Simon Willison’s Weblog

Two major new model releases today, within about 15 minutes of each other.

Anthropic released Opus 4.6. Here's its pelican:

OpenAI release GPT-5.3-Codex, albeit only via their Codex app, not yet in their API. Here's its pelican:

I've had a bit of preview access to both of these models and to be honest I'm finding it hard to find a good angle to write about them - they're both really good, but so were their predecessors Codex 5.2 and Opus 4.5. I've been having trouble finding tasks that those previous models couldn't handle but the new ones are able to ace.

The most convincing story about capabilities of the new model so far is Nicholas Carlini from Anthropic talking about Opus 4.6 and Building a C compiler with a team of parallel Claudes - Anthropic's version of Cursor's FastRender project.