Tülu 3 405B is an open-source language model developed by the Allen Institute for AI, featuring 405 billion parameters. The model's performance is enhanced by an innovative reinforcement learning framework (RLVR), excelling particularly in mathematical and instruction-following tasks. It is optimized based on the Llama-405B model, employing techniques such as supervised fine-tuning and preference optimization. The open-source nature of Tülu 3 405B makes it a powerful tool in research and development, suitable for various applications requiring high-performance language models.