Claude Sonnet 4.5 vs 4.6: What Changed and Which Should You Use?

Anthropic shipped Claude Sonnet 4.6 in February 2026, roughly five months after Sonnet 4.5 launched in September 2025. Both carry the same API pricing ($3 input / $15 output per million tokens), but the gap in capability is meaningful. If you're picking a model to build on right now, the choice matters.

This post breaks down exactly what changed, which use cases favor each model, and how to connect either one to a real content layer using the Cosmic JavaScript SDK.

What Changed: Sonnet 4.5 to 4.6

Coding

Sonnet 4.5 was already a strong coding model when it launched. Anthropic called it "the best coding model in the world" at the time, and it led SWE-bench Verified at 77.2% (averaged across 10 trials). It introduced the Claude Agent SDK and showed it could maintain focus across 30+ hour autonomous coding sessions.