Which is better for agentic multi-file refactoring: Sonnet 5 or Opus 5?

For agentic multi-file refactoring specifically, reporting consistently points toward Opus 5 as the stronger fit, since this task shape stresses exactly the capabilities where Opus 5's advantage over Sonnet 5 is reported as largest: holding more architectural context across files, making fewer errors on work spanning many files, and sustaining coherent multi-step execution over a longer agentic session.

Sonnet 5 remains capable of handling refactoring work, and is well suited to smaller-scope refactors, but the gap between the two models is described as widening specifically as refactor scope and session length increase, rather than being a constant, fixed gap across all refactoring tasks regardless of size.

A practical approach for teams uncertain which category a given refactor falls into is to start with Sonnet 5 at an appropriate effort level and treat repeated failures, incomplete multi-file changes, or excessive retries as the signal to escalate to Opus 5, rather than trying to predict scope perfectly in advance and always defaulting to one model for all refactoring work.

This mirrors the general pattern across the two models: Sonnet 5 is the reasonable default given its lower cost, with escalation to Opus 5 reserved specifically for the subset of tasks - here, larger and more architecturally complex refactors - where the capability gap is largest and most likely to actually change the outcome.