A Chinese AI start-up's 12-hour outage left hundreds of millions of users without service, with the company investigating and issuing fixes, prompting complaints and gains by rivals.

Chinese artificial intelligence start-up DeepSeek suffered a prolonged outage overnight that extended into early Monday morning, disrupting service for hundreds of millions of users, according to a company notice and user feedback. The Hangzhou-based AI lab’s namesake chatbot website and app were offline from Sunday evening, with the company continuing to investigate while issuing fixes from between 1am and 9am on Monday, according to service maintenance records the company published online. The...