Opening arguments have begun in the $134 billion lawsuit filed by Elon Musk against OpenAI, with attorneys on both sides presenting their cases in federal court in Oakland.

Musk arrived at the courthouse Tuesday ahead of his anticipated testimony, while OpenAI CEO Sam Altman also appeared alongside his legal team as proceedings got underway.

The lawsuit centers on Musk’s claim that OpenAI strayed from its founding purpose. As a co-founder, he argues the organization was meant to operate as a nonprofit dedicated to advancing artificial intelligence for the public good. His attorney told jurors that Musk contributed roughly $38 million to help establish the company, with the expectation that its technology would remain open source and focused on safety rather than profit.

To support its mission, OpenAI later set up a for-profit arm—something Musk’s lawyer likened to a museum gift shop funding a nonprofit institution. However, Musk contends that the company has since pivoted toward a profit-first approach, bolstered by billions in funding from Microsoft, which is also named in the lawsuit. “No one should be allowed to steal a charity,” his attorney argued.

OpenAI’s legal team strongly rejected those claims, describing the case as a “pageant of hypocrisy.” They assert that Musk’s motivations stem from rivalry with his own AI company, xAI, and maintain that no binding commitments were tied to his early contributions.

Defense attorneys also claimed that Musk previously attempted to convert OpenAI into a fully for-profit business under his control, an idea that other founders opposed. Additionally, they argue the lawsuit was filed too late and falls outside the legal time limits.

The trial is expected to run for about three weeks before being handed to an advisory jury. Ultimately, a federal judge will decide on liability and any potential damages.