Ultravox (formerly Fixie.ai) vs Retell AI
Detailed side-by-side comparison to help you choose the right tool
Ultravox (formerly Fixie.ai)
🔴DeveloperVoice AI Tools
Real-time, speech-native voice AI platform that processes audio directly without text conversion, enabling fast, natural voice conversations for AI agents with sub-second latency and preservation of paralinguistic signals.
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FreeRetell AI
🟡Low CodeVoice AI
Retell AI is an end-to-end platform for building, deploying and monitoring voice AI agents that handle phone calls at production scale.
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$0.07/minFeature Comparison
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Ultravox (formerly Fixie.ai) - Pros & Cons
Pros
- ✓Speech-native model processes audio directly, eliminating STT→LLM→TTS pipeline latency and producing sub-second response times that feel conversational rather than transactional.
- ✓Preserves paralinguistic information (tone, pace, hesitation) that traditional cascaded pipelines discard, leading to more natural turn-taking and barge-in handling.
- ✓Open-source Ultravox model published on Hugging Face gives teams the option to self-host for cost, latency, or compliance reasons instead of being locked into a proprietary API.
- ✓First-class integration path with telephony providers like Twilio plus WebRTC support, making it practical to ship real phone-call agents and in-app voice without building media plumbing from scratch.
- ✓Tool/function calling is supported inside live voice sessions, so agents can take real actions (lookups, transfers, bookings, CRM writes) rather than only chatting.
- ✓Developer-first surface area: API, JavaScript SDK, and clear primitives for building agents, which suits engineering teams already comfortable with LLM tooling.
Cons
- ✗Pure developer platform with no visual builder or no-code flow designer, so non-engineers cannot stand up an agent without writing code.
- ✗Voice and language coverage is narrower than long-established TTS/STT vendors that have spent years accumulating locales, accents, and voice libraries.
- ✗Speech-native architecture is newer than the cascaded STT+LLM+TTS approach, so tuning, debugging, and observability tooling around it is less mature than the pipeline ecosystem.
- ✗Costs at scale can be hard to predict for high-volume telephony workloads because pricing combines model usage with telephony minutes from third-party providers.
- ✗Branding/identity churn (Fixie.ai → Ultravox) means older documentation, blog posts, and integration guides on the public web can be inconsistent or outdated.
Retell AI - Pros & Cons
Pros
- ✓Compliance posture (SOC 2 Type II, HIPAA, PCI) is genuinely turnkey for regulated industries.
- ✓No-code flow editor lets ops teams own the agent without engineering tickets.
- ✓Latency tuning is audibly better than rolling your own STT-LLM-TTS pipeline.
Cons
- ✗Per-minute pricing can spike on unbounded outbound campaigns without budget caps.
- ✗Knowledge base ingestion is simpler than dedicated RAG platforms.
- ✗Deep customisation still requires the SDK and engineering work.
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