Arena raises $200M Series B at $3.1B valuation, launches Alignment Index

Arena announced a $200 million Series B at a $3.1 billion valuation, alongside a new Alignment Index that measures whether AI agents behave safely, truthfully, and within the bounds of user requests.

The company has surpassed $100 million in annualized revenue, facilitated 350 million sessions and 62 million votes, and led by Felicis and PXD from the seed and Series A stages.

Arena positions the index as a way to assess trustworthiness as AI systems increasingly take real actions.