Opus is the right call when you are stuck on something genuinely hard — a complex refactor, a multi-file architectural change, or an agent that needs to reason through ambiguity without hand-holding. The quality jump is real. The cost jump is also real.

Sonnet handles 90% of what most builders do day-to-day. In Claude Code sessions it is the default for good reason — fast enough to feel responsive, capable enough to write production code without constant corrections.

Haiku is what you reach for when you are running something thousands of times: a nightly cron that classifies leads, a pipeline that routes content, a webhook handler that needs to reply in under a second. Not built for depth — built for throughput.

The practical rule: start with Sonnet. Upgrade to Opus only when Sonnet gets it wrong twice on the same problem. Use Haiku for anything you would not want to pay Sonnet prices to run at scale.