Hayden Field is The Verge’s senior AI reporter. An AI beat reporter for more than five years, her work has also appeared in CNBC, MIT Technology Review, Wired UK, and other outlets. After months of speculation about whether OpenAI or Anthropic would be first in their race to IPO, Anthropic on Monday reached a key milestone: filing to kick off the process with the U.S. Securities and Exchange Commission. The filing sets the stage for what’s sure to be a massive IPO. As of its fundraise last week, Anthropic is being called the world’s most valuable startup, with a post-money valuation of $965 billion. That tops the $852 billion post-money valuation of OpenAI, which is its biggest rival.…