Simon Willison’s Weblog

Subscribe

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

Anthropic released Opus 4.6. Here's its pelican:

Slightly wonky bicycle frame but an excellent pelican, very clear beak and pouch, nice feathers.

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

Not nearly as good - the bicycle is a bit mangled, the pelican not nearly as well rendered - it's more of a line drawing.

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.

Monthly briefing

Sponsor me for $10/month and get a curated email digest of the month's most important LLM developments.

Pay me to send you less!

Sponsor & subscribe