Arena, the developer of AI evaluation tools, secured $200 million in a Series B funding round on September 22, 2026. This investment brings the company's post-money valuation to $2.88 billion. Previous funding includes a $100 million seed round in May 2025 and a $150 million Series A in January 2026.
The startup's commercial AI Evaluations product reached an estimated annualized run-rate of $100 million by June 2026, up from $30 million in January 2026. Backers of the company include Andreessen Horowitz, Felicis, UC Investments, Kleiner Perkins, and Lightspeed.
On October 8, 2026, Arena launched its Alignment Index to measure the safety and behavior of AI agents during practical tasks. The index is based on data from over 90,000 agent sessions across 27 models. It tracks metrics such as false attribution, where agents incorrectly credit information sources.
Early index results show OpenAI's GPT-6.1-Sol leading with a score of 87.9. Anthropic's Claude-Opus-5.5 follows with 83.2, and Grok-4.7 ranks third with 82.7. Arena originated as an open-source project at UC Berkeley in April 2023 and formally incorporated in April 2025.
