THE VERDICT
● Opus 5 at $5/$25 for almost everything. Frontier capability at half the price.
● Fable 5.1 at $10/$50 only where the hardest reasoning is the job, not a step in it.
● The number that changes the maths: Fable cache reads at $0.25 per million, 40x below fresh input.
Head to head
| Fable 5.1 | Opus 5 |
|---|
| Input / 1M | $10 | $5 |
| Output / 1M | $50 | $25 |
| Cache reads | $0.25 | Check current rate |
| Availability | GA since 1 September | GA since July |
| Consumer plans | API tier | Default on Claude Max |
THE HONEST TEST
Take twenty real tasks from your workload. Run them on both. Count how many Opus 5 got wrong that Fable 5.1 got right.
If that number is under three, you are paying double for a difference you cannot observe. A benchmark that doubles and a workload that does not improve is a common outcome.
Where the cache price matters
Fable cache reads at $0.25 per million are 40 times cheaper than fresh input. If your workload sends the same large context repeatedly — a codebase, a document corpus, a long system prompt — the effective per-call cost is nothing like $10.
That is the scenario where Fable can end up cheaper in practice than a rate-card comparison suggests. It is also the scenario most people never model, because comparing headline rates is easier.
Which one
| If you are... | Pick |
|---|
| Doing general work at volume | Opus 5. Half the price, and you will not notice the gap |
| Running hard scientific reasoning | Test Fable 5.1. That is what the benchmark gain claims to cover |
| Sending a large fixed context repeatedly | Model both with cache reads. Fable may come out cheaper |
| On a consumer plan | Opus 5 is the Claude Max default. No decision needed |
FAQ
Is Fable 5.1 better than Opus 5?
On Anthropic's own testing it scores higher on scientific reasoning. Whether that shows up in your work is a separate question, and the only way to know is to run your own tasks on both.
Why is Fable twice the price?
It is a higher tier rather than a newer version of the same tier. Opus 5 remains the general-purpose frontier option.
What is Mythos 5.1?
The same underlying model as Fable 5.1 with a different safeguard set, available through trusted access. A governance distinction rather than a capability one.