Claude Opus 4.5, and why evaluating new LLMs is increasingly difficult by Simon Willison

Anthropic released Claude Opus 4.5 this morning, which they call “best model in the world for coding, agents, and computer use”. This is their attempt to retake the crown for best coding model after significant challenges from OpenAI’s GPT-5.1-Codex-Max and Google’s Gemini 3, both released within the past week!

I did not have preview access to Opus4.5. Nor do I need it for the things I generally use LLMs for.

With the base text only models, I guess there is no more step change now. They may show benchmarks that they are the best model for coding, but it’s single decimal points. It does not really matter.

What matters more is the features they add - like when Anthropic added the skills feature. What you can do is more important. And yes I still believe it will be human in the loop situation. Will we be centaurs of reverse-centaurs is an open question.