The whole system card is worth reading, it's 120 pages and I couldn't put it down. It's like the (excellent) TV show Person of Interest come to life: https://www-cdn.anthropic.com/4263b940cabb546aa0e3283f35b686...

I wrote up my own highlights here: https://simonwillison.net/2025/May/25/claude-4-system-card/