Claude Sonnet 4.5

Claude Sonnet 4.5 was released September 29, 2025 as Anthropic's frontier coding and agent model — notably the first time Anthropic anchored its best agent capabilities on a mid-tier Sonnet rather than the premium Opus line. It set the state of the art on SWE-bench Verified at 77.2% (82% with parallel test-time compute) and on OSWorld computer-use tasks, and it can sustain coherent autonomous work for 30+ hours. The launch shipped alongside the Claude Agent SDK and a native VS Code extension, and it integrates deeply with Amazon Bedrock, Slack, and GitHub Copilot.\n\nThe honest picture: the 200K context window is smaller than GPT-5's 400K, and at $3 input / $15 output per million tokens it's pricier than OpenAI's flagship. It's API-only with no open weights. Anthropic has since released Claude Sonnet 5 (June 2026), which is both cheaper and stronger — Sonnet 4.5 is worth knowing as the model that defined the 2025 coding-agent standard, but new projects should evaluate Sonnet 5 first.