Claude 4 System Cards Highlights

Summary

System Card: Claude Opus 4 & Claude Sonnet 4. Direct link to a PDF on Anthropic's CDN because they don't appear to have a landing page anywhere for this document. Anthropic's system cards are always worth a look, and this one for the new Opus 4 and Sonnet 4 has some particularly spicy notes. It's also 120 pages long - nearly three times the length of the system card for Claude 3.7 Sonnet! If you're looking for some enjoyable hard science fiction and miss Person of Interest this document absolutely has you covered. It starts out with the expected vague description of the training data: Claude Opus 4 and Claude Sonnet 4 were trained on a proprietary mix of publicly available information on the Internet as of March 2025, as well as non-public data from third parties, data provided by data-labeling services and paid contractors, data from Claude users who have opted in to have their data used for training, and data we generated internally at Anthropic. Anthropic run their own crawler, which they say "operates transparently—website operators can easily identify when it has crawled their web pages and signal their preferences to us." The crawler is documented here, including the robots.txt user-agents needed to opt-out. I was frustrated to hear that Claude 4 redacts some of the chain of thought, but it sounds like that's actually quite rare and mostly you get the whole thing: For Claude Sonnet 4 and Claude Opus 4, we have opted to summarize lengthier thought processes using an additional, smaller model. In our experience, only around 5% of thought processes are long enough to trigger this summarization; the vast majority of thought processes are therefore shown in full. There's a note about their carbon footprint: Anthropic partners with external experts to conduct an analysis of our company-wide carbon footprint each year. Beyond our current operations, we're developing more compute-efficient models alongside industry-wide improvements in chip efficiency, while recognizi...