Which is better for long-horizon autonomous coding: Opus 5 or Fable 5?

For a task genuinely at this scale and duration, Anthropic's own guidance points toward Fable 5 as the recommended choice, since it explicitly calls out Fable 5 as better suited to more advanced projects, including ones a model may handle autonomously over multiple days, while positioning Opus 5 as the everyday, cost-efficient option for the bulk of typical work.

The practical reasoning is that a long-horizon autonomous task compounds small capability gaps over many sequential decisions - a model that's close to frontier-level on a single-turn benchmark can diverge more meaningfully from the frontier model's reliability once errors have many opportunities to accumulate across days of unsupervised execution.

That said, the right choice isn't purely about model capability in isolation - it's worth weighing Opus 5's roughly half price against Fable 5's stronger track record on this specific use case, and considering whether the task can be broken into shorter, more supervised checkpoints where Opus 5's cost efficiency becomes the more decisive factor instead.

In short: default to Fable 5 specifically for genuinely long, unsupervised, high-stakes autonomous runs where reliability compounding matters most, and default to Opus 5 for everything else, including most agentic coding work that fits within a normal working session.

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