Claude Sonnet 5 was released on July 15, 2025 as Anthropic's new default model, replacing Sonnet 4 across the API, Claude.ai, and Claude Code CLI. Key improvements include stronger agentic task decomposition (multi-step planning without re-prompting), more reliable tool/function calling with fewer hallucinated parameters, and better coherence on long outputs. The guide covers API integration with Node.js (model ID: claude-sonnet-5-20250715), a full agentic loop implementation with file read/write tools and path traversal protection, Claude Code CLI usage, and a decision matrix comparing Sonnet 5 ($3/$15 per 1M tokens post-Aug 31) vs Opus 4 ($15/$75) vs DeepSeek V3. Introductory pricing of $2/$10 runs until August 31, 2025, after which a 50% increase takes effect. Practical cost mitigation strategies include prompt caching and request batching.
