The bitter feud between Elon Musk and OpenAI boss Sam Altman has raged for years, but has mostly played out online in the form of accusations, counter-accusations and jibes.
Musk was at it again on Monday, calling Altman "Scam Altman" in a post on X.
But starting on Tuesday, the beef between the two tech billionaires will shift to a much higher-profile forum: a federal courtroom in California, where their row will be the focus of a month-long trial.
Being considered is Musk's claim that Altman - with whom he founded OpenAI - has swindled him out of millions of dollars and reneged on the ChatGPT-maker's original non-profit mission.
Musk and Altman themselves will be among those to testify in a case in which the future of AI could be at stake. And while one will presumably emerge the winner, it's plausible that neither will emerge from the saga unscathed.
The brawl has been likened to two heavyweight boxers heading into the ring. One observer compares it to King Kong taking on Godzilla.
"Musk and Altman are so big, so larger than life, and so unrelatable," says University of San Diego professor Sarah Federman, who specialises in conflict resolution. "That's what makes them so delicious to watch as they clash."
Now, a nine-person jury sworn in on Monday will help determine the outcome under the oversight of Judge Yvonne Gonzalez Rogers, who has promised that the wealth, power and celebrity Musk and Altman bring to the federal courthouse in Oakland will afford them "no special treatment".
Musk is also suing OpenAI and its co-founder and president Greg Brockman, along with Microsoft, whom he said aided in the scheme to monetise the company - claims which Microsoft denies.
Musk is asking for billions of dollars in what his lawyers call "wrongful gains" that he wants used to fund OpenAI's non-profit arm, and wants to see a shake-up at the company including the ousting of Altman.
OpenAI says Musk is motivated by jealousy and regret for walking away from the company. And as the race to artificial general intelligence (AGI) barrels full steam ahead, OpenAI accuses Musk of trying to derail one of his key rivals.
Musk and Altman co-founded OpenAI as a non-profit in 2015 with the mission of ensuring that AGI benefits all of humanity.
AGI is loosely defined as AI that surpasses human intelligence.
When OpenAI was formed, Musk's star had fully ascended. He was widely viewed as a relentless technologist, bringing electric vehicles into the mainstream as the head of Tesla while also developing revolutionary reusable rocket technology at SpaceX.
Altman, meanwhile, was well-known in Silicon Valley but not beyond. From his perch heading the influential tech incubator Y Combinator, his oracle-like pronouncements on Twitter were eagerly consumed by budding start-up founders.
Musk and Altman were reportedly introduced by a Silicon Valley investor in 2012. Altman, still in his twenties, was 14 years younger than Musk and would go on to pitch the idea of OpenAI to the Tesla and SpaceX boss. He has previously called Musk his hero.
Developing AI responsibly constituted a key part of the pitch.
With OpenAI, the men were friendly collaborators with a shared belief in the technology's potential.
At a joint conference appearance in 2015, Musk said AI was the one technology that "could most change humanity" but added it was "really dodgy" and "fraught with difficulty".
But what began as a non-profit was shifted into a for-profit entity – illegally, according to Musk.
For its part, OpenAI contends that in 2017, the defendants and Musk agreed that a for-profit was the logical next step "to advance the mission" but that they rejected Musk's push to be CEO with "absolute control".
Musk left OpenAI in 2018 following a reported power struggle with Altman.
"Guys, I've had enough," Musk wrote in an email a few months prior to his departure.
"Either go do something on your own or continue with OpenAI as a nonprofit.
"I will no longer fund OpenAI until you have made a firm commitment to stay or I'm just being a fool who is essentially providing free funding for you to create a startup."
In 2022, OpenAI kicked off the consumer AI revolution with the release of ChatGPT which immediately surged in popularity, reaching 100 million monthly active users within months.
Musk has since started his own AI start-up, xAI, which makes the chatbot Grok and has lagged behind competitors.
When he sued in 2024, Musk alleged that OpenAI had strayed from its core mission and was instead focused on "maximising profits" for Microsoft.
He said he had donated around $40m (£30m) to OpenAI after being manipulated by the defendants who betrayed him by moving to turn it into a mostly for-profit entity.
Since the lawsuit was filed, the animosity between Musk and Altman has frequently spilled into public view.
Last year, Musk and a consortium of investors offered to purchase OpenAI's assets for $97.4bn.
The company had been valued at $157bn in a recent funding round. (It's now approaching a rumoured public listing, or IPO, with a value of about $850bn.)
OpenAI rejected the offer and Altman subsequently posted on Musk's social media platform X, formerly known as Twitter, "no thank you but we will buy twitter for $9.74 billion if you want".
"Swindler," replied Musk in a comment on Altman's post.
Private texts with Mark Zuckerberg show Musk asking if the Meta boss was "open to the idea of bidding on the OpenAI IP with me and some others?"
Musk's interest in buying the company risks muddying the waters when it comes to this trial, says Columbia Law School professor Dorothy Lund.
"Musk has tried to take over OpenAI multiple times. He's been spurned," Lund told the BBC.
"So it's not crazy to think his motives might be a little suspect here. Even the judge in this case, Judge Gonzalez Rogers, has called this out."
The court is also expected to hear testimony from Microsoft boss Satya Nadella, former OpenAI scientists Mira Murati and Ilya Sutskever, and former OpenAI board member Shivon Zilis, who is the mother of four of Musk's children.
Colourful details about the private lives of the brawling billionaires have trickled out in the run-up to trial as attorneys wrangle over the evidence and testimony that should and shouldn't be shared with the jury.
The Tesla boss's use of "rhino ket", as it's known in Silicon Valley parlance, for instance, will not be allowed in court, the judge has ruled.
Musk's legal representation has also made news. One of his lawyers has been moonlighting as a clown in his downtime, according to Business Insider.
Another, who's a Hollywood producer on the side, was recently profiled by Vanity Fair magazine.
The stakes in this case are high for Musk and OpenAI - and, potentially, all of us.
As recently as late 2023, Musk was advocating for a pause in AI development.
Amid that wave of concern about the frenetic pace of the technology's advancements, Altman was briefly ousted as chief executive of OpenAI on concerns that he misled members of the board.
Now, with xAI, which was recently acquired by his soon-to-IPO rocket company SpaceX, Musk is deeply invested in the multi-player dash towards AGI.
"If Musk wins, it could result in the defeat of a key competitor in the race to AGI," said law professor Rose Chan Loui, the executive director of the Lowell Milken Center for Philanthropy and Nonprofits at UCLA.
"Whoever wins that race will have a lot of power."
She says Musk is attempting to position himself as the right person to adequately and fairly represent the interests of the OpenAI non-profit.
"Even though I'm appreciative of his elevating this issue in the public's eye, I think a lot of us worry that he's not actually unbiased, given that he runs his own very large AI company," she said.
The character of the person bringing such claims matters, according to Federman, who recently authored the book Corporate Reckoning on how executives can take responsibility for corporate harm.
This Musk v Altman trial is transpiring as the public has only just started to grapple with AI's complicated entry into our lives.
Both men have been at the forefront of bringing it to consumers.
The trial could offer new insights into their ambition and intent for the development of a technology now used by a growing subset of the global population.
In King Kong vs. Godzilla, "all the little people below are scrambling as these giants hit each other," says Federman. "One ultimately wins – but what's really left is this path that the rest of us have to live with."
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