Sierra is a conversational AI platform that builds autonomous, action-taking AI agents for enterprise customer experience across chat, voice, email, SMS, WhatsApp, and ChatGPT. Co-founded in 2023 by former Salesforce co-CEO and OpenAI board chair Bret Taylor along with former Google AR/VR lead Clay Bavor, Sierra has rapidly become the category-defining vendor for outcome-based AI customer agents. As of May 2026, the company raised $950 million at a $15.8 billion valuation just months after a $350M round at $10B, and now powers customer-facing agents for roughly 40 percent of the Fortune 50 including WeightWatchers, SiriusXM, Sonos, ADT, Chime, Cigna, Nordstrom, Nubank, Ramp, Rivian, Rocket Mortgage, Singtel, Sutter Health, and Wayfair. Sierra's platform is organized around its Agent Playbook: Agent OS for omnichannel deployment, the Agent Data Platform for persistent cross-session memory and personalization, Agent Studio as a no-code builder for CX teams, the Agent SDK for developers, and Ghostwriter, an agent that builds other agents from SOPs, transcripts, photos, or plain-English instructions in over 30 languages. Sierra uses outcome-based pricing, charging customers only for successfully resolved interactions rather than per-message or per-seat, which has driven ARR from $100M in late 2025 to over $150M by early 2026. The platform emphasizes brand voice fidelity, safety guardrails, and continuous autonomous improvement through its Explorer evaluation system.