DeepSeek announced on Monday that it would temporarily limit user registrations due to large-scale malicious attacks on its services. On Monday, DeepSeek dethroned ChatGPT as the most-downloaded free app in the U.S. on Apple's App Store, sparking a tech stock sell-off. The company's newly released R1 model is an open-source reasoning model that has drawn praise from developers and analysts for its performance and cost-efficiency. DeepSeek reportedly trained its R1 model at a fraction of the cost of competing models from OpenAI, Anthropic, and Google.