On Monday, Anthropic released Claude Sonnet 4.5, a new AI language model the company calls its "most capable model to date," with improved coding and computer use capabilities. The company also revealed Claude Code 2.0, a command-line AI agent for developers, and the Claude Agent SDK, which is a tool developers can use to build their own AI coding agents.
Anthropic says it has witnessed Sonnet 4.5 working continuously on the same project "for more than 30 hours on complex, multi-step tasks," though the company did not provide specific details about the tasks. In the past, agentic models have been known to typically lose coherence over long periods of time as errors accumulate and context windows (a type of short-term memory for the model) fill up. In the past, Anthropic has mentioned that previous Claude 4.0 models have played Pokémon for over 24 hours or refactored code for seven hours.
To understand why Sonnet exists, you need to know a bit about how AI language models work. Traditionally, Anthropic has produced three differently sized AI models in the Claude family: Haiku (the smallest), Sonnet (mid-range), and Opus (the largest). Anthropic last updated Haiku in November 2024 (to 3.5), Sonnet this past May (to 4.0), and Opus in August (to 4.1). Model size in parameters, which are values stored in its neural network, is roughly proportional to overall contextual depth (the number of multidimensional connections between concepts, which you might call "knowledge") and better problem-solving capability, but larger models are also slower and more expensive to run. So AI companies always seek a sweet spot in the middle with reasonable performance-cost trade-offs. Claude Sonnet has filled that role for Anthropic quite well for several years now.
