Arena raises $200 million at a $3.1 billion valuation and launches an AI Alignment Index
Today, we're announcing our Series B: $200M at a $3.1B valuation, and the release of Arena's Alignment Index. AI is advancing faster than our ability to evaluate it. The world needs a neutral third party to measure how safe and aligned AI actually is once it's in the hands of real people. That's the role Arena is stepping into today. The Alignment Index is built from real-world agent traces, starting with three signals: Unauthorized Action, False Attribution, and Deceptive Completion, with results for over 20 frontier models launching today. Hear more from our CEO @ml_angelopoulos below, and read the full breakdown, and more details about our company growth in thread. This funding will allow us to go further on safety signals, agentic capabilities, and modalities. It will also allow us to grow the company to match the scale of our mission. Today, we’re still a nimble team of 90, and we're looking for people to join us across research, product, engineering, and more. Come build with us. As AI gets more capable and autonomous, understanding how well it can safely and effectively work for people only matters more. That's what Arena is built to measure and advance. Thank you to our community, our investors, and everyone building with us! Take a look at our open roles if you want to join us (link in bio).
Context
The best known AI leaderboard is moving from ranking which answers people prefer to measuring whether agents act safely and honestly.
Arena began in 2023 as Chatbot Arena, a research project at Berkeley's Sky Computing Lab, and ranks AI models through crowdsourced human comparisons. It raised $150 million at a $1.7 billion valuation in January. The new round was led by Lightspeed Venture Partners and Khosla Ventures, The Next Web reported.
Deep dive
Arena announced its Series B on October 8: $200 million at a $3.1 billion valuation. The company says it is still a team of 90 and will use the money to go further on safety signals, agentic capabilities and new modalities. The Next Web reported that Salesforce Ventures and Dell Technologies Capital also joined.
Alongside the round, Arena launched the Alignment Index, built from more than 90,000 real world agent sessions across 27 models. It tracks three signals: Unauthorized Action, when an agent goes beyond the user's instructions or permissions; False Attribution, when it attributes statements or actions contradicted by user evidence; and Deceptive Completion, when it claims a task is done when it is not.
According to Arena, OpenAI's GPT 6.1 Sol leads with a score of 87.9, followed by Anthropic's Claude Opus 5.5 at 83.2 and SpaceXAI's Grok 4.7 at 82.7. Arena says OpenAI models had the best observed rates on all three signals, that newer models beat their predecessors across all four labs, and that misalignment risk rises with conversation length.
The Next Web noted that the method is the open question: preference voting can be gamed, and the index is itself a grader of agent behavior. The Next Web also reported that Arena had $100 million in annualized revenue by June.
What to expect next
Arena says the index is a starting point and will add more safety signals and models over time. Watch for how labs respond to their scores, and for whether outside researchers can audit how sessions are classified.
The UK Information Commissioner's Office also opened a call for evidence on October 8 on data protection risks from AI agents, so demand for independent agent safety measures may grow. Arena is hiring across research, product and engineering.