Developer-first API for building custom AI voice agents
Quick verdict
Retell AI is a strong pick if you have engineers and want maximum control over your voice agent's behavior. Teams without dev resources will get more value from a no-code platform, but as a building block, Retell's voice pipeline is genuinely fast.
Retell AI is aimed at engineering teams that want to build a bespoke voice agent rather than configure a prebuilt one. It provides the real-time voice pipeline, speech-to-text, turn-taking, interruption handling, text-to-speech, as an API/SDK, and lets developers plug in their choice of LLM and define custom logic in code. That makes it powerful for teams with specific product requirements (embedding a voice agent inside an existing app, highly custom conversation logic) but it assumes engineering resources; there isn't a no-code builder equivalent to Synthflow. Retell also offers a web-based agent builder for simpler flows, narrowing the gap for smaller teams.
Retell AI charges per minute of voice usage on top of any LLM/telephony costs you bring, with volume discounts at higher usage tiers.
Engineering teams building a custom voice agent experience into their own product.