Retell AI
Developer-first voice agent API — sub-second latency, bring-your-own LLM, telephony built in.
About
Retell AI is a developer-centric platform for building production voice agents, and it treats latency as an engineering constraint: roughly 600-780ms end-to-end in production testing. Its LLM-agnostic architecture lets you plug in GPT, Claude, or fine-tuned models — even routing different conversation turns to different models — with built-in inbound and outbound telephony, call transfer, voicemail detection, and interruption handling. Pricing is transparent and itemized from about $0.07/min base, with $10 in free credits; it is SOC 2 certified and HIPAA-ready.
The honest picture: this is a developers-only tool — there is no true no-code path, and you will need real engineering to ship. The all-in cost stacks to roughly $0.13-$0.31/min once you add knowledge bases, denoising, guardrails, and telephony, silence and hold time are billable, and production voice agents demand extensive edge-case testing plus reliable human fallback.
What it can do
- 600-780ms voice pipeline latency
- Bring-your-own LLM architecture
- Built-in telephony and number provisioning
- Call transfer, voicemail detection, barge-in
- Itemized per-minute pricing
Setup
Sign up at retellai.com for $10 in free credits. REST API plus TypeScript SDK and a basic visual builder.
Prerequisites
Engineering resources. Phone numbers via Retell or your own SIP provider.
Use cases
Works with
Trust score — 8/10
Scored 8/10 (verified tier, metrics-v1). Evidence: Verified via vendor site/docs/GitHub and reputable coverage, wave 51; Trending score 80/100 from researcher assessment.
Sources
Every fact above was checked against these sources. Where a claim comes from the provider and couldn't be independently verified, the text says so.