Arena announces $200M Series B at $3.1B valuation and launches Alignment Index

Arena announced a $200 million Series B at a $3.1 billion valuation alongside its Alignment Index. The company says the index draws on real-world agent traces and launched with results for over 20 frontier models. Its initial signals are Unauthorized Action, False Attribution and Deceptive Completion. Arena says the funding will support more safety signals, agentic capabilities and modalities.