Claude Fable 5.1 vs GPT-6 Astra (Fast) vs Claude Mythos 5
Too close to call on our weighted score (GPT-6 Astra (Fast) 36, Claude Fable 5.1 36, Claude Mythos 5 36). The right pick depends on what you value most.
AnthropicClaude Fable 5.1
36/100- ECI164.8
- Price$10.00 / $50.00
- Context1M
OpenAIGPT-6 Astra (Fast)
36/100- ECI—
- Price$20.00 / $100.00
- Context1.05M
- Anthropic
Claude Mythos 5
36/100- ECI—
- Price$10.00 / $50.00
- Context1M
Too close to call
It is close. Our weighted score puts them within a point (GPT-6 Astra (Fast) 36/100, Claude Fable 5.1 36/100, Claude Mythos 5 36/100), so choose by what matters most for your work: Claude Fable 5.1 on price and GPT-6 Astra (Fast) for long inputs. The score weighs price 50%, inputs & features 30%, context window 20%. None of the shared benchmarks cover every model here yet, so capability is left out of this verdict.
- CapabilityNot enough dataNo independent benchmark covers every model here yet
- Lowest priceClaude Fable 5.1 and Claude Mythos 5Claude Fable 5.1 $20.00 · Claude Mythos 5 $20.00 · GPT-6 Astra (Fast) $40.00 per 1M tokens (3:1 blend)
- Longest contextGPT-6 Astra (Fast)GPT-6 Astra (Fast) 1,050,000 · Claude Fable 5.1 1,000,000 · Claude Mythos 5 1,000,000 tokens
- Widest inputsSame inputsClaude Fable 5.1: Text, Images, PDFs · GPT-6 Astra (Fast): Text, Images, PDFs · Claude Mythos 5: Text, Images, PDFs
- Self-hostingNo open weightsAll three are available only through APIs
| Measure | Weight | Claude Fable 5.1 | GPT-6 Astra (Fast) | Claude Mythos 5 |
|---|---|---|---|---|
| Price | 50% | 0 | 0 | 0 |
| Inputs & features | 30% | 80 | 80 | 80 |
| Context window | 20% | 60 | 61 | 60 |
| Overall | 100% | 36/100 | 36/100 | 36/100 |
Left out because at least one model lacks the data: capability. The remaining weights were rescaled.
Every spec in one table
Highlighted cells lead their row. Dashes mean the data is not published.
| Specification | Claude Fable 5.1Anthropic | GPT-6 Astra (Fast)OpenAI | Claude Mythos 5Anthropic |
|---|---|---|---|
| Capability | |||
| Capabilities Index (ECI) | 164.8 | — | — |
| ECI rank | #4 of 148 | — | — |
| FrontierMath Tiers 1–3Research-level mathematics | 90.2% | — | — |
| OTIS Mock AIME 2024–2025Competition mathematics | 100% | — | — |
| SimpleQA VerifiedShort factual questions | 70.8% | — | — |
| Price per million tokens | |||
| Input | $10.00 (best) | $20.00 | $10.00 (best) |
| Output | $50.00 (best) | $100.00 | $50.00 (best) |
| Cached input | $0.25 | — | — |
| Blended (3:1) | $20.00 (best) | $40.00 | $20.00 (best) |
| Long-context rate | Same rate | Same rate | Same rate |
| Price source | Official Anthropic API | Median of 1 providers | Median of 2 providers |
| 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 | Yes | Yes |
| Tool calling | Yes | Yes | Yes |
| Structured output | Yes | Yes | Yes |
| Availability | |||
| Weights | Proprietary | Proprietary | Proprietary |
| API model ID | claude-fable-5-1 | — | — |
| API providers | 29 (best) | 1 | 2 |
| Released | Sep 1, 2026 | Sep 4, 2026 | Jun 9, 2026 |
| Knowledge cutoff | Jun 2026 | — | Jan 31, 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 (Fast)$400.00
- Claude Mythos 5$200.00
Which should you choose?
Which is better: Claude Fable 5.1, GPT-6 Astra (Fast) or Claude Mythos 5?
It is close. Our weighted score puts them within a point (GPT-6 Astra (Fast) 36/100, Claude Fable 5.1 36/100, Claude Mythos 5 36/100), so choose by what matters most for your work: Claude Fable 5.1 on price and GPT-6 Astra (Fast) for long inputs. The score weighs price 50%, inputs & features 30%, context window 20%. None of the shared benchmarks cover every model here yet, so capability is left out of this verdict.
Which is cheaper, Claude Fable 5.1, GPT-6 Astra (Fast) or Claude Mythos 5?
Claude Fable 5.1 is cheaper at $10.00 input / $50.00 output per million tokens (official Anthropic API price). Claude Mythos 5 costs $10.00 input / $50.00 output per million tokens (median across 2 API providers); GPT-6 Astra (Fast) costs $20.00 input / $100.00 output per million tokens (median across 1 API provider). At a typical mix of three input tokens to one output token, that is $20.00 per million tokens for Claude Fable 5.1 versus $20.00 for Claude Mythos 5 (1× as much) and $40.00 for GPT-6 Astra (Fast) (2× as much).
Which scores higher on benchmarks?
There is no independent benchmark that covers all three models yet. Claude Fable 5.1 has an ECI of 164.8, GPT-6 Astra (Fast) has not been scored yet and Claude Mythos 5 has not been scored yet.
Which is better for coding?
There are no published SWE-bench Verified results for Claude Fable 5.1, GPT-6 Astra (Fast) and Claude Mythos 5 yet, so there is no like-for-like coding score. Test both on a sample of your own tasks. All three support tool calling for agent workflows.
Which has the bigger context window?
GPT-6 Astra (Fast) 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 Mythos 5. Maximum output per response: Claude Fable 5.1 up to 128,000, GPT-6 Astra (Fast) up to 128,000, Claude Mythos 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 (Fast) accepts text, images and PDFs; Claude Mythos 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 (Fast) and Claude Mythos 5 are proprietary and only available through APIs and apps.
Which is newer?
GPT-6 Astra (Fast) is the newest, released Sep 4, 2026. Claude Fable 5.1 came out Sep 1, 2026; Claude Mythos 5 came out Jun 9, 2026. Knowledge cutoff: Claude Fable 5.1 Jun 2026, Claude Mythos 5 Jan 31, 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.
Related comparisons
- Claude Fable 5.1 vs GPT-6 AstraOpenAI · ECI 166.5
- Claude Fable 5.1 vs Kimi K3Moonshot AI · ECI 157.6
- Claude Fable 5.1 vs Grok 4.6xAI · ECI 156.6
- Claude Fable 5.1 vs Qwen3.8 MaxAlibaba (Qwen) · ECI 156.6
- GPT-6 Astra (Fast) vs Kimi K2.8 PreviewMoonshot AI
- GPT-6 Astra (Fast) vs MiniMax-M3.1-Flash-PreviewMiniMax
