Arena Series B and Alignment Index

Arena announced a $200 million Series B at a $3.1 billion valuation on October 8, alongside a preview of its Alignment Index for AI agents. The company says the index uses real user-agent sessions to flag unauthorized actions, false attribution and claims of completed work contradicted by the record. Its funding announcement names Lightspeed Venture Partners and Khosla Ventures as co-leads. The index measures selected observable failures; Arena does not present it as a complete assessment of model safety.