Tesla on Thursday plans to unveil a product that Elon Musk, the company’s chief executive, has said will add trillions of dollars to its stock market value and fuel its growth.

The product is a prototype of a self-driving taxi that Mr. Musk has been promising for years. And it will be shown at an invitation-only evening event at the Warner Bros. studio near Los Angeles. Mr. Musk has promised that the cab, which he calls the Robotaxi, will be able to ferry passengers to any destination without human intervention, a feat that other companies have achieved in just a few places, like Phoenix and San Francisco.

But many experts are doubtful that such Tesla taxis will hit the roads anytime soon. Mr. Musk has for several years claimed that the company was months away from starting a Robotaxi service. In addition, the autonomous driving technology Tesla offers today can make basic mistakes, requiring drivers to intervene to avoid accidents or violating traffic laws.

Still, Mr. Musk’s supporters and fans believe that the Robotaxi will open a lucrative line of business that will more than make up for Tesla’s recent struggles in the electric car market, where it has lost market share to more established carmakers. Mr. Musk has said people will be able to purchase Robotaxis for personal use and earn extra money by allowing the vehicles to ferry passengers, the automotive equivalent of listing a home on Airbnb.

“An autonomous taxi platform will unlock a multitrillion-dollar market,” Tasha Keeney, director of investment analysis at ARK Invest, an asset management firm that owns shares in Tesla, said in a statement.

But other analysts and autonomous driving experts said there was little evidence that Tesla was close to perfecting the technology and making a profit from it anytime soon.