The Bad Blood Between Polymarkets Shayne Coplan and Kalshis Tarek Mansour

Early one morning in November 2024, Shayne Coplan, the founder of Polymarket, awoke to an unexpected visit.A group of F.B.I.

agents burst into his New York apartment with a battering ram, scouring for evidence of illegal betting on his platform.On social media, Mr.Coplan dismissed the raid as a politically motivated attack by the Biden administration.

But privately, he and his advisers voiced suspicions about a different culprit, whom they blame for many of Polymarket’s problems: Tarek Mansour, the chief executive of Kalshi, Mr.Coplan’s biggest rival in the booming business of prediction markets.Mr.

Mansour’s company had indeed played a role.In the months before the F.B.I.

raid, Kalshi’s lawyers met with federal prosecutors in Manhattan to raise concerns about Polymarket, according to four people with knowledge of the discussions, which have not been previously reported.The lawyers explained how Polymarket’s platform worked, one of the people said, and highlighted an important fact — that customers could use it from the United States, where it was supposed to be banned.It was a moment of maximal aggression in a long-simmering feud between Kalshi and Polymarket, two of the tech industry’s hottest start-ups.

A clash of ambitious founders is nothing new in Silicon Valley, where ego-driven squabbles are as familiar as corporate puffer vests.Uber and Lyft battled to rule the ride-hailing industry, and the billionaire-on-billionaire mudslinging between Sam Altman of OpenAI and Dario Amodei of Anthropic has become an intriguing subplot in the artificial intelligence world.But the competition between the 28-year-old Mr.

Coplan and the 30-year-old Mr.Mansour goes far beyond the normal bounds of corporate rivalry.

Both are newly minted billionaires, intent on dominating prediction markets, which offer betting odds on sports, politics, culture and nearly everything else.By most accounts, they also despise each other with a fervor that has seeped into how ...