Claude Sonnet 4

Overview

Anthropic's May 2025 mid-tier coding workhorse, offering near-Opus capability at lower cost and latency. Claude Sonnet 4 is now deprecated — the Sonnet line has moved through 4.5 and 4.6 to Claude Sonnet 5, the current tier.

Why it matters

Sonnet 4 established the pattern that still holds: the mid-tier Sonnet is the model engineering teams actually deploy, balancing coding strength against cost for always-on assistants, CI/CD review bots, and production RAG systems. The specific model is now legacy — it is deprecated on the API, and new builds should target Claude Sonnet 5. The lesson for CTOs: the Sonnet tier is a strategy (near-frontier capability at workhorse pricing), and the model ID underneath it changes roughly twice a year.