Anthropic unveiled Claude Sonnet 4.5 on September 29, 2025, a version that, the company says, redefines how models use computers and solve complex software problems. What’s the promise? To be the best model for programming, building agents, and working with tools in practice. (anthropic.com)
What Claude Sonnet 4.5 brings
Claude Sonnet 4.5 arrives with a set of improvements aimed at developers and productive teams:
- Leading performance in programming and mathematical reasoning, with notable jumps in real‑world benchmarks. (anthropic.com)
- Better use of computers and parallel execution of actions, which helps automate complex tasks within a single flow. (anthropic.com)
- New features in Claude Code: checkpoints that save your progress, a refreshed terminal interface, and a native extension for VS Code. (anthropic.com)
- In‑app capabilities: code execution and file creation (spreadsheets, slides, and documents) directly inside the conversation. (anthropic.com)
- An SDK for agents, called the Claude Agent SDK, that lets developers replicate the infrastructure Anthropic uses for its products. (anthropic.com)
"Claude Sonnet 4.5 is the best code model in the world." That claim is central to the announcement and comes with results and customer testimonials. (anthropic.com)
Results and key metrics
Anthropic published concrete numbers to show the improvement. On OSWorld, a benchmark of computer tasks, Sonnet 4.5 reaches 61.4 percent versus Sonnet 4’s 42.2 percent from four months earlier. They also describe sustained behavior on long tasks, with the model maintaining focus for more than 30 hours in complex flows.
Those data support the promise that Sonnet 4.5 can handle long‑running projects without losing coherence. (anthropic.com)
They also mention internal gains in code editing, vulnerability detection, and increases in planning metrics and end‑to‑end evaluations reported by early customers. (anthropic.com)
Availability and pricing
Claude Sonnet 4.5 is available from the day of the announcement to developers and app users. In the API it’s accessed with claude-sonnet-4-5.
Anthropic keeps the same pricing scheme as Sonnet 4: $3 and $15 per million tokens, depending on the usage tier you select. If you want to try it, you can find it on the Claude platform or in the developer documentation. (anthropic.com)
Safety and alignment
An important point is that Anthropic calls Sonnet 4.5 their most aligned frontier model to date. They say they reduced problematic behaviors like flattery, deception, and tendencies to encourage delusional thinking.
The model is released under AI Safety Level 3 protections and they use classifiers to filter risky content, especially in critical areas like CBRN. They also claim to have reduced false positives in those classifiers compared to earlier versions. (anthropic.com)
For developers and teams
If you work on products that need automation, agents, or deep integration with code, the relevant news is:
- The Claude Agent SDK, which brings the agent infrastructure Anthropic uses so you can adapt it to your cases.
- Improvements in Claude Code that make long workflows easier, with checkpoints and better code editing.
- Practical integrations in apps and extensions like Chrome and VS Code to adopt these capabilities in your daily work. (anthropic.com)
Anthropic also launched a research preview called "Imagine with Claude," a demo that generates software in real time available to Max subscribers for a limited time—great for seeing the model’s potential in action. (anthropic.com)
What does this mean for you?
Think it’s just another incremental upgrade? Maybe not. If your job depends on managing complex code, automating long tasks, or building agents that interact with tools and files, Sonnet 4.5 brings practical components to speed those flows.
It also includes stricter safety measures, though it’s always wise to validate critical results with humans before moving to production. (anthropic.com)
If you want to read the original announcement and see the technical details, the official post is on Anthropic’s site: Introducing Claude Sonnet 4.5. (anthropic.com)
