Simon Willison

If you’ve been struggling with a multitude of sandboxes for agents, MxC is a sandbox solution that works across Windows, Linux, and macOS.

A real platform opportunity for developers across the ecosystem. Process, session, micro VM, and deep integration into agent ID on Windows.

Meet Mistral Large 4, aka Le Chonk.

• 1T parameters, natively multimodal. 49B active.
It is the best open weights model from US or Europe on aggregated benchmarks.
• State-of-the-art on critical workloads, including cyber defense, manufacturing and finance and it surpasses closed frontier models on visual grounding.
• Forged in Europe end-to-end and is deployable from Europe via our own Mistral Cloud infrastructure.
• Available to all via API today. Working with cybersecurity partners privately.

Open weights release end of October.

A thread of early experiments with Claude Sonnet 5.5.

A fall foliage simulator by @_re_pete, made with Sonnet 5 vs Sonnet 5.5.

Today I'm parting ways with Fly.io. It's been a privilege. I'm off to tilt at a big old windmill with Kurt, again.

sockpuppet.org/blog/2026/09/2…

Legendary programmer Simon Willison says he's never so much change happen so quickly as he has in 2026, and there's no sign of it stopping.

🎤 Don't miss his closing Keynote, Friday (Sept 25) on the Main Stage at 5.30pm!

Bury your head in the ground at your own risk. I aim to not jump on any hype trains, but since Opus 4.6 and GPT-5.2 + the harnesses it was clear that these things can write code nearly as good as I can in my best language; better in other languages.

But we won't see non-devs push production code for a long time; possibly forever, if you ask me. Software still needs to be built in robust ways, and it's our profession to do this well, and use the new tools we have, which create new and interesting (+tough!) challenges