Sierra raises $950M Series E at $15.8B valuation as enterprise AI agent race accelerates

Sierra, the enterprise AI agent platform co-founded by OpenAI chair Bret Taylor and Clay Bavor, raised $950M in a Series E round led by Tiger Global and Google GV, with participation from Benchmark, Sequoia, and Greenoaks. The company achieved $150M ARR within eight quarters and serves more than 40% of Fortune 50 companies. Sierra has now raised over $1B total and launched Ghostwriter, an agent-as-a-service tool, in April 2026. The valuation jumped from $10B in fall 2025 to $15.8B.