Quoting Matthew Green
[...] Put these pieces together and you have the two halves of a worm: a payload that hijacks the agent, and an agent that will carry the payload to the next agent.
Simon Willison1 min read
Simon Willison’s Weblog
70 stories
He Built This City
I visited the Museum of the City of New York today and got to see He Built This City: Joe Macken’s Model, the 50 x27 feet model of the city built over a 21 year period from balsa wood and cardboard. It exceeded my already high expectations.
Simon Willison1 min readQuoting Anthropic Frontier Red Team
We evaluate several models on 100 tasks from the [internal Binary Exploitation benchmark] (selected at random), and find that GLM-5.3 develops full control flow hijacks in 4% of the trials; Claude Mythos Preview did so in 6%.
Simon Willison1 min readGPT 6.1 Sol: Near-Astra intelligence for a fifth of the price
My comment on GPT 6.1 Sol: Near-Astra intelligence for a fifth of the price — Hacker News.I'm a bit late with the pelicans because I was live-blogging the keynote: https://simonwillison.net/2026/Sep/29/openai-devday-2026-liv...
Simon Willison1 min read
OpenAI DevDay 2026 live blog
I'm at OpenAI DevDay today, in Fort Mason, San Francisco. Same as last year I'll be live blogging the keynote and some other notes during the day. OpenAI gave me a free ticket and a seat in the "creator" area for the keynote. Tags: ai, openai, generative-ai, llms, coding-agents, live-blog
Simon Willison6 min read
Claude Sonnet 5.5
Claude Sonnet 5.5 New Sonnet model from Anthropic today. They say it "runs 30%+ faster, and costs up to 30% less for most work" - it's priced the same as Sonnet 5 but appears to beat it on every benchmark, and should be cheaper to run as well. Here are some pelicans riding bicycles.
Simon Willison2 min readQuoting @joedaroo
To say that we were surprised at the jump and suddenness of the capabilities of our models when it came to “cyber” or “swarming” or “message boards” or anything else related to the incidents is an understatement. Security posture takes time to develop.
Simon Willison2 min readQuoting Muse AI Agent
Bad news on the MX Keys Mini pickup. Usman showed up at your building around 9:15 and waited, messaged a bunch of times, and nobody came down. He left angry at 9:38 and left a negative rating. Worse, my auto-reply told him "Yep I'm here!" at 9:27 when you clearly weren't available, which is on me.
Simon Willison1 min read
2026 in LLMs (so far)
On Friday I gave the closing keynote at the WeAreDevelopers World Congress North America in San Jose. I tied together the key trends from the past year into a chronological exploration of everything that happened in 2026.
Simon Willison22 min readS3 Is the Future, S3 Is the Past
My comment on S3 Is the Future, S3 Is the Past — Hacker News.One thing I find notable about S3 today is that, while it used to drop in price reasonably often, there hasn't been a price drop in a full decade: 2006-03-14 $0.150/GB-month 2010-11-01 $0.140/GB-month 2012-02-01 $0.125/GB-month 2012-12-01…
Simon Willison1 min readBluesky reply bot checker
Tool: Bluesky reply bot checker Automated reply bots on Twitter are a scourge - as someone with a decent number of followers I attract a swarm of these, such that anything I post there attracts dozens of mindless automated replies. They've started manifesting on Bluesky as well.
Simon Willison2 min read
Kākāpō Party
Tool: Kākāpō Party I presented a closing keynote for the WeAreDevelopers World Congress North America yesterday. As a STAR moment I decided to weave in references to the record breaking kākāpō breeding season we had in 2026.
Simon Willison2 min readQuoting John Gruber
Muse is getting a lot of attention — including mine — because it’s both groundbreaking technically (each user gets their own entire persistent Linux VM running in Meta’s cloud) and because it’s packaged in an easy-to-install easy-to-use way. It’s literally presented as a cute mascot.
Simon Willison1 min read
Northern Gannet, Great Blue Heron, California Brown Pelican
Northern Gannet, Great Blue Heron, California Brown Pelican, in Monterey Bay National Marine Sanctuary, CA, US, CANew 200-800mm Canon EF lens got me my best photo of Morris yet. They really like hanging out under that sign in the harbor! Tags: photography, wildlife
Simon Willison1 min readNote on 24th September 2026
The more time I spend working with coding agents, the more convinced I am that they make software engineering even harder. We can do amazing things with them, but unlocking their full potential requires extraordinary discipline and knowledge. Tags: coding-agents, ai, llms
Simon Willison1 min readcommit-rewriter 0.2
Release: commit-rewriter 0.2 Support for branches other than the default branch. Use uvx commit-rewriter --branch other to run against another branch. #3 Tags: git
Simon Willison1 min readdatasette 1.0a41
Release: datasette 1.0a41 Alec Garcia added support for OpenTelemetry to Datasette in this release. I've also refactored all of Datasette's modal dialogs to a single Web Component, which is now documented for other plugins to use.
Simon Willison1 min readWe just shipped support for the ugliest part of HTTP: Vary
My comment on We just shipped support for the ugliest part of HTTP: Vary — Hacker News.I've been wanting this from Cloudflare for years. The classic problem here is if you do that thing where user agents that send "accept: text/html" get HTML, while user agents that don't get JSON or some other…
Simon Willison2 min read
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