Simon Willison’s Weblog

30th April 2026 - Link Blog

Our evaluation of OpenAI's GPT-5.5 cyber capabilities. The UK's AI Security Institute previously evaluated Claude Mythos: now they've evaluated GPT-5.5 for finding security vulnerability and found it to be comparable to Mythos, but unlike Mythos it's generally available right now.

Posted 30th April 2026 at 11:03 pm

Recent articles

Notes on the xAI/Anthropic data center deal - 7th May 2026
Live blog: Code w/ Claude 2026 - 6th May 2026
Vibe coding and agentic engineering are getting closer than I'd like - 6th May 2026

This is a link post by Simon Willison, posted on 30th April 2026.

ai 2020 openai 419 generative-ai 1787 llms 1753 anthropic 282 claude 275 ai-security-research 17 gpt 124

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