Sierra raises $950M Series E at $15.8B valuation for enterprise AI agents

Sierra, the AI startup co-founded by OpenAI chair Bret Taylor and former Google executive Clay Bavor, raised $950 million in a Series E round led by Tiger Global and Alphabet's GV, reaching a $15.8 billion post-money valuation (up from $10 billion in fall 2025). Benchmark, Sequoia, and Greenoaks also participated. Sierra reports over $150M ARR, grew from $100M ARR in November 2025 to $150M by February 2026, and serves 40%+ of Fortune 50 companies including Prudential, Cigna, Blue Cross Blue Shield, and Rocket Mortgage. The company's AI agents handle billions of customer interactions monthly for enterprise customer service automation.