Claude Fable 5.1 or Claude Sonnet 5.5: which to choose?
Claude Fable 5.1 (Anthropic) and Claude Sonnet 5.5 (Anthropic) compared point by point: measured performance, pricing, context and capabilities.
Data as of October 22, 2026
Side-by-side comparison
| Criterion | Claude Fable 5.1 | Claude Sonnet 5.5 |
|---|---|---|
| Developer | Anthropic | Anthropic |
| Released | September 1, 2026 | September 28, 2026 |
| Capability index | 92.6 / 100 | 95.9 / 100 |
| Input price | $10 | $2 |
| Output price | $50 | $10 |
| Cost of a typical task | $0.30 | $0.06 |
| Context window | 1,000,000 tokens | 1,000,000 tokens |
| Maximum output | 128,000 tokens | 128,000 tokens |
| Reasoning mode | Yes | Yes |
| Tool calling | Yes | Yes |
| Accepted inputs | text, image, PDF | text, image, PDF |
| Open weights | No | No |
| Knowledge cutoff | June 2026 | June 2026 |
Two models head to head
This comparison pits two frontier models, the ones you pick when quality comes first against each other: Claude Fable 5.1 and Claude Sonnet 5.5, both built by Anthropic. The first dates from September 1, 2026, the second from September 28, 2026.
All figures below come from public sources checked on October 22, 2026: models.dev and OpenRouter for prices and capabilities, Epoch AI for scores.
Performance
On our capability index, Claude Fable 5.1 scores 92.6 out of 100 and Claude Sonnet 5.5 95.9. Claude Sonnet 5.5 leads by 3.3 points.
On the 5 benchmarks reported for both, Claude Fable 5.1 wins 2 times and Claude Sonnet 5.5 2:
- OTIS Mock AIME (competition mathematics problems): Claude Fable 5.1 100%, Claude Sonnet 5.5 100%
- FrontierMath (very hard research-level mathematics): Claude Fable 5.1 90.2%, Claude Sonnet 5.5 88.8%
- FrontierCode (advanced programming tasks): Claude Fable 5.1 50.9%, Claude Sonnet 5.5 52.1%
- APEX-Agents (professional tasks carried out by an agent): Claude Fable 5.1 68.6%, Claude Sonnet 5.5 75.5%
- Humanity's Last Exam (multidisciplinary expert questions): Claude Fable 5.1 46.5%, Claude Sonnet 5.5 46.1%
Which one costs less?
Claude Fable 5.1 costs $10 per million input tokens and $50 per million output tokens; Claude Sonnet 5.5 costs $2 and $10.
For a typical agent task (read 20,000 tokens, write 2,000), expect $0.30 with Claude Fable 5.1 versus $0.06 with Claude Sonnet 5.5. Claude Sonnet 5.5 is 5 times cheaper, a gap that adds up quickly over 1,000 tasks: $240 difference.
Run the numbers with your own volumes in the agent cost calculator.
Under the hood
Both models have the same context window: 1,000,000 tokens, roughly 1,500 pages.
Reasoning mode: Claude Fable 5.1 yes, Claude Sonnet 5.5 yes. Tool calling: Claude Fable 5.1 yes, Claude Sonnet 5.5 yes. Image input: Claude Fable 5.1 yes, Claude Sonnet 5.5 yes.
Neither publishes its weights: both are used through an API.
Knowledge cutoff: June 2026 for Claude Fable 5.1, June 2026 for Claude Sonnet 5.5.
In short
Our reading of the data:
- Choose Claude Sonnet 5.5 for a higher capability index and a lower cost.
This verdict relies on public figures; it does not replace a trial on your own tasks.
Other comparisons
Frequently asked questions
Claude Fable 5.1 or Claude Sonnet 5.5: which is better?
On our index (Epoch AI data), Claude Fable 5.1 scores 92.6 and Claude Sonnet 5.5 95.9 out of 100. The best choice also depends on price and on your use case.
Which one is cheaper?
Claude Sonnet 5.5. To read 20,000 tokens and write 2,000, expect $0.30 with Claude Fable 5.1 and $0.06 with Claude Sonnet 5.5 (prices as of October 22, 2026).
Which one has the larger context window?
Neither: both accept 1,000,000 tokens.
Can I run Claude Fable 5.1 or Claude Sonnet 5.5 on my own servers?
No: both are closed models, available through an API.
Data sources
- Models.dev — Model data: models.dev (open-source project)
- OpenRouter — Market prices cross-checked with OpenRouter
- Epoch AI — AI Benchmarking Hub — Epoch AI, “AI Benchmarking Hub”, epoch.ai, CC BY 4.0
Data as of October 22, 2026
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