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June 8 (Reuters) - ChatGPT-maker OpenAI confidentially filed for a U.S. initial public offering on Monday, joining rival Anthropic in a push toward the stock market as investors seek exposure to the artificial intelligence boom.

The move sets up a high-stakes test on whether investor appetite for the AI revolution that has reshaped white-collar work around the world can match the sky-high expectations surrounding OpenAI.

It did not disclose the size or terms of the offering, but Reuters has 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.

The crucial step toward a listing comes on the heels of SpaceX's mega-IPO, which is on course to rewrite the record books as the Elon Musk-led company pursues a $75 billion offering at a $1.75 trillion valuation.