Retell AI
Voice-agent platform with vendor focus on sub-800ms turn latency and proprietary voice models. Y Combinator S24; raised $4.6M seed Aug 2024 (BoxGroup + others); >$1M ARR by month 6 per founders. Direct competitor to Vapi with proprietary-model focus.
At a glance
- Type
- Developer platform for AI voice agents (low-latency)
- Tier
- T1
- Section
- Voice agent platforms
- Created
- 2024 (YC S24)
- Latest release
- not applicable — not OSS
- License
- not applicable — not OSS
- Pricing
- Per-minute usage ($0.07/min + provider passthrough)
- Funding
- $4.6M seed Aug-2024 (BoxGroup + others)
Taxonomy
- storage
- kv
- retrieval
- injection
- persistence
- session
- update
- agent-controlled
- unit
- turn
- governance
- opaque
- conflict
- stateless
When to use
Optimised for: Sub-800ms voice turn latency with vertically-integrated stack
Anti-fit: Telephony focus; smaller funding than Vapi; per-minute pricing
Pros & cons
Pros
Latency-first positioning; SOC 2 + HIPAA; YC distribution; clean SDKs.
Cons
Smaller than Vapi; per-minute economics at scale; proprietary stack reduces flexibility.
Claims & capabilities
YC S24; >$1M ARR by month 6; proprietary low-latency voice stack
Technical surface
- API surface
- REST API + Python/JS/Go SDKs; webhooks
- Backend storage
- Call state in Retell-managed DB
- Deployment
- Cloud-managed
- Embedding model
- not applicable — not a memory product
- Multi-tenancy
- Multi-tenant cloud
- MCP
- searched not found
- A2A
- searched not found
- OpenTelemetry
- searched not found
Compare Retell AI with…
Similar systems
Other voice agent platforms in the catalog, ranked by inbound references.
- Cartesia Sonic T1
Cartesia's Sonic TTS — state-space-model-based voice generation, lowest published TTS latency (~40ms). Cartesia raised $64M Series A Q1 2025 (Index + Lightspeed); founded by Mamba authors. Agent platform layer ('Cartesia Line') wraps Sonic with telephony + LLM.
- Vapi T1
Voice-AI developer platform — STT + LLM + TTS pipelines wired to telephony (Twilio / SIP). Used to build outbound + inbound phone agents at scale. Raised $20M Series A from Bessemer + Y Combinator (Dec 2024) at $130M valuation; revenue reported >$10M ARR by Q1 2025.
- Bland AI T1
Voice-AI platform with custom STT + LLM + TTS stack ('the whole pipeline'). Raised $40M Series A Apr-2024 (Lightspeed + Anthology). Distinguished by 'pathway' graphs (deterministic call flows) and aggressive outbound-calling positioning.
- CallRail Conversation Intelligence T2
CallRail's AI-driven conversation intelligence layer atop their call-tracking product. Marketing-vertical voice analytics — auto-tagging, sentiment, lead scoring. CallRail is profitable / private (Sageview-backed); not a voice-agent runtime but the BI / analytics adjacent layer.
- ElevenLabs Conversational AI T1
ElevenLabs launched Conversational AI Nov-2024 — STT + LLM + their own market-leading TTS in one pipeline. Combines vendor's voice cloning + multilingual TTS strengths with hosted call orchestration. ElevenLabs raised $180M Series C Jan-2025 at $3.3B val.
- Hume EVI T1
Hume's Empathic Voice Interface (EVI) — voice agent platform with vocal-affect understanding. Hume raised $50M Series B Mar-2024 (EQT). Distinguished from pure LLM voice agents by emotion/prosody modelling; uses Hume's proprietary speech-prosody models.
Related systems
References (3)
- Anthropic Claude (foundation models) depends on at runtime — adjacent-infrastructure cell: Twilio SIP; in-house STT/TTS; Anthropic / OpenAI LLMs
- OpenAI GPT family (GPT-5 / GPT-4o / o3 / o4) depends on at runtime — adjacent-infrastructure cell: Twilio SIP; in-house STT/TTS; Anthropic / OpenAI LLMs
- Vapi competes with — er founders. Direct competitor to Vapi with proprietary-model focus.