Claude 4’s system prompt is very similar to the 3.7 prompt we analyzed last month. Once 4.0’s training is done, new issues will emerge that must be addressed by the system prompt. Reviewing the changes above (and honestly, that’s the bulk of them from 3.7 to 4.0), we get a sense for how system prompts program chatbot applications. But in reality, the bulk of the UX is defined here, in the system prompt. First with system prompt hot-fixes, then with post-training when building the next model.