00 |
| Cached input | $0.20 (best) | $0.25 | $1.00 |
| Blended (3:1) | $8.00 (best) | $20.00 | $20.00 |
| Long-context rate | Same rate | Same rate | Over 272K: $20.00 / $75.00 |
| Price source | Official Anthropic API | Official Anthropic API | Official OpenAI API |
| Limits | |||
| Context window | 1,000,000 tokens | 1,000,000 tokens | 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 | claude-fable-5-1 | gpt-6-astra |
| API providers | 28 | 29 (best) | 26 |
| Released | Sep 22, 2026 | Sep 1, 2026 | Sep 4, 2026 |
| Knowledge cutoff | Jun 2026 | Jun 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
- Claude Fable 5.1$200.00
- GPT-6 Astra$200.00
Which should you choose?
Which is better: Claude Opus 5.5, Claude Fable 5.1 or GPT-6 Astra?
It is close. Our weighted score puts them within 2 points (Claude Opus 5.5 70/100, GPT-6 Astra 68/100, Claude Fable 5.1 66/100), so choose by what matters most for your work: Claude Opus 5.5 for raw capability and GPT-6 Astra for long inputs. The score weighs capability 50%, price 25%, inputs & features 15%, context window 10%.
Which is cheaper, Claude Opus 5.5, Claude Fable 5.1 or GPT-6 Astra?
Claude Opus 5.5 is cheaper at $4.00 input / $20.00 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 $8.00 per million tokens for Claude Opus 5.5 versus $20.00 for Claude Fable 5.1 (2.5× as much) and $20.00 for GPT-6 Astra (2.5× as much).
Which scores higher on benchmarks?
Claude Opus 5.5 scores higher on the Capabilities Index (ECI): Claude Opus 5.5 167.4 (#1 of 148), GPT-6 Astra 166.5 (#2 of 148) and Claude Fable 5.1 164.8 (#4 of 148). The confidence ranges of the top two overlap (164.0–172.0 vs 163.1–171.1), so treat the gap as small. On individual benchmarks: FrontierMath Tiers 1–3 — GPT-6 Astra 93.7%, Claude Opus 5.5 91.2%, Claude Fable 5.1 90.2%; OTIS Mock AIME 2024–2025 — Claude Opus 5.5 100%, Claude Fable 5.1 100%, GPT-6 Astra 100%; SimpleQA Verified — GPT-6 Astra 75.6%, Claude Opus 5.5 72.2%, Claude Fable 5.1 70.8%.
Which is better for coding?
There are no published SWE-bench Verified results for Claude Opus 5.5, Claude Fable 5.1 and GPT-6 Astra yet, so there is no like-for-like coding score. On overall capability, Claude Opus 5.5 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 Opus 5.5 and 1,000,000 for Claude Fable 5.1. Maximum output per response: Claude Opus 5.5 up to 128,000, Claude Fable 5.1 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; Claude Fable 5.1 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, Claude Fable 5.1 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 Astra came out Sep 4, 2026; Claude Fable 5.1 came out Sep 1, 2026. Knowledge cutoff: Claude Opus 5.5 Jun 2026, Claude Fable 5.1 Jun 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.
