Claude Fable 5.1 vs Gemini 3.8 Flash vs GPT-6 Astra
Gemini 3.8 Flash comes out ahead, 75 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
- Our pick
GoogleGemini 3.8 Flash
75/100- ECI156.9
- Price$0.75 / $3.75
- Context1.05M
OpenAIGPT-6 Astra
68/100- ECI166.5
- Price$10.00 / $50.00
- Context1.05M
Gemini 3.8 Flash is our pick
Gemini 3.8 Flash is the better all-round choice, scoring 75/100 against GPT-6 Astra (68) and Claude Fable 5.1 (66). It leads on price and inputs & features. 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 Fable 5.1 164.8 · Gemini 3.8 Flash 156.9
- Lowest priceGemini 3.8 FlashGemini 3.8 Flash $1.50 · Claude Fable 5.1 $20.00 · GPT-6 Astra $20.00 per 1M tokens (3:1 blend)
- Longest contextGPT-6 Astra and Gemini 3.8 FlashGPT-6 Astra 1,050,000 · Gemini 3.8 Flash 1,048,576 · Claude Fable 5.1 1,000,000 tokens
- Widest inputsGemini 3.8 FlashClaude Fable 5.1: Text, Images, PDFs · Gemini 3.8 Flash: Text, Images, PDFs, Audio, Video · GPT-6 Astra: Text, Images, PDFs
- Self-hostingNo open weightsAll three are available only through APIs
| Measure | Weight | Claude Fable 5.1 | Gemini 3.8 Flash | GPT-6 Astra |
|---|---|---|---|---|
| CapabilityCapabilities Index (ECI) | 50% | 97 | 87 | 99 |
| Price | 25% | 0 | 42 | 0 |
| Inputs & features | 15% | 80 | 100 | 80 |
| Context window | 10% | 60 | 61 | 61 |
| Overall | 100% | 66/100 | 75/100 | 68/100 |
Every spec in one table
Highlighted cells lead their row. Dashes mean the data is not published.
| Specification | Claude Fable 5.1Anthropic | Gemini 3.8 FlashGoogle | GPT-6 AstraOpenAI |
|---|---|---|---|
| Capability | |||
| Capabilities Index (ECI) | 164.8 | 156.9 | 166.5 (best) |
| ECI rank | #4 of 148 | #15 of 148 | #2 of 148 (best) |
| GPQA DiamondGraduate-level science questions | — | 95.4% | 95.8% (best) |
| FrontierMath Tiers 1–3Research-level mathematics | 90.2% | 68.4% | 93.7% (best) |
| OTIS Mock AIME 2024–2025Competition mathematics | 100% (best) | 98.9% | 100% (best) |
| SimpleQA VerifiedShort factual questions | 70.8% | 69.7% | 75.6% (best) |
| Price per million tokens | |||
| Input | $10.00 | $0.75 (best) | $10.00 |
| Output | $50.00 | $3.75 (best) | $50.00 |
| Cached input | $0.25 | $0.075 (best) | $1.00 |
| Blended (3:1) | $20.00 | $1.50 (best) | $20.00 |
| Long-context rate | Same rate | Same rate | Over 272K: $20.00 / $75.00 |
| Price source | Official Anthropic API | Official Google API | Official OpenAI API |
| Limits | |||
| Context window | 1,000,000 tokens | 1,048,576 tokens | 1,050,000 tokens (best) |
| Max output | 128,000 tokens (best) | 65,536 tokens | 128,000 tokens (best) |
| Inputs and features | |||
| Text | Yes | Yes | Yes |
| Images | Yes | Yes | Yes |
| PDFs | Yes | Yes | Yes |
| Audio | No | Yes | No |
| Video | No | Yes | No |
| Reasoning | Yeslow · medium · high · xhigh · max | Yeslow · medium · high | 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 | gemini-3.8-flash | gpt-6-astra |
| API providers | 29 (best) | 21 | 26 |
| Released | Sep 1, 2026 | Sep 2, 2026 | Sep 4, 2026 |
| Knowledge cutoff | 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 Fable 5.1$200.00
- Gemini 3.8 Flash$15.00
- GPT-6 Astra$200.00
Which should you choose?
Which is better: Claude Fable 5.1, Gemini 3.8 Flash or GPT-6 Astra?
Gemini 3.8 Flash is the better all-round choice, scoring 75/100 against GPT-6 Astra (68) and Claude Fable 5.1 (66). It leads on price and inputs & features. 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, Gemini 3.8 Flash or GPT-6 Astra?
Gemini 3.8 Flash is cheaper at $0.75 input / $3.75 output per million tokens (official Google 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 $1.50 per million tokens for Gemini 3.8 Flash versus $20.00 for Claude Fable 5.1 (13× as much) and $20.00 for GPT-6 Astra (13× 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 Fable 5.1 164.8 (#4 of 148) and Gemini 3.8 Flash 156.9 (#15 of 148). The confidence ranges of the top two overlap (163.1–171.1 vs 161.8–169.1), so treat the gap as small. On individual benchmarks: FrontierMath Tiers 1–3 — GPT-6 Astra 93.7%, Claude Fable 5.1 90.2%, Gemini 3.8 Flash 68.4%; OTIS Mock AIME 2024–2025 — Claude Fable 5.1 100%, GPT-6 Astra 100%, Gemini 3.8 Flash 98.9%; SimpleQA Verified — GPT-6 Astra 75.6%, Claude Fable 5.1 70.8%, Gemini 3.8 Flash 69.7%.
Which is better for coding?
There are no published SWE-bench Verified results for Claude Fable 5.1, Gemini 3.8 Flash 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 Astra and Gemini 3.8 Flash have the largest context windows (1,050,000 and 1,048,576 tokens), against 1,000,000 for Claude Fable 5.1. Maximum output per response: Claude Fable 5.1 up to 128,000, Gemini 3.8 Flash up to 65,536, GPT-6 Astra up to 128,000 tokens.
Which can read images, PDFs, audio or video?
Claude Fable 5.1 accepts text, images and PDFs; Gemini 3.8 Flash accepts text, images, PDFs, audio and video; GPT-6 Astra accepts text, images and PDFs. Gemini 3.8 Flash handles the widest range of inputs.
Are any of these open source?
No. Claude Fable 5.1, Gemini 3.8 Flash and GPT-6 Astra are proprietary and only available through APIs and apps.
Which is newer?
GPT-6 Astra is the newest, released Sep 4, 2026. Gemini 3.8 Flash came out Sep 2, 2026; Claude Fable 5.1 came out Sep 1, 2026. Knowledge cutoff: 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.
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