What is the difference between Sonnet 5 and Opus 5 on long-horizon coding?

On the hardest, longest-horizon coding benchmarks - the kind involving extended multi-step work across many files or a long agentic session - reporting shows Sonnet 5 trailing both Opus 4.8 and Opus 5 by a meaningfully wider margin than the gap seen on shorter, everyday coding tasks.

This is consistent with the broader framing that Sonnet 5 narrows the gap with Opus-class models specifically on typical, well-scoped work, while the gap remains more pronounced specifically on tasks that stress long-horizon planning, deep multi-file reasoning, and sustained autonomous execution.

Practically, this supports the pattern of defaulting to Sonnet 5 for everyday coding work and escalating specifically when a task's shape, not just its category, involves this kind of extended, high-complexity, multi-step execution where the capability gap is largest.

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