Vapi is a developer-centric platform for building, testing, and deploying advanced voice AI agents with sub-251ms response times that enable near real-time, human-like conversational experiences. The platform provides a comprehensive API with thousands of configuration options, supporting multiple AI models including GPT-4, Claude, and Gemini for natural language understanding, along with various text-to-speech providers for voice synthesis. Key capabilities include the Assistants API for configuring voice agents, Squads for chaining multiple specialized agents with intelligent handoffs, Function Calling to trigger external APIs mid-conversation, and Knowledge Base Support using RAG to let agents answer questions from uploaded documents during live calls. Vapi supports deployment across web apps, mobile applications, CRMs, and customer service systems through integrations with telephony providers like Twilio, Vonage, and Telnyx. The platform handles the entire voice AI pipeline from speech-to-text transcription through LLM processing to text-to-speech output, abstracting away the complexity of building conversational voice systems. Vapi is particularly well-suited for building AI phone agents, customer support bots, appointment scheduling assistants, and outbound calling campaigns, giving developers fine-grained control over every aspect of the voice experience.