June 8 (Reuters) - OpenAI confidentially filed for a U.S. initial public offering recently, the ChatGPT maker said on Monday, joining rival Anthropic in a push toward a stock market listing as it looks to tap into insatiable investor demand for AI shares.

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 U.S. initial public offering, weeks after raising $65 billion in a funding round that valued it at $965 billion.

"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.

In March, OpenAI said it was generating $2 billion in monthly revenue and growing roughly four times faster than companies that defined the internet and mobile eras, including Alphabet and Meta.

That compares with about $1 billion in quarterly revenue at the end of 2024.

OpenAI told investors during its most recent fundraising round that it did not expect to be profitable until 2030, according to a source familiar with the matter.

CHALLENGERS GAIN MOMENTUM

Yet the industry OpenAI launched has quickly become crowded and investors are scrutinizing whether the AI sector's meteoric rise can be sustained.

Anthropic has emerged as one of the biggest rivals, with soaring demand for its Claude AI from software developers to handle their computer programming, and some firms deploying its top-shelf model Mythos to unearth vulnerabilities in their code.

While the blockbuster offerings could inject fresh momentum into the U.S. IPO market, some bankers warn they might also soak up capital that could otherwise flow to smaller deals.

"What OpenAI does not want is for the public market capital to exhaust itself," said Gil Luria, managing director of D.A. Davidson. "Not only are SpaceX and Anthropic ahead of it in line to IPO, large public competitors could also raise tens of billions of dollars each in public market secondary issuances, as Google just completed last week."

Musk-led SpaceX goes public this week.

NONPROFIT ROOTS SPARK LEGAL DISPUTE

OpenAI was founded in 2015 as a research-focused nonprofit, but created a for-profit arm four years later to help fund the soaring costs of developing artificial intelligence systems.

Its unusual structure, which gave the nonprofit control over the for-profit entity, came under intense scrutiny in late 2023 when CEO Sam Altman was briefly ousted before returning days later after employees revolted.

In December 2024, OpenAI unveiled plans to overhaul its structure by creating a public benefit corporation, saying the move would help it raise far more capital while easing restrictions imposed by its nonprofit parent.

OpenAI's overhaul quickly became controversial after sharp criticism from its early backer, Musk, who later sued OpenAI and accused Altman and other executives of turning the nonprofit into a vehicle for private enrichment.

A U.S. jury in May ruled against Musk in his lawsuit, finding the AI ​company not liable to the world's richest person for having allegedly strayed from its original mission to benefit humanity.

The unanimous verdict removed a key overhang on the IPO, with analysts saying it cleared a major legal hurdle.