Researchers have developed a method to diagnose the pretraining data mixture of Large Language Models, a crucial aspect of their architecture that influences their behavior and capabilities. This approach, known as Data Mixture Surgery, enables the estimation of domain-level data composition using only generated text from a target model1. By analyzing the output of a Large Language Model, it is possible to infer the mixture of data used during its pretraining phase, which can be useful for auditing and understanding potential biases or vulnerabilities. The lack of transparency in data mixture composition has made it challenging to assess the provenance and combination of data in these models. This breakthrough has significant implications for the development and deployment of Large Language Models, as it can help identify potential security risks and inform policy decisions. So what matters to practitioners is that this method can facilitate a deeper understanding of Large Language Models' inner workings, ultimately leading to more informed decision-making.