Opus 5 vs Haiku 4.5

The two ends of the current lineup

These two models sit at opposite extremes of the current range, and there's rarely genuine ambiguity about which one a given task calls for — the gap in price, capability, and even basic request-shape support (Haiku 4.5 doesn't accept an effort parameter at all) is wide enough that this comparison is more useful for confirming an obvious choice than for weighing a close call.

Where a real decision does exist

The genuine judgment call isn't which model handles a task better in isolation — it's whether a workload sitting near Haiku's ceiling of complexity is better served by Haiku at volume, or by routing that specific slice to Opus and accepting the cost, if the failure cases at Haiku's ceiling are expensive enough to justify it. That's a workload-shape question, not a general capability comparison.

What to check before assuming Haiku is sufficient

If you're defaulting to Haiku for cost reasons on a workload you haven't fully characterised yet, sample-check a portion of its output against what Opus would have produced on the same inputs — a cost saving that comes with a quality regression nobody measured isn't actually a saving once the downstream cost of that regression is accounted for.

Related

See the model picker for a task-specific recommendation, and Opus 5 vs Fable 5 for the comparison at the other, more expensive end of the lineup.