Vapi Ai

Vapi is purpose-built for AI voice agents and handles the full telephony pipeline — inbound/outbound call routing, real-time STT, LLM turn management, TTS voice synthesis, and call lifecycle events — through a single cohesive API. For agents automating phone interactions: create an assistant configuration (LLM, voice, prompts), initiate outbound calls, receive webhook events per call, and retrieve transcripts. Strong positioning for autonomous calling agents and AI receptionist use cases. Confidence is docs-derived.

REST API at api.vapi.ai. Resources: assistants, calls, phone-numbers, logs. POST /assistant creates an assistant config (model, voice, system prompt, tool definitions). POST /call initiates an outbound call ({assistantId, customer: {number}}). GET /call/{id} retrieves call status, transcript, recording URL. Webhook: configures serverUrl on assistant to receive real-time events (call-started, speech-update, function-call, call-ended). WebSocket streaming available for real-time audio. JavaScript/Python SDKs available.

API key auth: Authorization: Bearer {api_key} header. Keys from Vapi Dashboard. HTTPS enforced. Webhook signature verification for call event payloads. Phone number provisioning via Vapi (US/CA numbers) or BYOP (bring your own provider — Twilio, Vonage). No OAuth2.

Call state machine: created → ringing → in-progress → ended (completed/failed/busy/no-answer). Webhook delivery: Vapi retries failed webhook calls. Recording and transcript available after call ends. Rate limiting: per-account concurrent call limits (plan-based). Vapi uptime at status.vapi.ai. STT/LLM/TTS provider failures surface in call logs; retry or fallback provider configurable.

docs.vapi.ai is the most comprehensive voice AI developer documentation in the segment — quickstart, assistant creation, call management, tool calling, webhook integration, and provider configuration guides. Getting started: sign up, create assistant, make first call in under 15 minutes. Active community via Vapi Discord. Free trial credits.