“Instead of starting a company together, go on vacation together,” Amodei recently said during a talk at the Stanford Graduate School of Business. “Share a room with them. Be like, ‘How did that go?’ And if you’re like, ‘Man, all I wanna do is spend more time with you,’ great.”

“If you’re like, ‘Really, I’m gonna need a vacation to recover from my vacation,’ it might be the wrong choice,” Amodei continued. 

The AI magnate says she got “extremely lucky” with her cohort of fellow founders at Anthropic. And when it comes to picking the best right-hand man, Amodei looks for strong interpersonal relationships. She’s known her brother Dario Amodei her entire life, and some of the other cofounders have been in her orbit for 15 years. Plus, the majority had also already reported to her or her brother while they were all working at OpenAI. The Anthropic entrepreneurs thrived off an established framework of how to give each other feedback—and importantly, she says they knew “who we were as people.”

The only other key component is having a consensus of what the business is trying to do. Amodei said that Anthropic’s cofounders were all united on wanting to achieve something else; the core group had all previously at OpenAI, but broke off for various reasons, including differing views on company vision. When launching Anthropic, the Claude creators were “in the same zone already,” Amodei says.

“If you locked yourself and your co-founder in another room, and you wrote down or drew a picture of what it is you’re trying to build, you’re not gonna walk out and one has drawn a unicorn and the other has drawn a platypus,” the cofounder continued. “That’s the type of situation where you think you’re doing the same thing, but I think it just doesn’t end well.”

How entrepreneurs pick their cofounders—from stress tests to writing exercises

Paul Graham, the cofounder of Silicon Valley start-up accelerator Y Combinator, has warned entrepreneurs to be careful who they pick as a cofounder. Just like Amodei, he agrees that good character is immensely important in a partnership—it’s tested “severely” when times get tough.

“What people wished they’d paid more attention to when choosing cofounders was character and commitment, not ability. This was particularly true with startups that failed,” Graham wrote in a 2009 essay. “The lesson: don’t pick cofounders who will flake.”

Andy Dunn, the cofounder and former CEO of Bonobos, laid out five specific tests entrepreneurs should complete with a prospective partner before entering “the intense commitment of starting a company together.” 

First, the stress test, by practicing having difficult conversations; next, they should administer the time test, spending lots of time together to bond as business parents. Partners should also engage in the role exercise, ironing out who will do what, and how decisions change at scale; and the “when things go south” test will set up a gameplan for their inevitable departures. Finally, Dunn recommended examining their differences to make sure they’re not too alike—”very divergent skill sets” are key.

“You need to spend enough time with each other socially, getting to know each other’s friends and loved ones, and treating it like a marriage,” Dunn wrote for Fortune in 2024. 

VC cofounder Jeff Rosenthal and his business partner, Patrick Maloney, both handed each other an exam before jumping into the deep end. 

Before creating CIV, an investment platform that invests in and scales tech companies, the duo sat down and tested their dynamics through a written exercise. They each outlined their company vision in writing—from their office schedules, to their outlook for the business—exchanging notes “like pen pals” to ensure “nothing was lost in translation.” 

“We wanted to make sure that we were aligned in our vision for the type of business we would build, the type of culture we wanted, our expectations of each other,” Rosenthal told Business Insider last year. “And we didn’t wanna leave anything unsaid.”