> Claude Sonnet 4.6 was trained on a proprietary mix of publicly available information from the internet up to May 2025, 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 generated internally at Anthropic. Throughout the training process we used several data cleaning and filtering methods including deduplication and classification. ... After the pretraining process, Claude Sonnet 4.6 underwent substantial post-training and fine-tuning, with the intention of making it a helpful, honest, and harmless1 assistant.