When should you use Fable 5 instead of Opus 5?

Reach for Fable 5 when a task is at the edge of what's currently possible - the most demanding reasoning, or the largest and most ambiguous multi-day autonomous agentic projects - where Opus 5's slightly smaller capability margin relative to Fable 5 is more likely to actually matter to the outcome.

For the large majority of everyday coding, analysis, and business-automation tasks, Opus 5 is the more cost-efficient default, since it approaches Fable 5's intelligence on many of these tasks at roughly half the price, making the extra Fable 5 cost hard to justify unless the task specifically needs that margin.

It's also worth noting that Fable 5 and Mythos 5 always run with adaptive thinking on with no way to disable it, and carry their own distinct safety classifier behavior, so a task with very specific thinking-control requirements is another factor in the choice, not just raw capability.

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