Anthropic releases Claude Sonnet 4.5 and the Claude Agent SDK

Background

Anthropic sells its Claude models to individuals through the Claude apps and Claude Code, and to developers through an API (application programming interface), a standard way for one program to call another. Claude Code is Anthropic's agentic coding tool, in which the model carries out programming tasks in a developer's project with some autonomy.

What happened

On 29 September 2025 Anthropic released Claude Sonnet 4.5 at $3 per million input tokens and $15 per million output tokens.^[1] The model scored 77.2% on SWE-bench Verified, a coding benchmark, and 61.4% on OSWorld, a benchmark of computer use. Anthropic observed it working for more than 30 hours on complex tasks.^[1]

Anthropic called it its “most aligned frontier model”, and released it under AI Safety Level 3 (ASL-3) protections.^[1]

Key details

Claude Code gained checkpoints, which let a user save and return to earlier states of a session, a refreshed terminal interface and a native VS Code extension.^[1]

The infrastructure behind Claude Code was offered to developers as the Claude Agent SDK, a toolkit for building agents for purposes beyond coding.^[1]

The API gained context editing and a memory tool. The Claude apps gained code execution and file creation, so that paid users could produce spreadsheets, slides and documents.^[1]

Claude for Chrome, a browser extension that lets Claude act in the user's browser, was extended to Max users on the waitlist. “Imagine with Claude”, a research preview that generates software on the fly, ran for five days for Max subscribers.^[1]

Impact

The launch combined a new model with a set of developer tools. The Agent SDK in particular made the software that powers Claude Code available for building other agents, so developers could use the same foundation for a wide range of tasks.^[1]

The release also included Anthropic's own assessment of the model's alignment, which the company described as its most aligned frontier model to date.^[1]