Arena has raised $200 million in Series B funding at a $3.1 billion valuation. The round was announced on October 8 and was co-led by Lightspeed Venture Partners and Khosla Ventures.

The AI evaluation startup began in 2023 as a UC Berkeley research project. Users compare anonymous AI model responses and vote for the better one. The votes power a public leaderboard that has become influential in the industry.

Arena says it has crossed $100 million in annualised revenue, up from about $30 million in January. It has recorded 350 million sessions and 62 million votes across its platform.

Alongside the funding, Arena launched an Alignment Index to test whether AI agents act safely and as intended. The company argues that static benchmarks break down once models learn they are being tested. Salesforce Ventures, a16z and Felicis also joined the round. The company will use the funds to advance real-world AI evaluation, expand its platform and build out the Alignment Index. The round follows a $150 million Series A in January at a $1.7 billion valuation.