When Reflection AI set out to raise $2.5 billion early this year, one of its existing venture backers, B Capital, was eager to invest again in the startup, which has emerged as the most high-profile U.S. maker of open-source AI models. But Reflection’s valuation had more than tripled in just six months, forcing investors to write big checks for a sizable stake.

Such a check would have been too big for B Capital’s main venture fund, a $500 million vehicle raised in July. To come up with the cash, the company turned to a different source of capital: a pool of money apart from its regular fund, designed to give limited partners—the pension funds, endowments and wealthy individuals that back VC firms—more direct access to hot startups like Reflection at reduced fees.