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

Publishing WASM wheels to PyPI for use with Pyodide - 13th June 2026
Claude Fable is relentlessly proactive - 11th June 2026
Initial impressions of Claude Fable 5 - 9th June 2026

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

ai 2,077 openai 423 generative-ai 1,834 llms 1,802 anthropic 299 claude 284 ai-security-research 22 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