Elon Musk weighs in on competitor Rivian after the company's blockbuster IPO, says high production and breakeven cash flow are the 'true test'

• Rivian made its debut on the Nasdaq on Wednesday, closing 30% up from its offering price.
• Tesla CEO Elon Musk tweeted that production and breakeven cash flow would be Rivian's "true test."

Elon Musk weighed in on Rivian a day after the Tesla competitor's blockbuster initial public offering.

In a tweet, the Tesla CEO said that high production and breakeven cash flow would be the "true test" for Rivian and that Tesla was the only American carmaker to have achieved those benchmarks in the past 100 years.

Musk was responding to a tweet pointing out that Tesla - which is cash-flow positive - had already been selling its Roadster model for two years with plans to roll out the Model S when it went public in 2010.

Rivian, in comparison, just started selling its vehicles and reported a net loss of almost $1 billion in the first half of this year.

Rivian made its public debut on the Nasdaq on Wednesday, closing 30% up from its IPO price. The startup's shares extended gains Thursday, finishing the day 22% higher. The company's market cap is now over $100 billion, making it more valuable than the legacy automakers Ford and General Motors (both of which are over 100 years old).

It's not the first time Musk has commented on his company's competitor. Last month, in response to another tweet about Rivian's electric-truck production rate, he tweeted that "prototypes are trivial compared to scaling production & supply chain."

Rivian did not immediately respond to a request for comment on Musk's latest tweet.