Claude Opus 4.8 vs Claude Fable 5.1 vs GPT-5.5
Claude Fable 5.1 comes out ahead, 66 to 63 and 63 on our weighted score, though Claude Opus 4.8 is 2× cheaper per token.
AnthropicClaude Opus 4.8
63/100- ECI158.3
- Price$5.00 / $25.00
- Context1M
- Our pickAnthropic
Claude Fable 5.1
66/100- ECI164.8
- Price$10.00 / $50.00
- Context1M
OpenAIGPT-5.5
63/100- ECI159.2
- Price$5.00 / $30.00
- Context1.05M
Claude Fable 5.1 is our pick
Claude Fable 5.1 is the better all-round choice, scoring 66/100 against GPT-5.5 (63) and Claude Opus 4.8 (63). It leads on capability. Claude Opus 4.8 wins on price. The score weighs capability 50%, price 25%, inputs & features 15%, context window 10%.
- CapabilityClaude Fable 5.1Capabilities Index (ECI): Claude Fable 5.1 164.8 · GPT-5.5 159.2 · Claude Opus 4.8 158.3
- Lowest priceClaude Opus 4.8Claude Opus 4.8 $10.00 · GPT-5.5 $11.25 · Claude Fable 5.1 $20.00 per 1M tokens (3:1 blend)
- Longest contextGPT-5.5GPT-5.5 1,050,000 · Claude Opus 4.8 1,000,000 · Claude Fable 5.1 1,000,000 tokens
- Widest inputsSame inputsClaude Opus 4.8: Text, Images, PDFs · Claude Fable 5.1: Text, Images, PDFs · GPT-5.5: Text, Images, PDFs
- Self-hostingNo open weightsAll three are available only through APIs
| Measure | Weight | Claude Opus 4.8 | Claude Fable 5.1 | GPT-5.5 |
|---|---|---|---|---|
| CapabilityCapabilities Index (ECI) | 50% | 89 | 97 | 90 |
| Price | 25% | 2 | 0 | 0 |
| Inputs & features | 15% | 80 | 80 | 80 |
| Context window | 10% | 60 | 60 | 61 |
| Overall | 100% | 63/100 | 66/100 | 63/100 |
Every spec in one table
Highlighted cells lead their row. Dashes mean the data is not published.
| Specification | Claude Opus 4.8Anthropic | Claude Fable 5.1Anthropic | GPT-5.5OpenAI |
|---|---|---|---|
| Capability | |||
| Capabilities Index (ECI) | 158.3 | 164.8 (best) | 159.2 |
| ECI rank | #12 of 148 | #4 of 148 (best) | #10 of 148 |
| GPQA DiamondGraduate-level science questions | 91.0% | — | 94.0% (best) |
| FrontierMath Tiers 1–3Research-level mathematics | 80.0% | 90.2% (best) | 85.3% |
| OTIS Mock AIME 2024–2025Competition mathematics | 98.3% | 100% (best) | 100% (best) |
| SWE-bench VerifiedFixing real GitHub issues | — | — | 80.6% |
| SimpleQA VerifiedShort factual questions | 53.0% | 70.8% (best) | 63.0% |
| Price per million tokens | |||
| Input | $5.00 (best) | $10.00 | $5.00 (best) |
| Output | $25.00 (best) | $50.00 | $30.00 |
| Cached input | $0.50 | $0.25 (best) | $0.50 |
| Blended (3:1) | $10.00 (best) | $20.00 | $11.25 |
| Long-context rate | Same rate | Same rate | Over 272K: $10.00 / $45.00 |
| 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 |
| Tool calling | Yes | Yes | Yes |
| Structured output | Yes | Yes | Yes |
| Availability | |||
| Weights | Proprietary | Proprietary | Proprietary |
| API model ID | claude-opus-4-8 | claude-fable-5-1 | gpt-5.5 |
| API providers | 45 (best) | 29 | 42 |
| Released | May 28, 2026 | Sep 1, 2026 | Apr 23, 2026 |
| Knowledge cutoff | Jan 2026 | Jun 2026 | Dec 1, 2025 |
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 4.8$100.00
- Claude Fable 5.1$200.00
- GPT-5.5$110.00
Which should you choose?
Which is better: Claude Opus 4.8, Claude Fable 5.1 or GPT-5.5?
Claude Fable 5.1 is the better all-round choice, scoring 66/100 against GPT-5.5 (63) and Claude Opus 4.8 (63). It leads on capability. Claude Opus 4.8 wins on price. The score weighs capability 50%, price 25%, inputs & features 15%, context window 10%.
Which is cheaper, Claude Opus 4.8, Claude Fable 5.1 or GPT-5.5?
Claude Opus 4.8 is cheaper at $5.00 input / $25.00 output per million tokens (official Anthropic API price). GPT-5.5 costs $5.00 input / $30.00 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 $10.00 per million tokens for Claude Opus 4.8 versus $11.25 for GPT-5.5 (1.1× as much) and $20.00 for Claude Fable 5.1 (2× as much).
Which scores higher on benchmarks?
Claude Fable 5.1 scores higher on the Capabilities Index (ECI): Claude Fable 5.1 164.8 (#4 of 148), GPT-5.5 159.2 (#10 of 148) and Claude Opus 4.8 158.3 (#12 of 148). The confidence ranges of the top two overlap (161.8–169.1 vs 156.9–162.4), so treat the gap as small. On individual benchmarks: FrontierMath Tiers 1–3 — Claude Fable 5.1 90.2%, GPT-5.5 85.3%, Claude Opus 4.8 80.0%; OTIS Mock AIME 2024–2025 — Claude Fable 5.1 100%, GPT-5.5 100%, Claude Opus 4.8 98.3%; SimpleQA Verified — Claude Fable 5.1 70.8%, GPT-5.5 63.0%, Claude Opus 4.8 53.0%.
Which is better for coding?
There are no published SWE-bench Verified results for Claude Opus 4.8 and Claude Fable 5.1 yet, so there is no like-for-like coding score. On overall capability, Claude Fable 5.1 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-5.5 has the largest context window at 1,050,000 tokens, against 1,000,000 for Claude Opus 4.8 and 1,000,000 for Claude Fable 5.1. Maximum output per response: Claude Opus 4.8 up to 128,000, Claude Fable 5.1 up to 128,000, GPT-5.5 up to 128,000 tokens.
Which can read images, PDFs, audio or video?
Claude Opus 4.8 accepts text, images and PDFs; Claude Fable 5.1 accepts text, images and PDFs; GPT-5.5 accepts text, images and PDFs. They handle the same number of input types.
Are any of these open source?
No. Claude Opus 4.8, Claude Fable 5.1 and GPT-5.5 are proprietary and only available through APIs and apps.
Which is newer?
Claude Fable 5.1 is the newest, released Sep 1, 2026. Claude Opus 4.8 came out May 28, 2026; GPT-5.5 came out Apr 23, 2026. Knowledge cutoff: Claude Opus 4.8 Jan 2026, Claude Fable 5.1 Jun 2026, GPT-5.5 Dec 1, 2025.
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.
