What's Actually New
Claude Opus 4 represents Anthropic's most significant model update in over a year. The headline improvements are in three areas: complex multi-step reasoning, code generation and understanding, and the ability to follow long, nuanced instructions without losing track of early requirements.
In our testing, Opus 4 handles tasks that would previously require breaking into multiple prompts in a single coherent response. It's particularly impressive at legal document analysis, where it maintains awareness of earlier clauses when reasoning about later ones — something that tripped up previous models on long contracts.
Sonnet 4.5: The New Default
Sonnet 4.5 is arguably the more practically significant release. It brings Opus-level writing quality to the mid-tier model, making the cost-to-quality ratio substantially better than the previous Sonnet. For the vast majority of everyday use cases — writing, coding, research, business tasks — Sonnet 4.5 should now be your default.
What This Means for Your Workflow
The practical implication: if you were previously using Opus for most tasks because Sonnet felt insufficient, you can now switch to Sonnet 4.5 for most work and save significantly on API costs. Reserve Opus 4 for genuinely complex tasks — PhD-level research, advanced legal/financial analysis, or long documents requiring sustained reasoning.