Diffusion and Lightspeed Venture Partners have co-led a $550m funding round in Harvey, the legal artificial intelligence company, valuing the business at $15.5bn.

Diffusion and Lightspeed both come in as new investors, joined by fellow newcomers Sapphire Ventures and Whale Rock. They sit alongside an existing roster that returned for the round, including Sequoia, Kleiner Perkins, a16z, Coatue, Conviction, Elad Gil, Evantic, GIC, Goldman Sachs Alternatives, Verified Capital, and WNDR.

The proceeds are earmarked for product development and hiring, as Harvey looks to help law firms, in-house legal teams, and professional services firms build and manage their own AI capabilities at scale. The round follows the company’s release of its first post-trained open-weight model and the launch of Harvey LAB, its legal agent benchmark.

“The opportunity for companies to accelerate their competitive advantage with AI has never been higher,” said co-founders Winston Weinberg and Gabe Pereyra. “Harvey wants to be the global partner legal teams turn to for this work, and we plan to hire and develop the best team in the space to support our clients through the next wave of AI transformation.”

The investor interest is underpinned by Harvey’s traction among large legal buyers. The company said 80% of Am Law 100 firms now use its platform, alongside leading in-house legal teams that include five of the Fortune 10.