Claude Sonnet 4 vs. 4.5: A Real-World Comparison (www.cosmicjs.com)

Anthropic’s Claude Sonnet 4.5 was put through a controlled one-shot test—both Sonnet 4 and 4.5 were asked identically to “Build me a modern blog application” via the Cosmic AI Platform—to compare real-world output. Sonnet 4 produced a clean, functional, production-ready blog with solid component structure and good UX. Sonnet 4.5, however, generated more sophisticated architecture (refined component hierarchy, better separation of concerns, and more elegant state management), anticipated extra features (improved filtering, richer metadata and content relationships), and delivered a noticeably more polished design and smoother interactions. The team also observed faster build times (1.5–2x) and stronger cross-file coherence in 4.5, consistent with Anthropic’s claims of 77.2% on SWE-bench Verified and longer multi-step focus. For AI/ML practitioners and engineering teams, the takeaway is practical: both models can generate deployable applications in minutes (Cosmic’s GitHub/Vercel integrations handled instant deployment), but Sonnet 4.5 materially improves maintainability, reasoning about unspecified requirements, and performance on complex, long-running tasks. Use Sonnet 4 for straightforward, budget-conscious projects where stability matters; opt for Sonnet 4.5 when architectural quality, UX polish, or multi-step development coherence provide competitive advantage.