General Catalyst leads $1.1B round into 2-month-old River AI

River AI, a startup founded by a former co-founder of xAI, secured $1.1 billion in funding from investors including General Catalyst and Nvidia. The company aims to reinvent how AI models are trained so that AI agents can become personally trainable assistants rather than worker replacements. River offers an API that allows developers to fine-tune open-source models through reinforcement learning and low-rank adaptation, positioning this as an alternative to prompt engineering. The funding reflects growing enterprise interest in controlling their AI systems by using open-weight models rather than relying solely on closed-source alternatives.