Claude Fable 5.1 vs Claude Opus 5.5
Which one to choose
Pick Claude Fable 5.1 for the hardest reasoning and long-horizon agent work, when evaluations at higher effort still fall short on Opus 5.5. Pick Claude Opus 5.5 as the default for agentic coding and knowledge work, since Anthropic says it performs at Fable 5.1 level on most tasks and suggests it first. The difference is tier: Fable is the slower, deeper model, Opus the faster everyday choice.
Pricing comparison
| Claude Fable 5.1 | Claude Opus 5.5 | |
|---|---|---|
| Model maker | Anthropic | Anthropic |
| Delivery availability | Available | Available |
| Context window | 1M | 1M |
| Max output | 128K | 128K |
| Official price | Input$10.00per 1M tokensOutput$50.00per 1M tokens | Input$4.00per 1M tokensOutput$20.00per 1M tokens |
| TokenLab price | Input$3.00per 1M tokensOutput$15.00per 1M tokens | Input$1.20per 1M tokensOutput$6.00per 1M tokens |
| Model performance |
|
|
| Capabilities | JSON modePrompt CacheReasoningTool useVision | JSON modePrompt CacheReasoningTool useVision |
Choose Claude Fable 5.1 when
- Your Opus 5.5 evaluations still miss the bar after you raise the effort setting.
- The work is sustained, demanding reasoning across a long agent run.
- Your use case does not trigger Fable's cybersecurity or biology safeguards.
Choose Claude Opus 5.5 when
- You want a first model to try on agentic coding and knowledge work before moving up a tier.
- You value faster output and fewer tokens and turns for equal work.
- Your work touches security research that Fable's safeguards may interrupt.
How they differ
| Aspect | Claude Fable 5.1 | Claude Opus 5.5 |
|---|---|---|
| Tier | Anthropic's top Claude model as of September 2026, and the same model as Mythos 5.1 with stricter safeguards. | The current Opus, one tier below Fable. |
| Speed | The slowest tier in the lineup, so poor for latency-sensitive chat. | Output is more than 30% faster than Opus 5, and it needs fewer tokens and turns. |
| Effort defaults | Adaptive thinking is always on with a high default effort. | Adaptive thinking is always on with a medium default effort. |
| Safeguards | Safeguards still block exploit generation and restrict some biology work, though false positives are lower than on Fable 5. | Anthropic points to Fable 5.1 for the hardest reasoning; Opus 5.5 carries no such restriction. |
| Recommended order | Anthropic suggests it when evaluations at higher effort still fall short. | Anthropic suggests starting with this model. |
Summary
- Claude Fable 5.1: Input $3.00 / Output $15.00 per 1M tokens; Claude Opus 5.5: Input $1.20 / Output $6.00 per 1M tokens. Rates depend on the billing unit, specification, and usage. Compare matching conditions in the model's detailed pricing; a single rate does not determine the total cost.
FAQ
Is Fable 5.1 better than Opus 5.5?
Fable 5.1 is the higher tier for demanding reasoning, but Anthropic says Opus 5.5 matches it on most tasks and suggests it first. Choose Fable when Opus falls short on your own tests.
Which of the two is faster?
Opus 5.5 is faster. Fable 5.1 is the slowest tier in Anthropic's lineup, so it fits long, deep tasks better than interactive chat, where the shorter wait of Opus 5.5 is easier to live with.
Do both support images and tools?
Yes. Both read text and images, call tools and use adaptive thinking that cannot be turned off, so the same agent loop and screenshot inputs work with either model.
How do I use them together?
Run routine agent steps on Opus 5.5 and send only the stages that fail evaluation, such as hard planning or final review, to Fable 5.1.
Which is cheaper, Claude Fable 5.1 or Claude Opus 5.5?
Claude Fable 5.1: Input $3.00 / Output $15.00 per 1M tokens; Claude Opus 5.5: Input $1.20 / Output $6.00 per 1M tokens. Rates depend on the billing unit, specification, and usage. Compare matching conditions in the model's detailed pricing; a single rate does not determine the total cost.
What are the key differences between Claude Fable 5.1 and Claude Opus 5.5?
Both models share similar capabilities.
Sources
- Introducing Claude Fable 5.1 and Claude Mythos 5.1
- Claude Fable 5.1 model overview
- Introducing Claude Opus 5.5
- Claude Opus 5.5 model overview
Reviewed Oct 2, 2026