Arena raises $200M Series B and launches an AI Alignment Index

Arena, the startup behind one of the most watched AI leaderboards, has closed a $200 million Series B at a post-money valuation of $2.88 billion. The round closed on September 22, 2026. Weeks later, the company followed up with a new product: an Alignment Index that scores how AI agents behave when given real tasks. The money and the math The Series B lifts Arena’s total funding to approximately $450 million. Arena raised $100 million in seed funding in May 2025, then a $150 million Series A in January 2026. The Series B arrived about eight months after that. Investors who have backed Arena across its rounds include Andreessen Horowitz, Felicis, UC Investments, Kleiner Perkins, and Lightspeed, among others. Arena’s commercial AI Evaluations product, introduced in September 2025, reached an estimated annualized run-rate of $100 million by June 2026. That figure stood at $30 million in January 2026. Arena charges on a consumption basis rather than through fixed subscriptions. Grading the agents The Alignment Index launched on October 8, 2026. It is designed as a benchmark for measuring AI agent safety and alignment in practical settings. Arena built the index from more than 90,000 ag...