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

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Initial impressions of Claude Fable 5 - 9th June 2026
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Claude Opus 4.8: "a modest but tangible improvement" - 28th May 2026

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

ai 2,066 openai 422 generative-ai 1,824 llms 1,792 anthropic 294 claude 280 ai-security-research 20 gpt 124

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