claude-sonnet-4-5-20250929-thinking

Claude Sonnet 4.5 Thinking is the extended reasoning variant of Claude Sonnet 4.5, designed to keep the base model's balance of speed and capability while layering in step-by-step internal reasoning that is enabled by default. The design goal is improved accuracy on complex, multi-step problems without giving up the quick response times that make the standard Sonnet tier attractive for production use. It is positioned as a general-purpose mid-tier model aimed at coding, analysis, and agent-style workflows where cost control matters as much as raw quality, which is why it sits in the Sonnet line rather than the heavier Opus family.

In practice the model is built for tasks that benefit from deliberate deliberation: code generation, refactoring, debugging, and multi-step reasoning chains where a quick one-shot answer is not enough. The extended thinking mode runs internally and is exposed through standard chat and tool-use APIs, including an OpenAI-compatible interface for easier drop-in adoption. Long context handling makes it suitable for large documents and extended multi-turn sessions, while streaming support keeps interactive applications responsive. The combination of reasoning by default, strong coding performance, and a mid-tier price profile makes it a practical fit for agent systems, developer assistants, and analytics workloads that need reliable answers on hard problems without paying flagship-model rates.