ChatGPT maker OpenAI confidentially filed for a US initial public offering recently, the company said on Monday, joining rival Anthropic in a push towards the stock market as investors seek exposure to the artificial intelligence boom. OpenAI did not disclose the size or terms of the offering, and said a timeline has not yet been determined. "It may be a while because there are things we want to do that are likely easier as a private company," it said in a statement. Reuters had reported that the AI giant is targeting a valuation of up to $1 trillion in a stock market debut that could come as early as September. At that valuation, OpenAI would set the stage for a trio of trillion-dollar-valuation companies debuting rapidly, which together are seen as the most consequential test of investor appetite for high-growth technology stocks in the last 10 years. Elon Musk's SpaceX was the first off the block, filing for an IPO that would rank as the largest in history if completed, with the company pursuing a $75 billion offering at a $1.75 trillion valuation. Anthropic, the company behind the viral coding assistant Claude Code, said on June 1 it had confidentially filed for a US initial public offering, weeks after raising $65 billion in a funding round that valued it at $965 billion. Read more'Human role narrowing': Anthropic calls for global AI slowdown as systems could escape control "OpenAI is keeping options open as Anthropic edged ahead with its filing after a monster funding round," said Michael Ashley Schulman, a partner at Cerity Partners. On prediction markets, where traders wager on the outcome of future events, most participants had expected OpenAI to file for an IPO before Anthropic. The AI era The IPOs of Anthropic and OpenAI would crystallize a transformative period for the technology industry and global markets, with artificial intelligence rapidly emerging as the defining investment theme of the decade. OpenAI said earlier this year that it was raising $110 billion at an $840 billion valuation from a roster of heavyweight backers including SoftBank, Amazon and Nvidia. At the time, it also disclosed that ChatGPT had more than 900 million weekly active users and over 50 million consumer subscribers. The IPO filing follows OpenAI renegotiating its partnership with Microsoft, one of its earliest investors, which allowed the AI pioneer to forge new partnerships with firms such as Amazon.com and Alphabet's Google. The Windows maker's early investment, totaling $13 billion since 2019, helped pave the way for OpenAI's rapid rise and powered growth at Microsoft's Azure cloud-computing business.