OAKLAND, Calif. — The internal debate at tech startup OpenAI got so heated back in 2017 that one of its co-founders, Greg Brockman, feared co-founder Elon Musk would hit him, Brockman testified Tuesday in a trial that will determine the future of the organization behind ChatGPT.

“I truly thought he was going to physically attack me,” Brockman said during the fifth day of the trial.

Brockman said the tense encounter happened in August that year, shortly after the OpenAI co-founders decided they needed far more money to develop artificial intelligence than they thought they’d be able to raise as a charity. He testified that they were batting around ideas, including creating a for-profit arm.

“The meeting started out very pleasant,” Brockman testified.

Musk was there and had just given Tesla vehicles as gifts to his fellow OpenAI co-founders, Brockman said. Musk is the longtime CEO of Tesla and SpaceX, and, years later, he would found his own AI startup, xAI, which competes with OpenAI. Ilya Sutskever, another of the co-founders, reciprocated at the meeting by giving Musk a Tesla-related painting as a gift.

But Brockman testified that the tone of the meeting changed when the topic of equity shares in the proposed for-profit OpenAI arm came up. He said Musk wanted majority control of the organization and rejected a proposal that all the co-founders get equal shares.

“He said, ‘I decline,’” Brockman said. He said Musk refused to accept others’ having a say. “He said, ‘When are you going to be departing OpenAI?’ He said, ‘I will withhold funding until you decide what to do.’”

By then, he said, Musk was visibly angry, and he stood up, stormed around the room, grabbed the painting and left. No one ended up exchanging blows, he said.

The meeting was part of a chaotic period in OpenAI’s history that may be central to the trial. Musk is suing Brockman and OpenAI CEO Sam Altman, alleging they breached the duty they owed OpenAI as a charity and have enriched themselves. Brockman and Altman counter that OpenAI is still controlled by a nonprofit foundation and that Musk agreed with them years ago about the need to raise money from investors to attract and retain the best AI researchers.

Testimony began last week with Musk, who said Brockman and Altman misled him about their intentions as they gradually made changes at OpenAI before he sued in 2024. Musk testified that he made a $38 million founding donation to OpenAI. In 2024, OpenAI emphasized that it had raised $90 million from sources besides Musk.

OpenAI was founded in 2015. It added a capped-profit arm in 2019 and announced a restructuring last year that resulted in the nonprofit foundation’s having a 26% ownership share.

Musk has said that if the jury and the judge find Altman and Brockman liable, he wants the court to bar them from working at OpenAI and order other changes to undo its restructuring.

Brockman testified for four hours Tuesday and described other unusual moments from OpenAI’s early days. Just weeks before the meeting where Musk became angry, Brockman said, there was a meeting at a San Francisco-area house that Musk owned and called his “haunted mansion” because of its dated decor and lack of maintenance. Musk warned his co-founders before the meeting to expect “party carnage” in the house, Brockman testified.

“It was clear that there had been a party there the night before. There was confetti and cups all around,” Brockman said.

The meeting also included two women who had or would later have personal connections to Musk, Brockman said: actor Amber Heard, whom Musk was dating at the time, and Musk adviser Shivon Zilis, who now has several children with him.

Zilis is expected to testify Wednesday. She has worked for Musk’s brain science startup, Neuralink, and she served on the OpenAI board of directors until 2023, operating as a bridge between the firm and Musk after he had earlier left the board.

Brockman said on the stand Tuesday that although he admired some parts of Musk’s business record, he became disillusioned with Musk’s understanding of AI. He said Musk appeared underwhelmed when he was presented with GPT-1, an early version of the AI technology that would later underpin ChatGPT. Musk called the technology “stupid” and said “kids on the internet” could do a better job, statements that dismayed an early OpenAI employee who nearly left the industry over the incident, Brockman testified.

“He knows rockets, he knows electric cars, and I believe he did not — and does not — know AI,” Brockman said, adding, “I did not believe he would spend enough time to get good at it.”

He said he and fellow co-founders “considered voting to remove Elon from the board” in 2017 before they decided against the idea. He left the board voluntarily in 2018.

Brockman faced numerous questions about a journal he kept on his laptop to document and think through both personal and business matters. In one entry from September 2017 that has repeatedly come up during the trial, Brockman wrote about his desire to “get out from Elon” and wondered, “Financially, what will take me to $1B?”

Brockman testified that it was “very painful” to have the journal quoted in open court but added, “There’s nothing in there I’m ashamed of.” He said in testimony Monday that his stake in OpenAI is now worth nearly $30 billion, a figure that would make him among the wealthiest 100 people in the world, according to Bloomberg.

U.S. District Judge Yvonne Gonzalez Rogers told jurors Tuesday that presentation of evidence in the case could wrap up early next week, clearing them to begin deliberating. She said some jurors had been submitting questions about the case to her.

“I have been receiving your questions, and I have been sharing them with the lawyers. The lawyers have been trying to incorporate those questions into their examinations,” she said. She did not reveal the contents of the questions.